Ovulation Calculator
Enter the first day of your last period and your average cycle length to see an estimated ovulation day, fertile window, and next expected period. This is a general estimate based on typical cycle timing — not medical advice or a guarantee of when you'll actually ovulate. It updates instantly, and the link in your address bar always reflects your numbers so you can save or share it.
How this calculator works
This calculator uses the standard calendar method for estimating ovulation. Rather than assuming ovulation always lands on day 14, it works from the fact that the luteal phase — the stretch between ovulation and the start of your next period — is fairly consistent at around 14 days, no matter how long your overall cycle runs. So estimated ovulation day = first day of your last period + (average cycle length − 14) days. The fertile window covers the 5 days before ovulation through ovulation day itself, based on published estimates that sperm can survive around 5 days in the body and a released egg stays viable for about 24 hours. Your next period is estimated simply as your last period date plus your average cycle length. All of this is a general estimate — actual ovulation timing varies from cycle to cycle, even for the same person, and can shift with stress, illness, travel, or other everyday factors.
Worked example
Last period: May 1, 2026, with an average cycle length of 30 days — a bit longer than the typical 28. Estimated ovulation day = May 1 + (30 − 14) = May 1 + 16 days = May 17, 2026. Notice that's day 16, not day 14 — the two extra days of this longer cycle land before ovulation. The fertile window runs from May 12 through May 17, 2026 (5 days before ovulation through ovulation day). The next period is estimated at May 1 + 30 days = May 31, 2026.
Frequently asked questions
How accurate is this ovulation estimate?
It's a general estimate based on average timing, not a precise prediction. The calendar method assumes a fairly typical luteal phase and can't know about cycle irregularity, stress, travel, illness, or the other everyday things that shift real ovulation earlier or later. For general awareness of where you are in your cycle, that's usually plenty. But for family planning purposes beyond general awareness — trying to conceive or trying to avoid pregnancy — more precise methods like ovulation predictor kits (which detect the LH surge) or basal body temperature tracking give a much closer read on what your body is actually doing, and a doctor's guidance is more reliable still. Treat this calculator as a helpful starting point, not the final word.
Why does the fertile window start 5 days before ovulation?
Sperm can survive inside the body for up to about 5 days while waiting for an egg to be released, while the egg itself stays viable for only around 24 hours afterward. Put those two windows together and intercourse anytime from 5 days before ovulation through the day of ovulation itself can potentially lead to pregnancy — which is why the fertile window spans that whole stretch rather than a single day.
Why isn't ovulation always exactly 14 days after my period starts?
That's only true if your cycle happens to match the 28-day average. What actually stays fairly consistent is the luteal phase — the days between ovulation and your next period — which tends to run around 14 days regardless of how long your overall cycle is. So instead of counting 14 days forward from your last period, this calculator counts back 14 days from your next expected period. That's why, for example, someone with a 32-day cycle tends to ovulate around day 18, not day 14 — the extra days happen before ovulation, not after it.
Can I use this to avoid getting pregnant?
We'd gently steer you away from relying on it that way. As a calendar estimate, this tool is built for general awareness, not as a stand-alone method of contraception. Real fertility awareness-based methods for pregnancy prevention involve more careful daily tracking — temperature, cervical mucus, or hormone tests — and ideally guidance from a doctor or trained educator, since the margin for error matters much more when the goal is prevention rather than curiosity. This is a general estimate, not medical advice.