Conception Calculator
Most likely conception date: --
How the Conception Window Is Estimated
This calculator works backward using the same 266-day, full-term-from-ovulation logic behind Naegele's rule, just run in reverse. If you enter a due date, it subtracts 266 days (38 weeks) to estimate the date of ovulation/conception, and separately subtracts the full 280 days (40 weeks) directly from the due date to back into the implied last menstrual period. If you instead enter the first day of your last menstrual period, it assumes ovulation and conception happened around cycle day (cycle length minus 14) — 14 days before your next expected period — rather than always on day 14, so it adjusts sensibly for cycles longer or shorter than the 28-day average. Because sperm can survive up to five days and the egg is viable for about 24 hours, the "window" shown spans roughly five days before ovulation through one day after, not a single fixed date.
Why This Is an Estimate, Not a Fact
Ovulation timing varies cycle to cycle even for people who track carefully, and actual gestational length varies too — only a minority of pregnancies deliver on the exact estimated due date. Fertilization and implantation can also occur days apart. Treat the date range here as a planning aid, not a certainty, and remember this tool is for general education, not a substitute for medical advice from your doctor or midwife.
Going the Other Direction
This calculator works backward from a due date or LMP to estimate when conception likely occurred. If you instead want to go forward — starting from your last period to estimate your due date — use the due date calculator. If you're also tracking your fertile window for planning purposes, the ovulation calculator can help identify upcoming fertile days.
Frequently Asked Questions
How does this calculator estimate the conception date?
If you enter a due date, it subtracts 266 days (38 weeks) to estimate ovulation/conception, and subtracts 280 days total to back into an implied last menstrual period. If you enter your last menstrual period date instead, it adds (cycle length minus 14) days, which places ovulation about 14 days before your next expected period rather than always assuming a fixed day 14 - this keeps the estimate accurate for cycles that run longer or shorter than 28 days.
Why is a date range shown instead of one exact conception date?
Sperm can survive in the reproductive tract for up to five days, and a released egg is typically viable for about 24 hours, so conception can realistically occur anywhere from about five days before ovulation to one day after it. The calculator reports that window alongside the single most likely date so the estimate isn't presented as more precise than reproductive biology actually allows.