Ovulation Calculator: Estimate Your Fertile Window
Enter the first day of your last period and your cycle length – the calculator uses the calendar method to estimate ovulation, your fertile days and your next period for three cycles. Important: this is only an estimate and not a method of contraception. Ovulation varies from cycle to cycle, and the calculator does not replace medical advice.
Source: Wilcox AJ, Weinberg CR, Baird DD: Timing of sexual intercourse in relation to ovulation. N Engl J Med 1995;333:1517–21. Updated: .
How it is calculated
How the ovulation calculator works
The second half of the cycle – from ovulation to the next period – is fairly constant at about 14 days for many people. The calculator therefore adds your cycle length to the first day of your last period and subtracts 14 days: ovulation ≈ next period − 14 days. The fertile window is taken as the 5 days before ovulation up to 1 day after – sperm can survive for up to five days, the egg for about 12 to 24 hours.
How to calculate ovulation
- Enter the first day of your last period.
- Enter your average cycle length (usually 21 to 35 days).
- Read the estimated ovulation day, fertile days and next period for three cycles.
Example: last period started on 1 March, 28-day cycle – the next period is expected around 29 March, ovulation 14 days earlier, around 15 March.
Example: period started on 14 September 2026, 28-day cycle → next period on 12 October 2026, ovulation around 28 September 2026, fertile days roughly 23 to 29 September. The table repeats this for three cycles.
Not a method of contraception
The calendar method on its own is not suitable for birth control. Ovulation can shift because of stress, illness, travel or for no obvious reason – even in otherwise regular cycles. A calculation based on past cycles cannot foresee that. If you want to use natural family planning, you need a method that tracks body signs (such as the symptothermal method) with proper teaching – or another reliable contraceptive. Ask a doctor, nurse or a sexual health clinic for advice.
Trying to conceive
For planning, the calculator gives a rough guide: pregnancy is most likely from intercourse in the days just before ovulation. Ovulation tests, basal body temperature or cervical mucus pin it down more precisely. With irregular cycles the estimate is often off.
Important: this is an estimate, not a diagnosis, and it does not replace medical advice – including for fertility problems or cycle disorders. Once you are pregnant, the due date calculator takes over.
More pregnancy calculators: the due date calculator, the pregnancy week calculator and the date calculator for counting days.
Frequently asked questions
How do I calculate when I ovulate?
With the calendar method: first day of your period + cycle length − 14 days. In a 28-day cycle, ovulation is around day 14. This is only an estimate.
When are my fertile days?
About 5 days before ovulation up to 1 day after – roughly 6 days per cycle. Conception is most likely in the days just before ovulation.
Can I use the ovulation calculator for contraception?
No. The calendar method is not reliable birth control because ovulation can shift at any time. This calculator is explicitly not a method of contraception.
What if my cycle is irregular?
Then the estimate is uncertain. You can use an average cycle length, but do not rely on it. If your cycles vary a lot, it is worth seeing a doctor.
Why does the calculator subtract 14 days?
The time from ovulation to the next period (the luteal phase) is usually fairly constant at about 14 days. Most variation in cycle length happens in the first half of the cycle.
Sources and legal basis
- Wilcox AJ, Weinberg CR, Baird DD: Timing of sexual intercourse in relation to ovulation. N Engl J Med 1995;333:1517–21
- WHO – Family Planning: A Global Handbook for Providers (Fertility awareness methods)
- NHS – Natural family planning (fertility awareness)
- MedlinePlus – Fertility awareness: identifying fertile days
As of: · Editorial policy
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