Conception Calculator
Work backwards from a due date to the estimated conception date.
Quick answer
- Formula: Conception ≈ due date − 266 days · Estimated LMP ≈ due date − 280 days
- Worked example — Estimated conception date: January 15, 2026 (full example below).
Conception Calculator
| Estimated conception date | January 15, 2026 |
| Estimated last period (LMP) | January 1, 2026 |
For a due date of October 8, 2026, conception is estimated around January 15, 2026, with the last period starting around January 1, 2026.
A full-term due date falls 280 days after the last menstrual period (LMP) and about 266 days (38 weeks) after conception, because ovulation and conception occur roughly two weeks after the LMP in a typical cycle. This calculator reverses Naegele's rule from your due date to estimate both dates.
The estimate assumes a standard 28-day cycle and can be off by several days with longer or shorter cycles — it is educational information, not medical or legal proof of conception timing.
The formula
Worked example
For a due date of October 8, 2026, conception is estimated around January 15, 2026, with the last period starting around January 1, 2026.
The example is pre-loaded in the result panel — press Calculate with your own numbers to replace it.
Frequently asked questions
Why 266 days and not 280?
Gestational age is counted from the LMP, but the egg is only released — and fertilised — about 14 days later. 280 − 14 = 266 days from conception to the due date.
How exact is the conception date?
Treat it as the centre of a window of about a week: sperm survive up to five days, and cycle variation shifts ovulation. Ultrasound dating narrows it, but no method gives an exact day retrospectively.
Category: Health & Fitness calculators
Sources & methodology
This calculator applies the formula shown on this page exactly as written, using the standard definitions for its inputs. Results are computed from the values you enter — no data is stored — and are rounded only for display.
Content last reviewed by CalcHub Editorial.
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