Ovulation calculator
Estimated ovulation and 6-day fertile window, based on your cycle.
- Instant
- Free
- Private (processed locally)
- No sign-up
Spot the days that count in your cycle
When trying to conceive, it all comes down to a few days. The fertile window covers the 5 days before ovulation plus the day itself — because sperm can wait up to 5 days. The tool locates this window and projects the next three cycles.
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Enter your last period
The first day of bleeding.
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Set the cycle length
From 21 to 35 days — 28 by default.
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Read your window
Estimated ovulation + 6 fertile days, across 3 cycles.
Ovulation day by cycle length
| Cycle | Ovulation (≈ day) | Fertile window |
|---|---|---|
| 26 days | Day 12 | Day 7 → 12 |
| 28 days | Day 14 | Day 9 → 14 |
| 30 days | Day 16 | Day 11 → 16 |
| 32 days | Day 18 | Day 13 → 18 |
A pregnancy-planning aid, never a contraceptive method: real ovulation varies. To confirm, observe cervical mucus, basal body temperature or use ovulation tests.
Frequently asked questions
Why a 6-day fertile window?
Because sperm survive up to 5 days in the reproductive tract and the egg about 24 h. The window therefore runs from the 5 days before ovulation to the day itself: intercourse within this range can lead to conception.
How is ovulation estimated?
By the stable luteal phase rule: ovulation occurs about 14 days before the next period, i.e. day (cycle length − 14). For a 28-day cycle that is day 14; for 32 days, day 18.
Is it reliable as contraception?
No, absolutely not. Ovulation varies from cycle to cycle, stress and illness shift it, and the calculation cannot “see” these variations. To avoid pregnancy, use a real contraceptive method — not this calculator.
How do I confirm my actual ovulation?
Three signals: cervical mucus turns clear and stretchy (like egg white), basal body temperature rises 0.3-0.5°C after ovulation, and urine tests detect the LH surge 24-36 h before. The calculation only tells you where to look.