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Ovulation Calculator. Fertile window and next period.

The first day of your last period and your average cycle length in — an estimated ovulation date, fertile window and next expected period out.

Live calculation

When are you fertile?

days Most cycles run 21–35 days. A cycle outside that range still gets an estimate — just a flag that it's less typical.
An estimate based on average cycle timing, not medical advice. Actual ovulation varies cycle to cycle, so use this for general planning only.
Estimated ovulation date —
Fertile window
—
Next expected period
—

Your figures are kept on this device only.

How it works

This calculator works backward from your next period rather than forward from your last one. Ovulation reliably happens about 14 days before a period starts — the part of the cycle after ovulation (the luteal phase) is fairly fixed in length, while the part before it (the follicular phase) is what actually varies. So the estimate takes your cycle length, subtracts the fixed 14-day luteal phase, and adds what's left to the start of your last period.

ovulation date = last period start + (cycle length − 14) days
fertile window = ovulation date − 5 days through ovulation date + 1 day
next expected period = last period start + cycle length

The fertile window is wider than a single day because sperm can survive in the reproductive tract for up to 5 days, while the egg itself is viable for only about a day after release. That gives a window of roughly a week each cycle where conception is possible, centered on the estimated ovulation date.

Worked example

A last period that started August 1, with a 28-day average cycle.

Step by step
  1. Luteal offset: cycle length − 14 = 28 − 14 = 14 days.
  2. Estimated ovulation date: Aug 1 + 14 days = Aug 15.
  3. Fertile window: Aug 15 − 5 days to Aug 15 + 1 day = Aug 10 – Aug 16.
  4. Next expected period: Aug 1 + 28 days = Aug 29.

With a last period starting Aug 1 and a 28-day cycle, ovulation is estimated around Aug 15, the fertile window runs Aug 10 – Aug 16, and the next period is expected around Aug 29.

Common questions

How accurate is this estimate?

It's based on the average case — ovulation occurring 14 days before the next period starts — which holds reasonably well for a regular cycle. Actual timing can shift by several days even in a normally regular cycle, so treat this as a planning estimate, not a guarantee.

Why is the fertile window several days, not just one day?

An egg survives about 24 hours after release, but sperm can survive up to 5 days in the reproductive tract — so conception is possible from intercourse up to 5 days before ovulation through about a day after it.

What if my cycle length varies a lot month to month?

This calculator assumes a consistent cycle length. If yours varies by more than a few days, the estimate becomes less reliable — tracking ovulation with symptoms (basal temperature, cervical mucus) or a dedicated tracking method will be more accurate than a date calculation alone.