Calculator · Last updated May 20, 2026
Conception calculator
Work backward from your due date or last period to estimate the most likely day you conceived — and the 6-day window when it could have happened.
Written and reviewed by the babybumpkit editorial team.
How a conception date is calculated
Conception almost always happens at ovulation — when an egg is released and meets sperm in the fallopian tube. The math works in two directions:
- From a due date: conception = due date − 266 days. Pregnancy averages 266 days from conception to birth.
- From a last menstrual period (LMP): conception = LMP + (cycle length − 14). Ovulation lands ~14 days before the next expected period, regardless of cycle length.
Both give you the same answer when the inputs are accurate — they just start from different anchors. Use the due-date mode when an ultrasound has confirmed your dating; use the LMP mode when you're working from your period.
Why the calculator shows a window
Conception happens on a single day, but the sex that led to it could have happened over a 6-day window. Sperm can survive in the reproductive tract for up to five days waiting for ovulation. So fertilization can result from intercourse anywhere from 5 days before ovulation through ovulation day itself.
This matters most for people trying to work out timing — whether for paternity questions, planning, or just understanding what happened. The calculator gives you both the most likely conception day and the wider 6-day window when the fertilizing intercourse could have occurred.
How accurate is the estimate?
Accuracy depends on which input you use:
- Ultrasound-confirmed due date: very accurate — an early dating scan measures the embryo directly, and the calculator just reverses that with simple subtraction.
- LMP + regular cycles: usually within 1–3 days of actual conception.
- LMP + irregular cycles: can be off by a week or more, because ovulation timing varies.
For higher precision, an early dating ultrasound (8–12 weeks) is the most accurate way to confirm conception timing. ACOG guidance is that ultrasound dating supersedes LMP-based dating when the two differ by more than 5–7 days in the first trimester.
When this calculator can and can't help
The conception calculator is useful for:
- Figuring out roughly when you conceived (a common early-pregnancy curiosity)
- Understanding the gap between LMP-based gestational age and actual fetal age
- Narrowing down the timing window for personal or relationship clarity
It cannot reliably determine paternity. The 6-day fertile window is genuinely a window — if you had different partners within that range, a calculator can't tell you whose sperm fertilized the egg. The only definitive answer is DNA testing (prenatal NIPT-based paternity from 8+ weeks, or postnatal). Anyone selling certainty from dates alone is selling something they can't deliver.
Frequently asked questions
Sources and medical references
The conception-timing math on this page comes from published guidance and peer-reviewed research.
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