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Pregnancy & Baby
Conception Date Calculator — Estimate When You Conceived
Reverse the pregnancy math to find your likely conception window — from a due date or your last period
Whether you're piecing together a timeline, curious about which weekend things lined up, or trying to match a pregnancy back to a specific date, the conception date calculator runs the standard pregnancy math in reverse. A due-date tool counts forward to the birthday; this one counts backward to the moment fertilization most likely happened. Because the same American College of Obstetricians and Gynecologists (ACOG) dating rules drive both, this page estimates the conception/ovulation date, not the birth date.
There are two ways in, depending on what you already know:
- From a due date. A full pregnancy in ACOG dating runs 280 days (40 weeks) from the first day of your last period, but fertilization happens about 14 days later — so conception sits roughly 266 days before the due date. Subtract 266 days from your estimated due date to land on your likely conception day. A due date of December 6 points to conception around March 15.
- From your last menstrual period (LMP). Conception clusters around ovulation, which on a textbook 28-day cycle falls about 14 days after the period starts. The formula is conception ≈ LMP + (cycle length − 14), so a longer cycle pushes ovulation — and conception — later. An LMP of March 1 on a 28-day cycle gives conception around March 15; a 32-day cycle shifts it to roughly March 19.
Why a window, not a single day. An egg lives only 12–24 hours after release, but sperm can survive up to 5 days in the reproductive tract, so the fertile window spans several days. This calculator reports a likely conception window of about ±4–5 days around the central estimate rather than pretending to pinpoint one date.
Worked example. With a due date of December 6, 2026, conception ≈ Dec 6 − 266 days = March 15, 2026, with a likely window of roughly March 10–20. Starting instead from an LMP of March 1, 2026 on a 28-day cycle gives the same March 15 — the two paths agree, which is exactly what you'd expect.
Keep in mind that ACOG dates pregnancy from the LMP, so your "weeks pregnant" number already runs about two weeks ahead of true conception — don't confuse gestational age with the age of the embryo. A first-trimester ultrasound, which measures the baby directly, gives the most accurate dating and may shift any of these estimates by a few days.
This is an informational estimate of when conception likely occurred — not a medical or legal determination of a conception date, and not something for settling paternity or precise-timing disputes. For anything that matters legally or medically, talk to your provider and rely on an early ultrasound.
Calculator
Fill in the fields and click "Calculate" for instant results.
📰 Formula
• From due date: conception ≈ due date − 266 days • From LMP: conception ≈ LMP + (cycle length − 14) days • Ovulation day ≈ first day of LMP + (cycle length − 14) • Likely conception window ≈ central estimate ± 4–5 days (sperm survive up to 5 days; egg lives 12–24 hours) • Method follows ACOG pregnancy dating (Naegele's rule run in reverse)
📰 Formula
• From due date: conception ≈ due date − 266 days • From LMP: conception ≈ LMP + (cycle length − 14) days • Ovulation day ≈ first day of LMP + (cycle length − 14) • Likely conception window ≈ central estimate ± 4–5 days (sperm survive up to 5 days; egg lives 12–24 hours) • Method follows ACOG pregnancy dating (Naegele's rule run in reverse)
🧪 Worked examples
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Example 2
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Example 3
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Example 4
⚠️ Common mistakes
- Treating the conception date as exact instead of a several-day window around ovulation.
- Subtracting 280 days from the due date — conception is about 266 days before, not 280.
- Forgetting to adjust for cycle length, which moves ovulation (and conception) earlier or later.
- Confusing 'weeks pregnant' (counted from the LMP) with the embryo's true age since conception.
- Using this estimate to settle paternity or legal timing — only an early ultrasound and a provider can date a pregnancy with confidence.
💡 Tips
- If you have a due date from a dating scan, start from it — that path is usually the more reliable of the two.
- When starting from your last period, enter your real average cycle length so ovulation lands in the right place.
- Read the result as a window: the central day is most likely, but any day in the ±4–5 day range is plausible.
- If the due-date estimate and the LMP estimate disagree by more than a few days, trust the ultrasound-based due date.
- Remember sperm can live up to 5 days, so intercourse a few days before ovulation can still be the conception event.
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<iframe src="https://www.calcnimbus.com/embed/conception-date-calculator" width="100%" height="500" frameborder="0" style="border:1px solid #eee;border-radius:12px"></iframe>
❓ Frequently asked questions
How does a conception date calculator work?
It runs the standard ACOG pregnancy math backward. From a due date it subtracts about 266 days, since fertilization happens roughly 14 days after the last period began in 280-day (40-week) dating. From a last period it adds your cycle length minus 14 days to land on the likely ovulation and conception day. The result is an informational estimate, not a medical determination.
How accurate is an estimated conception date?
Treat it as a window of about four to five days, not a single date. An egg survives only 12 to 24 hours after ovulation, but sperm can live up to five days, so the fertile window — and therefore the exact moment of conception — spans several days. A first-trimester ultrasound gives the most accurate dating.
Should I count back 266 or 280 days from my due date?
Count back 266 days to estimate conception. The 280-day (40-week) figure is measured from the first day of your last period, which is about two weeks before you actually conceive. So conception sits roughly 280 minus 14, or 266 days, before the due date.
How do I find my conception date from my last period?
Add your cycle length minus 14 days to the first day of your last period. On a textbook 28-day cycle that's about 14 days after the period began, which is when ovulation — and conception — most likely occurred. A longer or shorter cycle moves that day later or earlier.
Does cycle length change the conception estimate?
Yes, because cycle length tells the calculator when you probably ovulated. Ovulation lands roughly 14 days before your next period, so a 32-day cycle pushes conception about four days later than a 28-day cycle would, and a 24-day cycle pulls it about four days earlier. Enter your true average for the best estimate.
Can this tell me exactly which day I conceived?
No — and no calculator honestly can. Because sperm can survive several days before an egg is released, the act of intercourse and the moment of fertilization can be days apart. This tool gives the most likely day plus a realistic window around it, which is the best estimate possible without medical testing.
Is the conception date the same as the implantation date?
No. Conception is when sperm fertilizes the egg, around ovulation. Implantation — when the embryo attaches to the uterine lining — happens about 6 to 12 days later. If you want that later milestone, use an implantation calculator instead of this one.
Can I use a conception date for paternity or legal timing?
You shouldn't rely on it for that. This is an informational estimate with a several-day margin, not a legal or medical determination of when conception occurred. Paternity and precise timing questions need a doctor, an early ultrasound, or formal testing — not a date from a web calculator.
Why does my 'weeks pregnant' number not match the conception date?
Because pregnancies are dated from the first day of your last period, not from conception. That makes your gestational age run about two weeks ahead of the embryo's true age. So at the moment you conceive, you're already counted as roughly two weeks pregnant under ACOG dating.