Ovulation Calculator
Enter cycle start and length – get your ovulation date and fertile window instantly
Calculated in your browser – your inputs are not transmitted and are kept on this device only if you choose
Days from the first day of your period to the day before the next one (21–45).
Expected ovulation
–
–How does the female cycle work?
Ovulation, the fertile window and the biology behind it
The female menstrual cycle is divided into two phases: follicular phase (from the first day of menstruation until ovulation) and luteal phase (from ovulation until the start of the next menstruation). For most women, the luteal phase is relatively constant at around 14 days. The follicular phase, however, can vary from cycle to cycle, which is why ovulation occurs correspondingly earlier or later in shorter or longer cycles.
The formula for ovulation is therefore: ovulation ≈ cycle start + (cycle length − 14) days. In a 28-day cycle it falls roughly at mid-cycle, in a 35-day cycle about a week later — in each case about 14 days before the next menstruation. From this ovulation date, our calculator determines the fertile window: 5 days before ovulation (because sperm can survive in the body for up to 5 days) until 1 day after ovulation (because the egg remains fertilizable for 12–24 hours).
For couples trying to conceive, the days of the fertile window – especially the 2–3 days around ovulation – are statistically the most favorable time for intercourse intended to result in conception. Current studies show that the highest probability of conception occurs on the day of ovulation and the day before. Regular intercourse every 2–3 days throughout the cycle can also be effective.
Important: the ovulation calculator provides a calculated estimate. Cycle variations caused by stress, illness, travel or hormonal changes can shift the actual ovulation date considerably. For reliable ovulation determination, gynecologists additionally recommend LH rapid tests, basal body temperature measurements or direct follicular ultrasound monitoring.
Methods for determining ovulation
Comparison of methods
- Calendar calculator (this calculator)
- Fast and free. Suitable for regular cycles. Inaccurate with cycle variations. No substitute for medical advice or reliable contraception.
- LH ovulation test
- Urine test from the pharmacy. Measures the rise in LH hormone 24–36 hours before ovulation. More reliable than calendar calculation, but affected by certain medical conditions.
- Basal body temperature measurement
- Measure your body temperature every morning (before getting up). A temperature rise of about 0.2–0.5°C indicates that ovulation has occurred. Requires consistent tracking over several cycles.
- Gynecological follicular ultrasound
- Ultrasound examination by a doctor. Direct observation of the growing follicle and ovulation. The most accurate method, but time-consuming and costly.
Calculation examples
- Cycle start
- 1 January 2026
- Cycle length
- 28 days
- Ovulation (cycle start + 14 days)
- 15 January 2026
- Fertile from (−5 days)
- 10 January 2026
- Fertile until (+1 day)
- 16 January 2026
- Cycle start
- 1 January 2026
- Cycle length
- 35 days
- Ovulation (cycle start + 21 days)
- 22 January 2026
- Fertile from (−5 days)
- 17 January 2026
- Fertile until (+1 day)
- 23 January 2026
Frequently asked questions about the ovulation calculator
Ovulation, fertile window and cycle variations
How is ovulation calculated?
Ovulation typically occurs about 14 days before the start of the next menstruation. Calculated from the beginning of the cycle, this gives: ovulation ≈ cycle start + (cycle length − 14) days. In a 28-day cycle, ovulation is then around mid-cycle; in a 35-day cycle, correspondingly later. This formula is based on the assumption that the luteal phase (after ovulation) remains relatively constant at about 14 days.
What is the fertile window?
The fertile window covers the days on which conception is fundamentally possible. After ovulation, the egg is fertilizable for only 12–24 hours. Sperm, however, can survive in the body for up to 5 days. That is why the period from 5 days before to 1 day after ovulation counts as the fertile window (6 days in total). The day of ovulation itself and the day before it are statistically the most fertile.
How accurate is the ovulation calculator?
The calculator provides a statistical estimate based on a regular cycle. In practice, ovulation can vary due to many factors: stress, illness, travel, weight fluctuations or hormonal influences. For reliable ovulation determination, we additionally recommend basal body temperature measurements, LH rapid tests or gynecological ultrasound checks.
Can I rely on the calculator for contraception?
No. The ovulation calculator is explicitly not suitable as a contraceptive method. Even the most structured method (the symptothermal method according to NFP) requires professional instruction and consistent application. Uncontrolled cycle irregularities make reliable contraception through calendar calculation alone impossible. Talk to your doctor about suitable contraceptive methods.
What should I do with an irregular cycle?
With a strongly fluctuating cycle, a calculated ovulation estimate is not very reliable. In that case, we recommend: LH ovulation tests (available at pharmacies), which measure the rise in luteinizing hormone shortly before ovulation; a basal body temperature chart over several cycles; or gynecological follicular ultrasound, which makes ovulation directly observable.