How to use the Conception Date Calculator
- Enter the first day of your last menstrual period — the first day of bleeding, not the last.
- Enter your average cycle length; if it varies, use the typical value rather than the shortest or longest.
- Read the most likely conception date, which is the estimated ovulation day.
- Note the fertile window: conception can result from intercourse up to five days before ovulation.
- Compare the due date shown with any dating scan — an early scan overrides calendar arithmetic.
How the calculation works
The luteal phase, from ovulation to the next period, is remarkably stable at about 14 days across women. The follicular phase before ovulation is what varies. So ovulation is estimated as cycle length minus 14 rather than as a fixed day 14, which is why a 32-day cycle puts ovulation near day 18, not day 14. Getting this wrong is the most common source of a due date that later shifts by a week.
The fertile window spans roughly the five days before ovulation plus the day itself. Sperm survive up to five days in favourable cervical mucus, while the egg is viable for only about 12 to 24 hours. Wilcox's landmark study found essentially no pregnancies from intercourse more than six days before ovulation, which sets the window's width.
Due date follows Naegele's rule — 280 days from the first day of the last period — adjusted for cycle length so that a longer cycle pushes the date later. Only about 4% of births occur on the estimated date, and the standard deviation of spontaneous labour is around 13 days, so the date is the centre of a distribution rather than a deadline. A first-trimester crown-rump-length scan is accurate to within five days and takes precedence over any calculation.
Ovulation = LMP + (cycle length − 14); fertile window = ovulation − 5 to ovulation + 1; due date = LMP + 280 + (cycle − 28)Source: Wilcox AJ, Weinberg CR, Baird DD, 'Timing of sexual intercourse in relation to ovulation', New England Journal of Medicine 333 (1995); ACOG Committee Opinion 700 on estimating due date.
Worked example
A last period starting 1 March with a 30-day cycle.
- Ovulation = 1 March + (30 − 14) = 17 March.
- Fertile window = 12 March to 18 March.
- Due date = 1 March + 280 + 2 = 8 December.
Conception most likely on 17 March, with a due date of 8 December.
Frequently asked questions
Why is the conception date later than day 14?+
Because ovulation is fixed relative to your next period, not your last. Longer cycles have longer follicular phases, so ovulation comes later.
Can I pinpoint the exact day?+
Rarely. Sperm survive several days, so conception can happen days after intercourse. The window is the honest answer.
My scan gives a different due date. Which is right?+
The scan. First-trimester measurements are more accurate than calendar dating and clinicians will re-date the pregnancy from them.
Does this work with irregular cycles?+
Less well. With cycles varying by more than a few days, ovulation prediction kits or basal temperature tracking give a far better estimate.
Last reviewed September 1, 2026. We review this page whenever the underlying formula, tax year, published rate or standard changes.