Period Calculator: Next Period Date & Cycle Forecast
See your next period and the six after it.
Conception Date Calculator: From Due Date or Last Period
Work backwards to ovulation, the fertile window and conception.
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About Conception Date Calculator: From Due Date or Last Period
Estimate your conception date from a due date or last period start, with the fertile window, ovulation day and reworked due date.
How is the conception date worked out?
Ovulation is assumed to fall about 14 days before the next period, and conception happens at or just after ovulation. From your last period start the tool counts back 14 days from the end of your cycle to find ovulation, then reports the six-day fertile window around it. The estimated due date is 280 days of gestation from your last period — the standard Naegele offset — and working back from a due date simply reverses that 280 days.
Why does conception not always match exactly?
Sperm survive up to five days and an egg for about a day, so conception can occur anywhere across the fertile window. Ovulation also shifts from cycle to cycle, which is why a missed or irregular period makes this estimate much weaker.
📐 Methodology & Reference Accuracy:
Calculations, formulas, and conversions execute locally in your web browser. Output models are benchmarked against official reference standards and updated for 2026.
🔒 This tool runs entirely in your browser. Nothing is uploaded or stored.
Background reading: Conception Date Calculator on Wikipedia. More: all women's health.
Frequently Asked Questions about Conception Date Calculator: From Due Date or Last Period
Can I use my due date instead of my last period?
Yes. Switch to the due-date option and the calculator works backwards by 280 days of gestation, the standard offset between a last period and a due date, giving you the implied last period, fertile window and conception date. Because a due date does not reveal your cycle length, the ovulation estimate assumes a 28-day cycle.
How does this differ from a due date calculator?
A due date calculator runs forwards from the last period. This one answers the reverse question: given a due date, when did conception most likely happen.
How accurate is conception dating from a due date?
It is exact maths on uncertain inputs: the tool reverses precisely 280 days of gestation and assumes ovulation 14 days before the next period on a 28-day cycle. Real cycles vary, so treat the fertile window as a range and confirm with a clinician dating scan, which measures the embryo directly.