Ovulation Calculator
Enter the first day of your last period and your usual cycle length. We estimate when you are most likely to ovulate and your fertile window.
This calculator gives general estimates for information only. It is not medical advice and does not replace a consultation with a doctor or other qualified health professional.
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The formula
Ovulation day = first day of last period + (cycle length - 14). Fertile window = 5 days before ovulation to 1 day after.
How the estimate works
Ovulation usually happens about 14 days before your next period starts, not 14 days after the last one began. In a 28-day cycle that is day 14, but in a 32-day cycle it is day 18. That is why cycle length matters.
Your fertile window
Sperm can live up to five days and the egg survives for roughly a day, so the fertile window runs from five days before ovulation to the day after. The two days before ovulation, and ovulation day itself, are the most fertile.
An example
If your last period began on 20 September and your cycle is 28 days, ovulation is estimated around 4 October, your fertile window runs from 29 September to 5 October, and your next period is due about 18 October.
Limits of a calculator
Stress, illness, travel and irregular cycles all move ovulation. If your cycles vary a lot, ovulation tests or tracking your cervical mucus and temperature will be more reliable. If you have been trying for a year (six months if you are over 35) without success, talk to a doctor.
Examples to check
Type these into the calculator above to see how it behaves. Every result below was produced by the same calculator code that runs on this page.
| What you enter | What you get |
|---|---|
| First day of your last period: 2026-09-20; Average cycle length (days): 28 | Estimated ovulation: Sunday, October 4, 2026 |
| First day of your last period: 2026-09-20; Average cycle length (days): 32 | Estimated ovulation: Thursday, October 8, 2026 |
| First day of your last period: 2026-09-20; Average cycle length (days): 26 | Estimated ovulation: Friday, October 2, 2026 |
Ovulation Calculator: frequently asked questions
How do I know my cycle length?
Count from the first day of one period to the day before the next. Do that for three months and average it. Cycles between 21 and 35 days are considered normal.
Can I rely on this result as medical advice?
No. It is a general estimate for information only. It cannot replace a check-up, and your doctor can interpret the numbers for your own situation.
Is what I enter in this calculator saved or sent anywhere?
No. The calculation runs in your browser, so what you enter is not sent to our servers. We only count, anonymously, whether a tool ran successfully.
Is this calculator free to use?
Yes. It is free, needs no sign-up and has no usage limit.
Last reviewed: October 3, 2026