Find your most fertile days. This free ovulation calculator estimates your ovulation date and fertile window from your last period and cycle length, to help when you’re trying to conceive.
Enter the first day of your last period and your usual cycle length (the number of days from one period’s start to the next — typically 21–35). The fertile window is the days you’re most likely to conceive.
Estimates only, not medical advice or contraception. Cycles vary month to month; ovulation predictor kits and tracking with your doctor give more precise timing.
How the Ovulation Calculator Works
This tool estimates the day you’re most likely to ovulate and the fertile days around it, based on the first day of your last period and your average cycle length. It’s aimed at people trying to conceive who want a general sense of timing, not a precise fertility diagnosis.
Formula: Ovulation day (counted from last period start) = cycle length − 14. Estimated ovulation date = last period start + (cycle length − 14) days. Fertile window = last period start + (cycle length − 14 − 5) days through last period start + (cycle length − 14 + 1) days. Estimated next period = last period start + cycle length. Estimated due date if conceived = ovulation date + 266 days.
- Last period start — the first day of your most recent period, the anchor date for every estimate below.
- Average cycle length — the number of days from the start of one period to the next, entered as a whole number between 20 and 45 days (there’s no default here — you must enter your own value).
Example Scenarios
| Last period start | Cycle length | Estimated ovulation | Fertile window |
|---|---|---|---|
| Jan 1, 2026 | 28 days | Jan 15, 2026 | Jan 10 – Jan 16, 2026 |
| Jan 1, 2026 | 32 days | Jan 19, 2026 | Jan 14 – Jan 20, 2026 |
| Feb 1, 2026 | 30 days | Feb 17, 2026 | Feb 12 – Feb 18, 2026 |
| Mar 1, 2026 | 24 days | Mar 11, 2026 | Mar 6 – Mar 12, 2026 |
| Nov 1, 2026 | 26 days | Nov 13, 2026 | Nov 8 – Nov 14, 2026 |
Ovulation Calculator FAQ
How is my ovulation day calculated from my cycle length?
The calculator assumes ovulation happens 14 days before your next period starts, so it subtracts 14 from your average cycle length to find which day of your cycle that is, then counts that many days forward from your last period’s start date.Why is the fertile window 5 days before and 1 day after ovulation?
Sperm can survive in the reproductive tract for several days waiting for an egg, while an egg is generally viable for about a day after release, so the days just before ovulation plus the day itself and the day after make up the window when conception is most possible.What if my cycle length is outside the 20–45 day range?
The calculator won’t produce an estimate outside that range, since the underlying 14-day-before-next-period assumption becomes much less reliable for very short or very long cycles. If your cycles are consistently outside that range, a healthcare professional can help assess what’s going on.Can I use ovulation timing like this to avoid pregnancy?
This tool is not designed or validated as a contraceptive method. Ovulation timing shifts from cycle to cycle, and relying on an estimate like this to prevent pregnancy carries real risk — speak with a healthcare professional about reliable contraception options.How is the “due date if conceived” figure calculated?
It adds 266 days to your estimated ovulation date, since gestational dating conventionally runs about 266 days from conception (or 280 days from the last period, which is roughly two weeks earlier).My cycles are irregular — how reliable is this estimate?
Less reliable than for someone with consistent cycles, since the calculation depends entirely on your average cycle length. Tracking basal body temperature, ovulation predictor kits, or cervical mucus changes alongside this estimate, or working with a doctor, can give a clearer picture.Related Calculators
If your fertile window has passed and you’re now wondering about a possible pregnancy, the due date calculator for estimating from your last period or conception and the conception date calculator can help you work out the timeline. If you’re watching for very early signs, the early pregnancy symptoms guide covers what people commonly notice in the first weeks.