Ovulation calculator
Enter the first day of your last period and your average cycle length. The calculator estimates your ovulation date and the fertile window around it. It is an estimate for regular cycles, not a guarantee.
Use it to estimate when you ovulate and your fertile days, based on your cycle.
Ovulation date
Jun 15, 2026
- Fertile window start
- Jun 10, 2026
- Fertile window end
- Jun 16, 2026
The result updates as you choose. The headline is the estimated ovulation date; the others mark the start and end of the fertile window.
How does it work?
This assumes a roughly regular cycle with ovulation about 14 days before the next period. It is an estimate, not a guarantee — cycles and ovulation timing vary. Not a contraceptive method.
Ovulation estimate
- last period
- First day of the last period.
- cycle
- Average cycle length in days.
- ovulation
- Estimated ovulation day.
With a 28-day cycle, ovulation is about 14 days after the period starts; the fertile window runs from 5 days before to 1 day after.
Method & sources
Ovulation is about 14 days before the next period. It is placed cycleLength − 14 days after the last period's first day. The fertile window runs from 5 days before to 1 day after ovulation.
Sources
Where this method comes from — use these references to understand the formula, assumptions, and limits.
- Fertility awareness — menstrual cycle — World Health Organization, verified 2026-06-10
- Periods and fertility — NHS — NHS (UK National Health Service), verified 2026-06-10
How we calculate
- Ovulation is about 14 days before the next period.
- It is placed cycleLength − 14 days after the last period's first day.
- The fertile window runs from 5 days before to 1 day after ovulation.
- An estimate for regular cycles, not a contraceptive method.
Rounding
Dates are exact day counts. The calculation is done in UTC.
What this calculator does
Ovulation usually happens about 14 days before the next period, regardless of cycle length. This calculator places it that far after your last period's first day, then marks the fertile window — the days when conception is most likely — around it.
How to use it
- Enter the first day of your last period.
- Enter your average cycle length in days.
- Read the estimated ovulation date.
- Note the fertile window dates.
A worked example
With a 28-day cycle, ovulation falls about 14 days after the period starts. The fertile window runs from about 5 days before ovulation to the day after.
Why it's an estimate
Cycle length and ovulation timing vary from cycle to cycle, and stress, illness, and other factors can shift them. The dates here are a guide; ovulation predictor kits or tracking signs give a closer picture.
Common mistakes
- Entering the period end date instead of the first day.
- Using an average cycle length that doesn't match yours.
- Treating the window as a contraceptive method — it isn't.
When it's useful
Planning around the fertile window when trying to conceive, or understanding your cycle — not for preventing pregnancy.
FAQ
- How is the ovulation date estimated?
- It's placed your cycle length minus 14 days after the first day of your last period, since ovulation is about 14 days before the next period.
- What is the fertile window?
- The days when conception is most likely — roughly 5 days before ovulation to the day after, because sperm can survive several days.
- How accurate is it?
- It's an estimate for regular cycles. Real timing varies; predictor kits or tracking physical signs are more precise.
- Can I use it as birth control?
- No. It is not a contraceptive method. Ovulation timing varies and the window is only an estimate.
- What cycle length should I enter?
- Your average — the days from the first day of one period to the first day of the next. Many cycles are 28 days, but yours may differ.
- Can I share a calculation?
- Yes. Use Share to copy a link that reopens the calculator with the same date and cycle length.
Related calculators
Embed this calculator
Add this calculator to your own site. The snippet includes the calculator iframe and a small attribution link:
<iframe src="https://wisecalcs.com/embed/en/ovulation-calculator" width="100%" height="520" style="border:0" loading="lazy"></iframe>
<p>Calculator from <a href="https://wisecalcs.com/en/health/ovulation-calculator">WiseCalcs</a></p>