Ovulation and Fertile Window Calculator
Enter the first day of your last period and your usual cycle length. These dates are estimates and cycles vary from month to month.
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Formula
- Ovulation day:
last period + cycle length − 14 days - Fertile window: 5 days before ovulation to 1 day after
- Next period:
last period + cycle length
Worked example
Last period began 1 March 2024 with a 28-day cycle: ovulation is 1 March + 14 days = 15 March, the fertile window is 10 to 16 March, and the next period is expected on 29 March.
Frequently asked questions
When do I ovulate?
Ovulation usually happens about 14 days before the next period. With a 28-day cycle starting 1 March, that is 15 March.
How is the fertile window worked out?
It runs from 5 days before ovulation to 1 day after, because sperm can survive several days. For ovulation on 15 March that is 10 to 16 March.
What if my cycle is not regular?
The estimate assumes a steady cycle. If yours varies by more than a few days, the real ovulation day can differ by a week or more, so this is not a reliable guide.
Can I use this to avoid pregnancy?
No. It is an estimate and not a method of contraception. Ask a health professional about reliable options.