Estimate your most likely conception date & fertile window
Pick the one anchor you actually have — you don't need the others.
Use the FIRST day of bleeding, not the last.
Default 28. Shifts ovulation & conception.
From your provider or an earlier scan. No cycle length needed.
First-trimester scans date most accurately (ACOG).
Only change this to see gestational age on a different date.
Most likely conception date
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For information and curiosity only — not medical advice. A conception date is inherently approximate: the egg is fertilizable for only ~12–24 hours but sperm survive up to ~5 days, so the actual day can fall anywhere in the ~6-day fertile window. A first-trimester ultrasound is the most accurate way to date a pregnancy — always confirm dating with your healthcare provider.
Conception Date Calculator — guide & how to use
What it does
This conception date calculator estimates the single most likely day you conceived
and the 6-day fertile / intercourse window around it, working from whichever one anchor you
actually have: your last menstrual period (LMP), your estimated due date (EDD),
or an early ultrasound with its gestational-age reading. It also shows your
estimated due date (in every mode, so you can cross-check), your
current gestational age in weeks + days, and your trimester — all calculated privately in your
browser, free.
How to use it
Choose what you know — LMP is the most commonly known value and the clinical default.
Enter that date. For LMP, set your average cycle length; for ultrasound, enter the
gestational age (weeks + days) the scan reported.
Read your most likely conception date, the fertile window, your EDD, and how
far along you are today.
How the math works
From LMP: conception = LMP + (cycle length − 14) days; due date = LMP + 280 days, shifted by
(cycle length − 28) days for non-28-day cycles.
From due date: conception = EDD − 266 days (38 weeks from conception); implied LMP = EDD − 280
days. Cycle length is not applied — it's already baked into the due date.
From ultrasound: dating-LMP = scan date − (weeks × 7 + days); conception = dating-LMP + 14
days; due date = dating-LMP + 280 days.
Fertile / intercourse window: the 6-day span from (conception − 5 days) through the conception
day itself, with the 2–3 days just before ovulation as the highest-probability sub-window.
Gestational age is counted from the LMP (or the equivalent LMP origin), so it runs about two
weeks ahead of true conceptional age — this is the normal clinical convention.
Worked example
Say your LMP was March 1, 2026 and your average cycle is 32 days (longer than
the textbook 28):
Ovulation offset = 32 − 14 = 18 days, so estimated conception = Mar 1 + 18 = March 19,
2026 (a naive "day 14" would have wrongly said March 15).
Fertile window = Mar 14 → Mar 19, 2026 (conception − 5 days through conception day).
Due date = Mar 1 + 280 + (32 − 28) = Mar 1 + 284 days ≈ December 10, 2026.
The 4-day cycle adjustment moved both the conception date and the due date — which is exactly why entering your
real cycle length matters.
Accuracy & safety notes
This is an estimate for information and curiosity only, not a medical diagnosis. Confirm dating
with your provider.
The "most likely" conception date is one day inside a roughly 6-day window — because sperm
survive up to ~5 days, an earlier act of intercourse is fully consistent with the same estimate. Treat it as a
best guess, not proof of a specific date.
The LMP method assumes a regular cycle and ovulation on (cycle length − 14). Irregular cycles,
early/late ovulation, and uncertain LMP recall all shift the true date; Naegele's rule can be off by a week or
more.
A first-trimester ultrasound (crown-rump length up to 13w6d) is the single most accurate dating
method per ACOG. If you have both LMP and an early scan, the ultrasound generally wins.
Only about 4% of births land on the exact due date; conception carries the same uncertainty
band.
Gestational age (from LMP) is about 2 weeks longer than true conceptional/embryonic age —
they're different clocks. This tool reports gestational age because that's the clinical standard.
All computation is client-side — no health data leaves your device.
Frequently asked questions
How do you calculate the conception date?
Conception ≈ ovulation. From LMP it's LMP + (cycle length − 14) days; from a due date it's due date − 266 days; from an early ultrasound the tool back-calculates the dating LMP, then adds 14 days. All with real calendar arithmetic.
Can it tell me the exact day I got pregnant?
No. The egg lives only ~12–24 hours but sperm survive up to ~5 days, so conception can occur anywhere across a ~6-day window. The tool gives the most likely day plus that window — it can't pin the exact act of intercourse.
Why does cycle length change the answer?
Ovulation falls about (cycle length − 14) days after your period starts. The classic "day 14" only fits a 28-day cycle; longer cycles ovulate later, so conception is later than a naive +14 suggests.
I only know my due date — should I enter cycle length?
No. The due date already encodes ovulation timing, so conception is simply due date − 266 days. Re-applying cycle length would double-count it and create a ~2-week error.
What if I entered the ultrasound GA as "8 weeks 9 days"?
Days must be 0–6 — "8 weeks 9 days" should be "9 weeks 2 days". The tool flags days ≥ 7 so you can normalize the entry.
Is it really free?
Yes. 100% free, and everything runs privately in your browser.