Conception Date Calculator
Estimate when conception likely happened — from your due date or your last period — with the fertile-window range around ovulation.
How it works
From a last period: conception ≈ LMP + (cycle length − 14), i.e. around ovulation · from a due date: conception ≈ due date − 266 days.
Conception happens at or just after ovulation, which is estimated as roughly two weeks before your next period was due — so about (cycle − 14) days after your last period began. Because sperm can survive up to five days and the egg about a day, any day in the fertile window could be the day, which is why this shows a range rather than a single certainty. It’s an average-timing estimate; only a clinician with an early ultrasound can date a pregnancy accurately.
Worked example
With a last period on June 1, 2026 and a 28-day cycle, ovulation — and most likely conception — falls around June 15, 2026, with the fertile window running June 10 to June 16.
Frequently asked questions
Can I know the exact day I conceived?
Rarely. Sperm can live up to five days and the egg about a day, so conception may happen days after intercourse. This gives the most likely date, not a certainty — a clinician can narrow it with an early scan.
Can I use this to prove timing for legal purposes?
No. It is an estimate from average cycle timing and should never be used to establish legal or medical facts such as paternity. Only a doctor with proper testing can determine those.
My cycles are irregular — is this still useful?
Less so. The estimate assumes ovulation about 14 days before your next period, which shifts with cycle length. If your cycles vary a lot, treat it as a rough guide and ask your provider.
Related calculators
Estimates based on standard formulas and average cycles — every pregnancy is different. This is not medical advice; your doctor or midwife, with an ultrasound, gives the accurate dates and guidance.