This calculator estimates when you are likely to ovulate and which days in your cycle you are most likely to conceive on, from the first day of your last period and your usual cycle length.
These dates are an estimate from an average cycle. Ovulation shifts from month to month, and stress, illness, PCOS or thyroid problems can move it a long way. If your cycles are irregular, or you have been trying for a year (six months over 35), a fertility evaluation will tell you far more than a calculator can.
How the fertile window is worked out
Ovulation is counted back from the next period rather than forward from the last one: the luteal phase — the stretch between ovulation and the next period — is the steadier half of a cycle, and is about 14 days for most people.
The fertile window opens about five days before ovulation, because sperm can survive several days in the reproductive tract while an egg is only viable for around 24 hours after release.
When an estimate is not enough
Ovulation moves from month to month, and conditions such as PCOS or a thyroid disorder can move it a long way. If your cycles are irregular, or you have been trying to conceive for a year — six months if you are over 35 — a fertility evaluation will tell you far more than any calculator can.