Ovulation Calculator
Enter the first day of your last period and your average cycle length to see your estimated ovulation date and fertile window for the next three cycles.
Enter your last period date and cycle length to see your fertile window.
Estimates only, based on the calendar method — not medical advice or a contraception method. Calculated on your device; nothing is sent anywhere.
How it works
- Select the first day of your most recent period in the date field.
- Set your average cycle length in days — 28 is typical, but anything from 21 to 35 is common.
- Read the estimated ovulation date, six-day fertile window, and next period start for each of the next three cycles.
Frequently asked questions
- How does the ovulation calculator estimate my fertile days?
- It uses the standard calendar method: ovulation is assumed to occur about 14 days before your next period, so it adds your cycle length to your last period start and subtracts 14. The fertile window covers the five days before ovulation plus the ovulation day itself, because sperm can survive up to five days while the egg is viable for about 24 hours.
- How accurate is a calendar-based ovulation calculator?
- It gives a reasonable estimate for people with regular cycles, but actual ovulation can shift by several days from month to month due to stress, illness, travel, or natural variation. If timing matters, combine the estimate with signs like basal body temperature, cervical mucus changes, or ovulation predictor kits, which detect the hormone surge directly.
- What if my cycles are irregular?
- The calendar method works poorly when cycle length varies by more than a few days, because the 14-day luteal phase assumption anchors to a next period date that itself is uncertain. In that case, track several cycles to find your shortest and longest lengths, run the calculator for both, and treat the combined range as your possible fertile window.
- Is my period data private or sent to a server?
- Everything is calculated locally in your browser with plain date arithmetic; no dates are transmitted, logged, or stored anywhere. Reproductive health data is sensitive, so this tool is deliberately built with no accounts, no cookies for your inputs, and no network requests during use.
- Can I use this calculator to avoid pregnancy?
- It is not reliable enough for contraception on its own. Sperm survival and ovulation timing vary, so conception is possible from unprotected sex outside the calculated window. Fertility-awareness methods used for birth control require daily observations and proper training; talk to a healthcare provider about options suited to you.
About this tool
This ovulation calculator estimates the days each month when pregnancy is most likely. From two inputs — the first day of your last menstrual period and your average cycle length — it projects your next three cycles, marking the estimated ovulation date, the six-day fertile window, and the expected start of each period. The results update instantly as you change either input, so you can compare scenarios for shorter or longer cycles in seconds.
The math follows the widely used calendar method. The luteal phase, the stretch between ovulation and the next period, is fairly constant at around 14 days for most people, while the first half of the cycle varies. So the calculator adds your cycle length to your last period date to find the next period, then counts back 14 days to place ovulation. The fertile window spans the five days before ovulation through ovulation day itself, reflecting how long sperm can survive in the reproductive tract and the roughly 24-hour lifespan of a released egg. All of this runs client-side in your browser: no dates are uploaded, stored, or shared, which matters for data this personal.
Typical uses include timing intercourse when trying to conceive, planning ahead for ovulation test strips so you start testing a few days before the predicted date, and simply anticipating when the next period will arrive. The three-cycle view is handy for scheduling around trips or appointments without recalculating each month.
For best results, use an averaged cycle length from at least three tracked cycles rather than a guess. If your cycles differ by more than seven days, calendar estimates become unreliable and hormone-based testing gives far better timing. Remember the output is an estimate, not a diagnosis — consult a clinician for fertility concerns.