This conception calculator works the pregnancy calendar backwards: from a due date (or last period) it estimates the conception date and the fertile window in which conception occurred.
Estimates use the standard 28-day-cycle model adjusted by your cycle length. Ultrasound dating in the first trimester can shift a due date; use the due date your clinician currently uses. Conception (fertilization) and implantation are different events — implantation follows about 6–10 days later.
The Arithmetic of the Estimate
Standard obstetric dating counts pregnancy as 280 days from the last menstrual period (LMP), with ovulation — and conception — at about day 14 of a 28-day cycle. So conception ≈ due date − 266 days, shifted later day-for-day when cycles run longer than 28 (ovulation moves, the 14-day luteal phase roughly doesn’t). Sperm survive up to ~5 days and the egg ~24 hours, which is why the fertile window for intercourse opens days before the conception date itself. Going the other direction is our Due Date Calculator; cycle tracking lives in the Period Calculator. Part of our Health Calculators collection.
Worked Example (Verified)
Due date October 1, 2027, 28-day cycle:
- Implied LMP: December 25, 2026
- Estimated conception: January 8, 2027 (LMP + 14 days = due date − 266)
- Fertile window: roughly January 3 – January 9, 2027 — intercourse dates that could lead to that conception
- With a 32-day cycle instead, everything after the LMP shifts +4 days: conception ≈ January 12
How Precise Is This, Honestly?
- ± several days at best: cycle variability, sperm survival (up to 5 days) and implantation timing make any single “conception date” an estimate family, not a timestamp.
- Due dates themselves carry uncertainty: LMP-based dating assumes a textbook cycle; early ultrasound redates a meaningful share of pregnancies.
- For IVF pregnancies, conception is known exactly (transfer date minus embryo age) and none of this estimation applies.
- Legal or medical determinations do not rest on calculators — this is orientation arithmetic, clearly labeled.
Related Tools
- Due Date Calculator — forward from LMP or conception
- Pregnancy Weight Gain Calculator — the journey after
- Health Calculators — the full collection
Frequently Asked Questions
How do I figure out when I conceived?
Subtract 266 days from the due date (for a 28-day cycle), or add 14 days to the implied last period. Due October 1, 2027 → conception around January 8, 2027. Adjust one day later per day your cycle exceeds 28.
Can conception happen days after intercourse?
Yes — sperm can wait up to about 5 days for ovulation, so the conception date and the relevant intercourse date often differ by several days. The fertile window shown here exists for exactly that reason.
Why does cycle length change the estimate?
Because ovulation sits ~14 days before the next period, not fixed at day 14 after the last one. Longer cycles ovulate later; the calculator shifts conception (and the implied LMP math from a due date) accordingly.
Is the conception date the same as implantation?
No: fertilization happens in the tube; the embryo implants in the uterus roughly 6–10 days later, when hCG begins (and pregnancy tests can first turn positive, a few days after that).
How accurate are conception calculators?
Within several days for regular cycles with a firm due date; wider for irregular cycles or LMP-uncertain pregnancies. First-trimester ultrasound is the most accurate dating tool clinicians have.
Is this conception calculator free?
Yes — free, instant, and nothing you enter leaves your browser.