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.
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.
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.
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:
Related tools
- Due Date CalculatorEstimate your due date from the last menstrual period (with cycle length) or conception date – plus current pregnancy week, trimester and days to go.
- Pregnancy Week CalculatorHow many weeks pregnant am I? Work out your pregnancy week from the last period or due date – weeks and days, trimester, month and days until the due date.
- BMI calculator (WHO categories)Calculate your body mass index in lb and ft/in or kg and cm. See your WHO category, from underweight to obesity class III, and the healthy weight range.
- BMR and daily energy needs calculatorCalculate your basal metabolic rate with the Mifflin-St Jeor equation and your total daily energy needs with FAO/WHO activity levels (PAL), in kcal and kJ.
- Body Fat Calculator (U.S. Navy Method)Estimate your body fat percentage from height, neck, waist and hip measurements using the U.S. Navy equations by Hodgdon and Beckett (1984). In inches or cm.
- Calories burned calculatorEstimate calories burned in 30 sports and activities from MET value, body weight and duration per the 2024 Compendium, in kg or lb, with a personal correction.