Due Date Calculator
Estimate your due date, ovulation date, fertile window, and track your cycle.
This calculator estimates your due date, ovulation date, and fertile window using menstrual cycle information and standard pregnancy dating methods. Results are educational estimates and should not replace professional medical advice.
Follicular Phase
Fertility is low. Ovulation is approaching.
When to Seek Medical Advice
- Your cycles are consistently irregular.
- You've been trying to conceive for an extended period without success.
- You have concerns about ovulation, menstrual health, or your pregnancy.
Pregnancy Milestones (if conception occurs)
These milestones apply only if conception occurs around the estimated ovulation date. Actual pregnancy timelines vary and should be confirmed by your healthcare provider.
| Week | Dates | Trimester | Milestones |
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The educational information on this page is based on guidance from recognized medical organizations and public health resources.
How This Due Date Calculator Works
This calculator estimates your due date, ovulation date, and fertile window using the first day of your last menstrual period (LMP) and your average menstrual cycle length. For a regular 28-day cycle, due date is estimated by adding 280 days (40 weeks) to the first day of LMP. Ovulation is estimated to occur about 14 days before the start of the next menstrual period. Because menstrual cycles vary between individuals, the estimated dates may differ from your actual dates. The results are intended for educational purposes and should not replace medical advice.
Real Example
If LMP is July 20, 2026 with a 28-day cycle:
- Estimated Due Date: March 29, 2027
- Estimated Ovulation: August 5, 2026
- Most Fertile: August 2–6, 2026
- Next Period: August 19, 2026
- Pregnancy Test: Around August 19, 2026
Limitations
This calculator provides educational estimates only. Ovulation timing, due dates, and fertile windows can vary due to irregular menstrual cycles, hormonal conditions, medications, breastfeeding, stress, illness, or other health factors. The estimated dates may not reflect your actual dates. If you have concerns about fertility, menstrual health, or your pregnancy, consult a qualified healthcare provider.
References
- American College of Obstetricians and Gynecologists (ACOG)
- National Health Service (NHS)
- World Health Organization (WHO)
- Mayo Clinic
- MedlinePlus
Related Pregnancy & Fertility Tools
Trust & Privacy
Educational content is reviewed periodically for clarity, accuracy, and consistency with established pregnancy guidance. Updates are made when recommendations or calculator functionality change.
Frequently Asked Questions
Your due date is estimated by adding 280 days (40 weeks) to the first day of your last menstrual period, or by using other methods like conception date or ultrasound.
Ovulation is estimated by subtracting 14 days from your average cycle length. For a 28-day cycle, ovulation typically occurs around day 14.
The fertile window is usually about 5–6 days: the 5 days before ovulation and the day of ovulation.
Yes. The calculator uses your average cycle length. If cycles are irregular, the estimate may be less accurate.
A pregnancy test can be taken about 12–14 days after ovulation, or around the first day of your missed period.
Yes. Your healthcare provider may adjust your due date based on ultrasound measurements, especially in the first trimester, for a more accurate estimate.
No. All calculations are performed locally in your browser. No personal information is stored or shared.
🌸 © 2026 CalculatorKits — Due Date Calculator · For educational use only
Due Date Calculator: Estimate Your Baby’s Due Date and Pregnancy Timeline
A Due Date Calculator gives you a simple way to estimate when your baby may be due. For many people, one of the first questions after discovering a pregnancy is when the baby is expected to arrive.
The CalculatorKits Due Date Calculator is designed to make this calculation easier. You can enter the first day of your last menstrual period and your average cycle length, then view an estimated due date and related pregnancy information. The calculator also provides useful dates such as estimated conception, ovulation, fertile window, next period, pregnancy test timing, and pregnancy progress.
A due date is only an estimate. It does not mean your baby will arrive on that exact day. Pregnancy length can vary, and your healthcare provider may adjust your estimated due date when better information becomes available.
This guide explains how a Due Date Calculator works, how to use the CalculatorKits tool step by step, how cycle length affects the result, and why ultrasound dating can sometimes provide a different due date.
Quick Answer
A Due Date Calculator estimates your expected delivery date from pregnancy dating information.
The standard method uses the first day of your last menstrual period.
Estimated due date = First day of last menstrual period + 280 days
This is approximately 40 weeks.
Another way to calculate the same estimate is the traditional calendar method:
Add 7 days to the first day of your last period, subtract 3 months, then add 1 year.
For a typical 28 day cycle, conception is often estimated to occur around two weeks after the start of the last menstrual period. The CalculatorKits tool also considers your average cycle length, which can make the calendar estimate more useful when your cycle is shorter or longer than 28 days.
The CalculatorKits result can include:
• Estimated due date
• Estimated conception date
• Estimated ovulation date
• Fertile window
• Next expected period
• Pregnancy test timing
• Pregnancy week
• Pregnancy progress
• Pregnancy milestones
The NHS explains that pregnancy normally lasts between 37 and 42 weeks from the first day of the last menstrual period, so your calculated date should be treated as a reference rather than an exact delivery date.
What Is a Due Date Calculator?
A Due Date Calculator is an online tool that estimates when a pregnancy may reach approximately 40 weeks.
The most common method begins with the first day of your last menstrual period, also known as the LMP.
Using the LMP makes pregnancy dating easier because many people can remember when their last period started, while the exact date of conception is usually much harder to identify.
Pregnancy dating may seem confusing because the pregnancy clock starts before conception. In a typical cycle, ovulation and conception happen around two weeks after the first day of the last period.
The CalculatorKits tool turns the date calculation into a simple result and presents additional information about the pregnancy and menstrual cycle.
Why Use This Due Date Calculator?
A Due Date Calculator can save you from manually counting calendar days.
It can help you:
- Estimate when your baby may be due
- Understand your current pregnancy week
- See approximately how much time remains
- Understand estimated conception timing
- See estimated ovulation timing
- Identify the estimated fertile window
- Follow pregnancy milestones
- Prepare for prenatal appointments
The CalculatorKits version is particularly useful because it does not stop at one date. Its result shows a pregnancy timeline and related cycle information, making it easier to understand how the dates connect.
The estimate can also be useful when you are still waiting for your first prenatal appointment and want a general idea of your pregnancy timeline.
How Does the Due Date Calculator Work?
The calculation begins with information about your menstrual cycle.
First Day of Your Last Period
The first day of your last menstrual period is the main starting point for the traditional pregnancy calculation.
This date is treated as day one of pregnancy dating.
Average Cycle Length
The CalculatorKits tool also asks for your average cycle length.
A 28 day cycle is a common reference, but not everyone has one.
If your usual cycle is longer or shorter, the estimated ovulation and related dates can change.
The NHS also allows cycle length adjustments in its pregnancy due date calculator.
Estimated Ovulation and Conception
Once the cycle length is considered, the calculator estimates when ovulation may occur.
Conception is generally estimated around the time of ovulation.
These are calendar based estimates, not measurements of what happened inside the body.
Pregnancy Timeline
The result then uses the calculated dates to show pregnancy progress, estimated pregnancy week, and related milestones.
How to Use the Due Date Calculator
Follow these steps to use the CalculatorKits tool.
- Open the CalculatorKits Due Date Calculator.
- Enter the first day of your last menstrual period.
- Enter your average cycle length in days.
- Check that the date and cycle length are correct.
- Review the information before calculating.
- Click Calculate Due Date.
- Review your estimated due date.
- Check the estimated conception date and ovulation date.
- Review the estimated fertile window and next period.
- Check the suggested pregnancy test date.
- Review your pregnancy week and pregnancy timeline when shown.
- Save or note the result if you want to compare it with your pregnancy records.
For the most useful estimate, use the actual date of your last period and an average cycle length based on several recent cycles when possible.
How Is a Due Date Calculated From Your Last Period?
The traditional formula is:
Estimated due date = LMP + 280 days
This represents approximately 40 weeks.
Another traditional approach is Naegele’s rule.
For example, suppose the first day of your last period was January 10.
Add seven days:
January 17
Subtract three months:
October 17
Then adjust the year if necessary.
The estimated due date would therefore be around October 17.
This calculation works as a general estimate, but it assumes a fairly typical menstrual cycle.
Mayo Clinic explains that healthcare professionals commonly count 40 weeks from the start of the last menstrual period when estimating a due date.
Why Is Pregnancy Counted as 40 Weeks?
This is one of the most confusing parts of pregnancy dating.
Pregnancy is usually counted from the first day of the last menstrual period rather than from conception.
In a typical cycle, ovulation occurs later.
This means the first part of the 40 week pregnancy timeline happens before conception.
For example, someone who is four weeks pregnant may have conceived approximately two weeks earlier.
This method provides a standard way for healthcare professionals to describe gestational age and organize prenatal care.
Mayo Clinic also explains why the 40 week pregnancy timeline includes the period before conception.
What Is Gestational Age?
Gestational age tells you how far along the pregnancy is.
It is usually expressed in weeks and days.
For example:
12 weeks and 3 days
means approximately 12 completed weeks and 3 additional days according to the dating method.
Gestational age is important because pregnancy development and prenatal care are organized around pregnancy weeks.
The CalculatorKits timeline uses this information to show your current pregnancy progress.
You can also use the Pregnancy Week by Week Calculator to explore general information according to your estimated pregnancy week.
What Is an Estimated Due Date?
An estimated due date, often called EDD, is the date when pregnancy is expected to reach approximately 40 weeks.
The key word is estimated.
It is not a prediction accurate to the exact day.
The NHS states that pregnancy normally lasts between 37 and 42 weeks from the first day of the last menstrual period.
This is why your baby may arrive before or after your calculated date.
The due date is still important because it gives your healthcare provider a reference point for monitoring the pregnancy and planning care.
Will My Baby Arrive on the Due Date?
There is no reason to expect your baby to arrive exactly on the estimated due date.
The date represents one point in a broader period when delivery may occur.
So rather than thinking of your due date as a fixed appointment, think of it as an estimated milestone.
Your healthcare provider will guide you as you approach or pass that date.
Why Does Cycle Length Matter?
The traditional calculation assumes a 28 day menstrual cycle.
But cycles can be shorter or longer.
For example, someone with a 24 day cycle may ovulate earlier than someone with a 32 day cycle.
Because ovulation timing affects conception timing, cycle length can also affect the estimated due date.
That is why the CalculatorKits tool includes an average cycle length field.
The NHS similarly advises adjusting the calculation when the cycle length is different from 28 days.
If your cycles are very irregular, however, a calendar calculation may be less reliable.
What If I Do Not Know My Last Period?
Not everyone remembers the exact first day of the last period.
This can happen when your cycles are irregular, you do not track periods, or you simply do not remember the date.
You can still discuss pregnancy dating with your healthcare provider.
The NHS recommends speaking with a midwife or GP if you do not know or are unsure of the first day of your last period.
An ultrasound may provide better information for establishing gestational age.
Do not choose an exact date simply to make the calculator produce a more precise looking result.
Due Date Calculator and Conception Date
If your approximate conception date is known, the due date can be estimated using approximately 266 days from conception.
The formula is:
Estimated due date = Conception date + 266 days
This represents about 38 weeks.
The problem is that conception is often difficult to pinpoint precisely.
Intercourse does not necessarily happen on the day fertilization occurs. Sperm may remain viable for several days, and ovulation can occur earlier or later than expected.
For this reason, conception based dating is still an estimate.
The Pregnancy Conception Calculator can help you explore the relationship between conception timing and pregnancy dates.
Due Date Calculator and Ovulation
Ovulation is closely connected to conception.
For a typical 28 day cycle, ovulation is often estimated around cycle day 14.
That does not mean everyone ovulates on day 14.
A longer cycle usually shifts the estimated ovulation date later, while a shorter cycle generally shifts it earlier.
The Ovulation Calculator can help estimate your likely ovulation date from your last period and average cycle length.
These dates are estimates and cannot confirm the exact day ovulation occurred.
Due Date Calculator and Fertile Window
The fertile window covers the days when pregnancy is most likely to occur around ovulation.
It is broader than one single day.
Sperm can survive for several days in the reproductive tract, which means intercourse before ovulation can result in pregnancy.
The CalculatorKits tool provides an estimated fertile window based on the cycle information entered.
You can also use the Fertile Window Calculator when you want to focus specifically on your likely fertile days.
Due Date Calculator and Ultrasound
An ultrasound can provide important information about pregnancy dating.
During an early ultrasound, measurements are used to estimate gestational age.
ACOG states that first trimester ultrasound measurement is the most accurate method for establishing or confirming gestational age when appropriate.
This can be especially useful when:
• Your LMP is uncertain
• Your periods are irregular
• Your cycle length is unusual
• Menstrual dating and ultrasound dating do not agree
If your healthcare provider establishes a different due date after an appropriate ultrasound, use the clinically established date for your pregnancy care.
Due Date Calculator and IVF
Pregnancy dating can be more precise after IVF because the embryo age and transfer date may be known.
ACOG recommends using assisted reproductive technology derived gestational age when assigning an estimated due date.
If your pregnancy resulted from IVF, follow the dating information provided by your fertility specialist.
A standard LMP calculation may not be the most appropriate method in this situation.
What Can the Calculator Show?
The CalculatorKits result provides more than a single date.
Depending on the information entered, it can provide:
- Estimated due date
- Estimated conception date
- Estimated ovulation date
- Fertile window
- Next period
- Pregnancy test timing
- Current cycle information
- Pregnancy week
- Pregnancy progress
- Pregnancy milestones
This makes the tool useful for understanding the wider pregnancy timeline instead of looking at the due date in isolation.
How Accurate Is a Due Date Calculator?
A Due Date Calculator can perform the date calculation accurately based on the information entered.
The biological estimate can still have uncertainty.
For example, if your last menstrual period date is incorrect, the calculator will still perform the calculation correctly from that incorrect date.
Cycle length can also affect the estimate.
This is why your clinically established due date may be different from an online calculation.
A useful way to think about it is:
The calculator can calculate the dates correctly, but the underlying pregnancy timing may not be known with complete certainty.
Why Can Two Due Date Calculators Give Different Results?
Two calculators may use different assumptions.
One may assume a 28 day cycle.
Another may adjust for your average cycle length.
One may calculate only from the LMP.
Another may allow conception, ultrasound, or IVF information.
Small differences can therefore occur.
When comparing results, check the calculation method rather than looking only at the final date.
For medical decisions, use the date established by your healthcare professional.
Common Mistakes When Using a Due Date Calculator
Entering the Wrong LMP Date
A wrong starting date can shift the estimated due date.
Confusing LMP With Conception
The first day of the last period is not the day conception occurred.
Assuming Everyone Has a 28 Day Cycle
Cycle length varies between people.
Assuming Ovulation Always Happens on Day 14
Day 14 is a common estimate for a 28 day cycle, not a universal rule.
Ignoring Cycle Length
A consistently shorter or longer cycle can affect the estimate.
Treating the Due Date as Exact
The estimated date is only a reference point.
Ignoring Ultrasound Dating
An appropriate early ultrasound may provide better information about gestational age.
Using the Calculator to Diagnose Pregnancy Problems
A date calculator cannot determine fetal health or pregnancy complications.
How to Get a Better Due Date Estimate
Use the most reliable information you have.
Record the first day of your last menstrual period.
Use your average cycle length when the calculator allows it.
Keep your ultrasound results available.
If you conceived through IVF, use the dating information supplied by your fertility specialist.
Once your healthcare provider establishes an official estimated due date, use that date for medical care.
The online calculation is most useful for understanding your timeline and preparing for appointments.
Frequently Asked Questions
What is a Due Date Calculator?
A Due Date Calculator is an online tool that estimates when your baby may be due using pregnancy dating information.
How is the due date calculated?
The traditional method adds approximately 280 days or 40 weeks to the first day of the last menstrual period.
What does LMP mean?
LMP means last menstrual period. In pregnancy dating, it usually refers to the first day of your most recent menstrual period.
Is the estimated due date exact?
No. It is an estimate. Babies can arrive before or after the calculated date.
How many weeks is pregnancy?
Pregnancy is commonly dated as approximately 40 weeks from the first day of the last menstrual period.
What if I do not know my LMP?
Tell your healthcare provider that the date is uncertain. An ultrasound may provide better pregnancy dating.
What if my cycle is longer than 28 days?
A longer cycle can shift the estimated ovulation and due date. Enter your average cycle length when the tool provides that option.
Can my due date change after an ultrasound?
Yes. Your healthcare provider may revise the estimated date if ultrasound provides better information about gestational age.
Can I calculate my due date from conception?
Yes. A general conception based calculation adds approximately 266 days to the conception date.
Is conception the same as intercourse?
No. Intercourse and fertilization do not necessarily happen on the same day.
Can IVF affect pregnancy dating?
Yes. IVF pregnancies can use embryo age and transfer information to establish gestational age.
Can a Due Date Calculator predict labor?
No. It estimates the due date but cannot predict exactly when labor will begin.
Can the calculator tell me whether my baby is healthy?
No. It only calculates pregnancy dates and timeline information.
Can I use the calculator with irregular periods?
Yes, but the estimate may be less reliable when cycles vary significantly.
What is gestational age?
Gestational age is the estimated length of pregnancy expressed in weeks and days.
How often should I use a Due Date Calculator?
You normally do not need to use it every day. Use it when checking your timeline or when reviewing corrected information.
Key Takeaways
A Due Date Calculator provides a convenient way to estimate your baby’s expected delivery date.
The traditional LMP method uses approximately 280 days or 40 weeks from the first day of the last menstrual period.
A conception based calculation generally uses approximately 266 days from conception.
Cycle length can affect the estimated ovulation and due date.
Your due date is an estimate rather than a guaranteed delivery date.
The CalculatorKits tool can also show estimated conception, ovulation, fertile window, next period, pregnancy test timing, pregnancy week, and pregnancy milestones.
First trimester ultrasound is the most accurate clinical method for establishing or confirming gestational age when appropriate.
IVF pregnancies can use embryo age and transfer information for pregnancy dating.
Use online calculations as an educational and planning aid.
For official pregnancy dating and medical decisions, follow the information provided by your healthcare professional.
Related CalculatorKits Tools
Pregnancy Calculator
Estimate your pregnancy due date and understand your current pregnancy week, progress, remaining time, and general timeline.
Pregnancy Conception Calculator
Estimate a possible conception date and understand how conception timing relates to pregnancy dates.
Ovulation Calculator
Estimate your likely ovulation date using your last period and average menstrual cycle length.
Fertile Window Calculator
Estimate the days when conception may be more likely around your predicted ovulation.
Pregnancy Week by Week Calculator
Explore general pregnancy development and timeline information based on your estimated gestational week.
References
- American College of Obstetricians and Gynecologists: Methods for Estimating the Due Date
Clinical guidance on estimating the due date using the last menstrual period, ultrasound findings, cycle information, and assisted reproductive technology. - NHS: Pregnancy Due Date Calculator
Explains how an estimated due date is calculated and why cycle length can affect the result. - Mayo Clinic: Pregnancy Due Date Calculator
Explains the 40 week pregnancy dating system and traditional due date calculation. - Mayo Clinic: Fetal Development During Pregnancy
Explains why pregnancy is dated from the last menstrual period even though conception occurs later. - Cleveland Clinic: Due Date Calculator
Provides information about LMP dating, conception dating, ultrasound dating, IVF dating, and estimated due dates.
Medical Disclaimer
This article and the CalculatorKits Due Date Calculator are provided for general educational and informational purposes only. The calculator provides an estimated pregnancy date and is not a substitute for prenatal care, ultrasound assessment, or professional medical advice. Pregnancy dating can vary depending on menstrual history, cycle length, conception timing, ultrasound findings, and other factors. If you are pregnant or think you may be pregnant, consult a qualified healthcare professional for appropriate prenatal care and confirmation of your pregnancy dating.
Written and reviewed by the CalculatorKits Editorial Team
Last Updated: September 6, 2026