About the Conception Calculator
A conception calculator estimates when conception likely occurred, based on your last menstrual period and cycle length. Conception happens at ovulation, which is roughly two weeks into a cycle, so the estimated date falls about a fortnight after your period began.
The formula
Conception ≈ LMP + (cycle length − 14) daysOvulation typically occurs 14 days before the next period, so subtracting 14 from the cycle length gives the day of the cycle on which conception was most likely.
How to use this calculator
- 1Enter your Last Period Year. The field starts at
2026, which you can overwrite. - 2Enter your Last Period Month (1–12). The field starts at
8, which you can overwrite. - 3Enter your Last Period Day. The field starts at
1, which you can overwrite. - 4Enter your Cycle Length (Days). The field starts at
28, which you can overwrite. - 5Read the result straight away — it recalculates as you type, so there is no button to press. Use Share to copy a link that reopens the page with your exact numbers filled in.
Worked example
| Input | Value |
|---|---|
| Last Period Year | 2026 |
| Last Period Month (1–12) | 8 |
| Last Period Day | 1 |
| Cycle Length (Days) | 28 |
Result
Ovulation Date: Aug 15, 2026
Fertile Window: Aug 10, 2026 – Aug 16, 2026
Estimated Due Date: May 8, 2027
Pregnancy Length: ~280 days (40 weeks)
Those are the values the page loads with, so you can reproduce this result yourself and then change one field at a time to see what drives the outcome.
Understanding your result
The luteal phase — from ovulation to the next period — is fairly consistent at around 14 days, while the follicular phase before ovulation varies much more. This is why cycle length is used to adjust the estimate: someone with a 32-day cycle ovulates around day 18, not day 14.
The estimate is a window rather than a date. Sperm survive up to five days in the reproductive tract and an egg remains viable for 12 to 24 hours, so conception could have occurred across a span of several days. Even with a known ovulation date, the exact day of fertilisation is not determinable.
Things worth knowing
- The fertile window spans roughly five days before ovulation through one day after.
- Ovulation tracking through basal body temperature, cervical mucus, or LH test strips gives a better estimate than cycle arithmetic.
- Gestational age is counted from LMP, not conception, so it runs about two weeks ahead of the embryo's actual age.
- Stress, illness, travel, and weight change can all delay ovulation in a given cycle.
- For legal or paternity purposes, only DNA testing is definitive. Conception estimates are not.
Frequently asked questions
When did I conceive?+
Most likely around ovulation, roughly 14 days before your next period was due. With a 28-day cycle that is about day 14; with a 32-day cycle, about day 18.
How accurate is a conception date estimate?+
It gives a window of several days rather than a single date. Sperm survive up to five days and the egg 12 to 24 hours, so the exact day of fertilisation cannot be pinned down.
Why is gestational age two weeks more than the embryo's age?+
Because pregnancy is dated from the first day of the last period, which precedes conception by about two weeks. At eight weeks gestational age, the embryo is around six weeks old.
Can I determine paternity from a conception date?+
No. The window is too wide and individual variation too large. DNA testing is the only reliable method.