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Ovulation Calculator

Enter the first day of your last period and your cycle length to estimate your ovulation day and fertile window. Estimate only, not medical advice.

Days from the first day of one period to the first day of the next (20 to 45). Usually 28.

This result is an estimate only. Check with your bank or the relevant authority before making decisions.

How it is calculated

  • Ovulation day = first day of last period + cycle length − 14 days
  • Fertile window = 5 days before ovulation to 1 day after ovulation
  • Next period = first day of last period + cycle length

Estimate only, not medical advice, and not a contraception method.

Worked example

First day of last period 10 January 2026, 28-day cycle:

  1. Ovulation: 24 January 2026
  2. Fertile window: 19 January to 25 January 2026
  3. Next period: 7 February 2026

Why your number may differ

Ovulation usually happens about 14 days before the next period, but irregular cycles make the estimate less reliable. This is not a contraception method. Estimate only, not medical advice.

Frequently asked questions

When does ovulation usually happen?

Ovulation usually happens about 14 days before your next period.

What if my cycle is irregular?

Irregular cycles make this estimate less reliable.

Can I use this for contraception?

No. This is not a contraception method. Estimate only, not medical advice.

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