About the Due Date Calculator
A due date calculator estimates when a baby is likely to arrive, using Naegele's rule: 280 days from the first day of the last menstrual period. It is the standard starting point in prenatal care, though only a small minority of babies arrive on the calculated date.
The formula
Due date = LMP + 280 days, adjusted for cycle lengthLMP is the first day of the last menstrual period. The 280-day figure assumes a 28-day cycle with ovulation on day 14; longer or shorter cycles shift the date accordingly.
How to use this calculator
- 1Enter your Last Period Year. The field starts at
2026, which you can overwrite. - 2Enter your Last Period Month (1–12). The field starts at
8, which you can overwrite. - 3Enter your Last Period Day. The field starts at
1, which you can overwrite. - 4Enter your Cycle Length (Days). The field starts at
28, which you can overwrite. - 5Read the result straight away — it recalculates as you type, so there is no button to press. Use Share to copy a link that reopens the page with your exact numbers filled in.
Worked example
| Input | Value |
|---|---|
| Last Period Year | 2026 |
| Last Period Month (1–12) | 8 |
| Last Period Day | 1 |
| Cycle Length (Days) | 28 |
Result
Estimated Due Date: May 8, 2027
Currently: 1 weeks pregnant
Trimester: 1st Trimester
Days Remaining: 269 days
Those are the values the page loads with, so you can reproduce this result yourself and then change one field at a time to see what drives the outcome.
Understanding your result
Only about 4 percent of babies are born on their due date, and roughly 80 percent arrive within two weeks either side. The due date is best understood as the centre of a distribution rather than an appointment — full term spans 37 to 42 weeks, and anywhere in that window is normal.
Naegele's rule assumes a regular 28-day cycle and known LMP, which is why an early ultrasound is more accurate. A first-trimester scan measuring crown-rump length dates a pregnancy to within about five days, and clinical practice is to revise the due date when the scan and the LMP calculation disagree substantially.
Things worth knowing
- A first-trimester ultrasound is the most accurate dating method and takes precedence over LMP calculations.
- Irregular cycles make LMP-based dating unreliable, so mention this at your first appointment.
- Full term is 39 to 40 weeks; 37 to 38 is early term and 41 plus is late term. All can be normal.
- First pregnancies tend to run slightly longer than subsequent ones.
- Most clinicians discuss induction between 41 and 42 weeks, since risks rise beyond that point.
Frequently asked questions
How accurate is a due date?+
Only about 4 percent of babies arrive on the exact date, though roughly 80 percent come within two weeks either side. Treat it as the middle of a range rather than a prediction.
How is a due date calculated?+
Naegele's rule adds 280 days to the first day of your last period, assuming a 28-day cycle. Longer or shorter cycles shift the date, and an early ultrasound gives a more accurate estimate.
What if my cycles are irregular?+
LMP-based dating becomes unreliable, because ovulation timing is uncertain. A first-trimester ultrasound is the accurate alternative and should be used for dating.
Can my due date change?+
Yes. Clinicians revise it when an early ultrasound differs substantially from the LMP calculation, usually by more than five to seven days in the first trimester.