Free Pregnancy Due Date Calculator
Estimate a due date from the first day of your last period, with trimester dates and weeks pregnant.
Enter the first day of your last menstrual period and the length of your usual cycle. The calculator adds 280 days (adjusted for cycle length) to estimate your due date, and shows how far along you are, your trimester and key dates.
How to use the Pregnancy Due Date Calculator
- Choose metric or US units and enter your own measurements.
- Read the result and the supporting numbers shown beneath it.
- Use the explanation below to see what the number means, and talk to a doctor about anything that worries you.
About the Pregnancy Due Date Calculator
Estimate a due date from the first day of your last period, with trimester dates and weeks pregnant.
Health numbers are easier to understand when you can see how they are worked out. These calculators use published formulas and explain what the result does and does not tell you. They give general estimates and are not medical advice.
How a due date is worked out
Doctors count pregnancy from the first day of your last menstrual period (LMP), because that date is usually known and conception is not. A full-term pregnancy then lasts about 40 weeks, or 280 days. The method is known as Naegele's rule: take the LMP, add one year, subtract three months and add seven days, which is the same as adding 280 days.
Adjusting for your cycle
The rule assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, ovulation comes later or earlier, so the calculator shifts the due date by the difference. A 35-day cycle moves the due date 7 days later, and a 24-day cycle moves it 4 days earlier.
- Worked example: LMP on 1 March 2026 with a 28-day cycle gives a due date of 6 December 2026.
- Conception is estimated at LMP + cycle length − 14 days. For the same example that is about 15 March 2026.
Weeks, trimesters and full term
Dates are shown as weeks and days since your LMP. The trimester dates below follow the commonly used definitions.
| Stage | Weeks |
|---|---|
| First trimester | Up to 13 weeks 6 days |
| Second trimester | 14 weeks 0 days to 27 weeks 6 days |
| Third trimester | 28 weeks 0 days until birth |
| Early term | 37 weeks 0 days to 38 weeks 6 days |
| Full term | 39 weeks 0 days to 40 weeks 6 days |
| Late term | 41 weeks 0 days to 41 weeks 6 days |
| Post term | 42 weeks 0 days and beyond |
Why the date is only an estimate
Only a small share of babies are born on their estimated due date. Most arrive within a couple of weeks either side. Irregular cycles, an uncertain LMP and natural variation all affect the result, and early ultrasound measurements are often used to confirm or correct the date, so your doctor or midwife's date takes priority over any calculator.
What to do next
Book your first antenatal appointment as early as you can, and bring the date of your last period. If you have bleeding, severe pain or any worry about your pregnancy, contact your doctor, midwife or local emergency service rather than relying on a calculator.
Getting started with the Pregnancy Due Date Calculator
Using the pregnancy due date calculator takes a minute or two. Your entries are saved in this browser, so you can come back later and carry on.
Tips for better results
A due date is an estimate. Only a small share of babies arrive on the day, and a midwife or doctor may adjust the date after an early ultrasound. Use this for planning, and follow your care team's dating.
Use measurements you have taken recently, not remembered ones.
These tools give general estimates and are not medical advice. Check anything important with a doctor or registered dietitian.
Privacy and where your data goes
Everything you type into the pregnancy due date calculator stays on your own device. Zapxa does not receive your numbers or documents. If you clear your browser data, your saved entries will be removed, so download a PDF of anything you want to keep.
Frequently asked questions
How is the due date calculated?
The standard method, called Naegele's rule, adds 280 days (40 weeks) to the first day of your last period. If your cycle is longer or shorter than 28 days, the date moves by the same number of days.
How many weeks pregnant am I?
Pregnancy is counted from the first day of your last period, not from conception, so you are already about 2 weeks pregnant at conception. Set the date to check to today's date to see your current week.
What if my cycle is irregular?
Cycle-based dates are less reliable when cycles vary. Your doctor or midwife will usually confirm or adjust the date with an early ultrasound.
How many babies are born on their due date?
Only a small minority. Most births happen within about two weeks either side of the estimated date.
When does each trimester start?
Commonly, the second trimester begins at week 14 and the third at week 28. Some sources count slightly different weeks, so follow your own care provider.
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