Fertile window and ovulation calculator

Estimate your most fertile days, ovulation, next period and estimated due date.

How the fertile window is calculated

The calendar method assumes ovulation happens about 14 days before the next period. Because the egg survives up to 24 hours and sperm up to five days, the fertile window runs from five days before to one day after ovulation.

The estimated due date uses Naegele's rule: 280 days from the first day of the last period. For cycles other than 28 days, ovulation and the fertile window shift by the same amount.

Based on the calendar method and Naegele's rule, as described in obstetric textbooks (for example, Williams Obstetrics).

Frequently asked questions

In most regular cycles, yes. In shorter or longer cycles ovulation shifts, but the second phase of the cycle (after ovulation) tends to stay around 14 days.

It is unlikely but possible. Ovulation may happen earlier or later than predicted, especially in irregular cycles.

No. The calendar method has a high failure rate and is not recommended as a stand-alone contraceptive. Talk to your gynaecologist about more effective methods.

Count from the first day of one period to the day before the next period. The average of your last three to six cycles is the most reliable.

Important notice

This calculator is an educational tool and does not replace a medical consultation. Results are estimates based on published models and may not apply to your case. Never make decisions about your health or your pregnancy without advice from your doctor.

Reviewed by: Dr. Peterson Fernandes — CRM-MG 42452 · RQE 37814

Last reviewed: 11/3/2026

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