Ovulation Calculator
Estimate your most fertile days and ovulation date based on your cycle length.
Reviewed October 7, 2026 Runs in your browser — inputs are never stored
This tool provides general educational estimates only and is not medical advice. It does not diagnose, treat, or replace guidance from a qualified healthcare provider. Consult a doctor for decisions about your health.
About the Ovulation Calculator
The ovulation calculator estimates the day you are most likely to ovulate and your fertile window from the first day of your last period and your usual cycle length. Ovulation typically happens about 14 days before the next period starts, so the tool subtracts 14 days from your cycle length. Because sperm can survive for up to five days, the fertile window covers the five days before ovulation and the day after. The estimate is most reliable for regular cycles. It is a planning aid for trying to conceive and must not be used as contraception.
How to use it
- 1Select the first day of your last period.
- 2Enter your average cycle length in days (count from the first day of one period to the first day of the next).
- 3Press Calculate to see your estimated ovulation date, fertile window and next period.
- 4Track a few cycles and use your average if your cycle length varies.
Worked example
if your last period started on 1 March and your cycle is 30 days long, ovulation is estimated for 17 March. Your fertile window runs from 12 to 18 March and your next period is expected around 31 March.
How it works
How it works
Ovulation typically occurs about 14 days before your next period. The fertile window spans the five days before ovulation plus the day itself.
Who it is for
People trying to conceive who want to time intercourse, and anyone learning about their menstrual cycle.
Why it helps
Knowing the likely fertile days improves the chance of conception in each cycle.
Common use cases
- Timing intercourse when trying to conceive
- Planning when to use ovulation test strips
- Understanding how cycle length shifts the fertile window
Common mistakes to avoid
- Assuming ovulation is always on day 14 — it depends on your cycle length.
- Using a calendar estimate as contraception.
- Using a single unusual cycle instead of your average cycle length.
Frequently asked questions
A:About six days: the five days leading up to ovulation and the day of ovulation.
A:No. Cycle-based estimates are not a reliable birth-control method.
Reviewed for accuracy · Last updated October 7, 2026
Formulas follow recognised clinical references — including WHO BMI classifications, the Mifflin-St Jeor equation, and Naegele’s rule — and are periodically checked against published reference values.
CalcWisee Editorial Team
Calculator development and formula review
CalcWisee is an independent site founded by Ayman. Every calculator is built from a published formula or official definition, tested against hand-worked examples before it goes live, and corrected promptly when a reader reports an issue to ayman@calcwisee.com.
Expertise: Financial mathematics, Health and fitness formulas, Applied mathematics, Unit conversion standards
References & sources
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