Ovulation Calculator
Estimate when you are most likely to ovulate and your fertile window, from the first day of your last period and your cycle length.
Result
Enter the first day of your last period.
How to use this calculator
- Enter the first day of your last period and your average cycle length.
- Change the luteal phase only if you know yours.
Formula
Ovulation ≈ period start + cycle length − luteal phase (usually 14 days). The fertile window runs from 5 days before ovulation to 1 day after.
Worked example
A 28-day cycle
With a period starting on 1 January, ovulation is estimated on 15 January and the fertile window is 10–16 January.
Assumptions and limitations
- Everything runs in your browser; nothing you enter is uploaded or stored.
- Estimates only, and not a method of contraception. Cycles vary; ovulation tests give more precise timing.
Frequently asked questions
How accurate is this?
It is most useful for regular cycles. Stress, illness and other factors can shift ovulation.
When should I take a pregnancy test?
From the day your period is due; the result shows the earliest reliable date.