Health
Conception Date Calculator
Estimate your most likely conception date and window from your due date or last menstrual period.
Default 28 days. Adjust if your cycle is longer or shorter.
Estimated Conception Date
Corresponding Due Date
Key Dates
How it works
There's no way to know your exact conception date with certainty — it can only be estimated from typical cycle and gestation timing. This calculator offers two methods, and the answer depends on which starting point you know.
From a due date: since due dates are usually set at 40 weeks from the last menstrual period, and conception typically occurs about two weeks into that cycle, gestation counted from conception itself averages 38 weeks (266 days). Subtracting 266 days from the due date lands on the estimated conception day.
From your last period: ovulation timing depends on cycle length, but the luteal phase — the stretch between ovulation and the next period — stays close to 14 days no matter how long or short the overall cycle is. So ovulation, and conception with it, is estimated as your cycle length minus 14 days after the LMP.
In both cases, the single date shown is a best estimate, not a certainty. The window below it — a few days wide, accounting for sperm survival and natural variation in ovulation timing — is the more honest way to think about when conception likely happened.
Formulas
From due date
Conception = Due date − 266 days
Standard 38-week gestation from conception
From LMP
Ovulation = LMP + (Cycle length − 14) days
Luteal phase is consistently ~14 days
Conception window (due date)
±3 days around estimate
Accounts for variability in ovulation timing
Conception window (LMP)
−5 / +1 days around ovulation
Sperm survives up to 5 days; egg survives ~24 hrs
Implied due date
Conception + 266 days, or LMP + 280 days
Cross-check figure shown alongside the result
Why a window, not a single day
Sperm survival: Sperm can live inside the reproductive tract for up to 5 days, so fertilization can happen days after intercourse.
Egg lifespan: Once released, an egg is viable for only about 24 hours, narrowing the window on the other side.
Cycle variation: Even regular cycles shift slightly month to month, so ovulation rarely lands on the exact same cycle day every time.
Related
Related calculators.
FAQ
Frequently asked questions.
How do you calculate conception date?
Conception date can be estimated two ways. From a known due date, subtract 266 days (38 weeks) — the typical gestation length counted from conception rather than from the last period. From your last menstrual period (LMP), add your cycle length minus 14 days to find the estimated ovulation date, since conception almost always happens within about a day of ovulation. Both methods give an estimate, not an exact date, because the precise timing of ovulation and fertilization can't be observed directly in most pregnancies.
Why is there a conception window instead of a single date?
A single 'conception date' implies more precision than biology actually allows. Sperm can survive in the reproductive tract for up to five days waiting for ovulation, and the egg itself is only viable for about 24 hours after release. Cycle length and ovulation timing also vary somewhat from month to month, even for people with generally regular cycles. Presenting a window — a few days on either side of the estimated date — is a more honest reflection of this natural variability than a single calculated day.
How accurate is a conception date calculator?
A conception date calculator gives a well-informed estimate based on average cycle and gestation lengths, but it cannot pinpoint the exact day fertilization occurred. Accuracy depends on how regular your cycle is and how precisely you know your LMP or due date. An early first-trimester ultrasound (ideally between 8 and 13 weeks) is generally considered the most accurate way to date a pregnancy, since it measures the embryo or fetus directly rather than relying on cycle assumptions.
Can I calculate conception date from my due date?
Yes. Subtract 266 days (38 weeks) from your due date to estimate the conception date. This works because due dates are usually calculated assuming a 40-week gestation from the last menstrual period, and conception typically occurs about two weeks after the start of that period — so gestation counted from conception itself averages 38 weeks rather than 40.
What is the difference between conception date and ovulation date?
Ovulation is when an egg is released from the ovary; conception (fertilization) happens when a sperm meets that egg, which typically occurs within about 24 hours of ovulation. In practice, most calculators treat the ovulation date and conception date as essentially the same day, since the gap between the two is small compared to the natural uncertainty in estimating either one.
Does cycle length affect my estimated conception date?
Yes. Ovulation is driven by a consistent luteal phase of about 14 days between ovulation and the next period, regardless of overall cycle length. That means someone with a 35-day cycle ovulates later relative to their LMP (around day 21) than someone with a 21-day cycle (around day 7). Entering your actual average cycle length, rather than assuming the default 28 days, produces a more accurate estimate of both ovulation and conception timing.
Can this calculator tell me the sex or exact moment of conception?
No. This calculator only estimates a likely date and window for conception based on standard obstetric timing conventions — it cannot determine or predict the sex of a baby, nor can it identify the precise hour or day fertilization occurred. For clinical dating, sex determination, or confirmation of pregnancy timing, consult an ultrasound or your healthcare provider.
Last updated: July 18, 2026