Work out an estimated conception date from a due date, or from your last period and average cycle length.

How to Use the Conception Calculator

Choose "From Due Date" if you already have one, or "From Last Period" if you'd rather work from your cycle directly — the second mode also asks for average cycle length, since that number changes the estimate more than people expect.

People ask this calculator two very different questions. One is curious and forward-looking: "when did this probably happen?" The other carries more weight — a paternity timeline, an insurance question, a legal date that needs defending. This tool can only ever answer the first kind of question with any confidence.

Conception date = Due date − 266 days
Conception date = LMP + (cycle length − 14) days

Both formulas point at the same underlying idea: conception tends to land around ovulation, and ovulation tends to land about 266 days before delivery. The LMP-based version is the more honest of the two, because it makes the assumption explicit instead of hiding it — a 28-day cycle plugs in 14, a 32-day cycle plugs in 18, and the estimate shifts by exactly that many days.

Why Cycle Length Changes the Answer

Ovulation isn't fixed at "day 14 of every cycle" — it's fixed relative to the next period, roughly 14 days before it starts. A longer cycle means a longer stretch before ovulation, not after, so someone who assumes a standard 28 days when their actual average is 34 will misplace the estimate by nearly a week. That's a meaningful gap in any context where the answer needs to hold up to scrutiny, which is exactly why this calculator asks for a real cycle length instead of quietly assuming one for everyone.

Even with an accurate cycle length, this is still a probability estimate, not a measurement. Sperm can survive several days inside the body before fertilization happens, and ovulation timing itself varies cycle to cycle for the same person — stress, illness, and travel all nudge it. Two calculators fed the same inputs a month apart could reasonably produce estimates a few days apart, because the biology itself isn't perfectly consistent.

This is a statistical estimate, not a precise determination. Nothing about menstrual-cycle math can pin conception to a single calendar day with legal or forensic certainty — actual ovulation timing varies by cycle and isn't directly observed here. Anyone who needs a conception date established with real precision, for a legal, medical, or paternity matter, needs clinical dating (early ultrasound) or genetic testing, not a calculator built on cycle averages.