Pregnancy due date calculator
Estimate the due date from the first day of your last menstrual period and your usual cycle length, plus the likely conception date and the end of the first trimester.
Estimate only. Dating ultrasound and clinical assessment take precedence.
How this calculator works
A pregnancy due date is estimated as 40 weeks — 280 days — from the first day of the last menstrual period. This is Naegele's rule, and it assumes a regular 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, the estimated ovulation day moves, so this calculator adjusts the due date by the difference between your cycle length and 28 days.
Because the estimate depends on ovulation timing, only about 4% of babies arrive exactly on the due date; most births happen within two weeks either side. An early ultrasound (dating scan) is more accurate than LMP dating, especially for irregular cycles, and clinicians usually revise the due date based on it. Treat the result as a planning window rather than an appointment: arrange leave and appointments around the range, not the single day.
The formula
Naegele's rule: due date = first day of last period + 280 days. Corrected for cycle length L:
due date = LMP + 280 + (L − 28) days
conception est. = LMP + (L − 14) days
first trimester ends = LMP + 97 days (13 weeks 6 days)The 280-day figure corresponds to 40 weeks from LMP, or 38 weeks from conception.
Worked example
With a last period starting Aug 13, 2026 and a 28-day cycle, the estimated due date is May 20, 2027. Estimated conception is Aug 27, 2026, and the first trimester ends around Nov 18, 2026.
Common mistakes
- Using the last day of the period. The calculation starts from the first day of bleeding; using the end date shifts the due date by several days.
- Ignoring cycle length. A 35-day cycle moves ovulation later and the due date about a week later than the unadjusted rule.
- Treating the due date as exact. Only a small share of births happen on the estimated date; a dating ultrasound is more accurate.
Frequently asked questions
How accurate is a due date calculated from LMP?
Reasonably accurate for regular cycles, but an early dating ultrasound is more precise. Most clinicians adjust the date if the scan differs from the LMP estimate.
How is the due date calculated?
Add 280 days (40 weeks) to the first day of your last period. If your cycle is not 28 days, shift the result by the difference between your cycle length and 28.
What if my cycles are irregular?
LMP dating becomes unreliable, and an early ultrasound or known ovulation date should be used instead. Ask your clinician to set the date from the scan.
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