How to use the Ovulation Calculator
Enter the first day of last period and average cycle length and the result appears straight away. Everything runs in your browser, so nothing you type is uploaded.
The formula
Ovulation ≈ cycle length − 14 days after the period starts; fertile window = 5 days before to 1 day after
Examples
- Period started 1 Nov 2026, 28-day cycle
Cycle 1 (period starts Sun, November 1, 2026) Ovulation: Sun, November 15, 2026 Fertile window: Tue, November 10, 2026 to Mon, November 16, 2026 Next period expected: Sun, November 29, 2026
- Same start date, shorter 26-day cycle
Cycle 1 (period starts Sun, November 1, 2026) Ovulation: Fri, November 13, 2026 Fertile window: Sun, November 8, 2026 to Sat, November 14, 2026 Next period expected: Fri, November 27, 2026
- Same start date, longer 32-day cycle
Cycle 1 (period starts Sun, November 1, 2026) Ovulation: Thu, November 19, 2026 Fertile window: Sat, November 14, 2026 to Fri, November 20, 2026 Next period expected: Thu, December 3, 2026
Limitations and assumptions
Predictions assume a regular cycle and are not a reliable method of contraception. General estimate for adults, not medical advice. Speak to a doctor or registered dietitian before making significant changes to diet, exercise or health routines.
Frequently asked questions
How is ovulation estimated?
Ovulation typically happens around 14 days before the next period starts, so a 28-day cycle points to about day 14 and a 32-day cycle to about day 18.
How long is the fertile window?
About a week: the five days before ovulation, the day itself and the day after, because sperm can survive several days.
Is this accurate if my cycle is irregular?
Less so. Irregular cycles move ovulation from month to month, so combine the estimate with ovulation tests or advice from a clinician.
Can I use this to avoid pregnancy?
No. It is an estimate for planning, not contraception.