What this calculator does
Estimated conception date works out back-calculates a likely conception date from a due date or the start of the last period. Enter your own figures above and the answer updates as you type: nothing is fixed in the code, so the result reflects exactly the numbers you supply.
The formula this calculator evaluates is printed under the tool and explained below, so you can check the working by hand or reuse it in a spreadsheet.
The formula
FormulaFrom due date: conception ≈ due date − 266 days. From last period: conception ≈ LMP + cycle length − 14 days
The inputs explained
| Field | What to enter |
|---|---|
| Calculate from | Choose from Due date, Last menstrual period. |
| Due date | A calendar date. |
| Last menstrual period start | A calendar date. |
| Average cycle length (days) | A number, measured in days. Starts at 28. |
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How the answer changes with average cycle length
Every other input is held at the calculator’s starting values while average cycle length varies. Select any row to load that scenario into the calculator.
How the answer changes with average cycle length
| Average cycle length (days) | Estimated conception date | Likely fertile window | Implied due date (266 days later) |
|---|---|---|---|
| 14 | 16 Mar 2026 | 13 Mar 2026 to 17 Mar 2026 | 7 Dec 2026 |
| 21 | 16 Mar 2026 | 13 Mar 2026 to 17 Mar 2026 | 7 Dec 2026 |
| 28 | 16 Mar 2026 | 13 Mar 2026 to 17 Mar 2026 | 7 Dec 2026 |
| 42 | 16 Mar 2026 | 13 Mar 2026 to 17 Mar 2026 | 7 Dec 2026 |
| 56 | 16 Mar 2026 | 13 Mar 2026 to 17 Mar 2026 | 7 Dec 2026 |
| 84 | 16 Mar 2026 | 13 Mar 2026 to 17 Mar 2026 | 7 Dec 2026 |