Ovulation calculator
Enter the first day of your last period and your average cycle length to get the ovulation day, the fertile window (5 days before to 1 day after ovulation) and the next period date, plus your current cycle day and phase when today is supplied. The luteal phase is fixed at 14 days.
Runs in your browserEvery computation happens in your browser — your data never leaves this device.
Cycle settings
Calendar method: ovulation = next period − 14 days; the fertile window runs from 5 days before to 1 day after ovulation.
Result
Enter the first day of your last period and your cycle length to see the ovulation day and fertile window.
What this tool does
- Planning a pregnancy: mark this cycle’s fertile window so effort lands on the days that actually matter.
- Trying to avoid the riskiest dates: see roughly where ovulation sits and how wide the window is.
- Irregular cycles: log each period start and compare how far the ovulation day drifts as the cycle length changes.
- Before a follicle-tracking appointment, get a reference ovulation date to discuss timing with your clinician.
Example
Input
Last period started 2024-01-01, average cycle 28 days, today 2024-01-15
Output
Predicted ovulation 2024-01-15; fertile window 2024-01-10 to 2024-01-16 (7 days); next period starts 2024-01-29; today is cycle day 15, inside the fertile window, 0 days to ovulation
Ovulation = last period + (cycle length − 14); the luteal phase is fixed at 14 days.
Frequently asked questions
How is the ovulation day calculated?
The calendar method assumes a fairly constant luteal phase of about 14 days, so ovulation ≈ next period − 14 days, that is, last period + (cycle length − 14). A 28-day cycle puts ovulation on day 14; a 30-day cycle pushes it to day 16.
Why is the fertile window seven days long?
Sperm can survive up to about five days in the reproductive tract and an egg stays fertilisable for roughly 24 hours, so the window runs from five days before ovulation to one day after. Conception is most likely in the two days before ovulation and on the day itself.
Can I use this for contraception?
Not on its own. The calendar method assumes regular cycles and ovulation exactly 14 days before the next period, but stress, poor sleep, illness and travel all shift it, and failure rates are far higher than with condoms or the pill. Use a reliable contraceptive method if you want to avoid pregnancy, and combine this with ovulation tests and basal body temperature if you are trying to conceive.
What if my cycle is irregular?
Record several period start dates and run the calculation twice, once with your shortest cycle and once with your longest; treat the union of the two windows as the plausible range. If your cycle is consistently under 21 or over 45 days, or suddenly becomes erratic, see a clinician.
Why do I have to enter “today” separately?
The ovulation day and fertile window follow from dates alone, but “which cycle day am I on” needs the current date. To keep the maths independent of time zones and the system clock, today is an explicit input — the page fills in the current date on load and you can change it to test any date.
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