Pregnancy Conception Calculator
Conception and Due Date, From One Input
A last period date and cycle length are enough to estimate two different things people usually look up separately: roughly when conception happened, and roughly when the baby is due. This calculator runs both calculations from the same pair of inputs, so you get the fertile window that likely produced the pregnancy alongside the projected due date it leads to.
The Formulas
Conception Window = Ovulation Day − 5 days to Ovulation Day + 1 day
Due Date = Last Period Start Date + 280 days + (Cycle Length − 28)
The conception window formula assumes a standard 14-day luteal phase and accounts for sperm survival (up to five days) and the egg's brief post-ovulation viability (about a day). The due date formula is Naegele's rule with the same cycle-length correction used elsewhere on this site.
How Cycle Length Moves Both Estimates
From a last period start of January 5, 2026:
| Cycle length | Conception window | Estimated due date |
|---|---|---|
| 24 days | Jan 10 – Jan 16 | Oct 08, 2026 |
| 26 days | Jan 12 – Jan 18 | Oct 10, 2026 |
| 28 days | Jan 14 – Jan 20 | Oct 12, 2026 |
| 30 days | Jan 16 – Jan 22 | Oct 14, 2026 |
| 32 days | Jan 18 – Jan 24 | Oct 16, 2026 |
| 35 days | Jan 21 – Jan 27 | Oct 19, 2026 |
Computed with the calculator's own formulas. Both estimates shift together as cycle length changes, since they're derived from the same underlying ovulation-day calculation.
Why a Combined View Helps
- Consistency check — if you have an independent estimate of when conception occurred, comparing it against this calculator's window is a useful sanity check on your entered cycle length.
- Single-visit convenience — rather than running a conception calculation and a due-date calculation separately, this returns both from one set of inputs.
- Early planning — useful before a dating scan, when both the conception timeline and the due date still rest on calendar math rather than clinical confirmation.
How to Use This Calculator
- Enter the Last Period Start Date.
- Enter your Average Cycle Length (days) — defaults to 28, accepts 20 to 45.
- Select Calculate for the likely conception window, estimated due date, and your current gestational status.
Related Calculations
For conception-only calculations that also work backward from a due date or birth date, see the Conception Calculator. For ovulation timing on its own, use the Ovulation Calculator.