CalcVitalsConception Calculator — When Did I Conceive?
Conception happens at ovulation — about 14 days after the last period in a 28-day cycle, and therefore about 266 days before the due date. A due date of October 8, 2026 points to a conception date around January 15, 2026, within a few days either side.
How it works
This calculator runs the pregnancy clock backwards. Because gestation is conventionally 266 days from conception to birth, a known due date minus 266 days lands on the likely conception date; from the other end, a last period plus 14 days (adjusted for cycle length) lands on the same spot. Both directions are printed so you can cross-check whichever dates you actually know.
The honest uncertainty is several days wide, for two separate reasons. First, ovulation itself varies — the day-14 assumption is an average. Second, fertilisation is not intercourse: sperm survive up to five days, so the plausible window of conception-causing intercourse extends five days before the conception date. Any tool claiming to name the day with certainty is claiming more than biology allows.
For most people this is curiosity or record-keeping. Where it carries weight — questions of paternity, or reconstructing timelines for medical care — treat the range, not the central date, as the answer, and know that early ultrasound dating narrows things far better than calendar reconstruction after the fact.
The method
From due date: conception = due date − 266 days; LMP = due date − 280 days. From LMP: conception = LMP + 14 + (cycle − 28) days; due date = LMP + 280 + (cycle − 28). Range ±3 days around the estimate reflects typical ovulation variability.
Frequently asked questions
Can this tell me exactly which day I conceived?
No — and no calculator can. Ovulation varies even in regular cycles, and sperm survival spreads the possibilities across about six days. The range shown is the scientifically honest answer.
Why 266 days and not 280?
280 days (40 weeks) is counted from the last period. Conception happens about 14 of those days in, so from conception to birth is 280 − 14 = 266 days.
Does an ultrasound change the estimate?
Yes — a first-trimester scan dates the pregnancy by embryo size and is more precise than LMP-based reconstruction. If a scan due date exists, run this calculator from that date.