Enter the first day of your last period and your cycle length to estimate ovulation day, fertile window and next period — cycle math, not contraception.
All math runs locally in your browser. The dates you enter never leave your device.
Estimates from calendar math are not reliable for contraception; if you have an IUD or are avoiding pregnancy, consult a clinician — cycle day can vary several days per cycle.
For general information — not medical advice.
This is calendar arithmetic on UTC day counts, so time zones never skew the dates. From the first day of your last period and your average cycle length it estimates ovulation as cycle length minus 14 days, then draws a fertile window around it.
No. This page estimates a fertile window from calendar math; it does not prevent pregnancy. Fertility-awareness methods based on calendar rules alone (the rhythm method) have among the highest typical-use failure rates of any contraceptive — roughly 24 per 100 women per year, because ovulation can shift by several days from cycle to cycle. If you are trying to avoid pregnancy, or you have an IUD and want to reason about your cycle alongside it, talk to a clinician about methods that fit your situation.
Treat the result as a rough sketch, not a map. The calculator assumes your average cycle length repeats, so the wider your cycle-to-cycle variation, the less useful a single estimated ovulation day is. With genuinely irregular cycles, calendar estimates can be off by a week or more — a healthcare provider can help you interpret your own pattern.
Some people use basal body temperature (measured before rising, which tends to shift after ovulation) or over-the-counter ovulation predictor kits (which detect the LH surge about one to two days before ovulation). These observe your body instead of assuming a calendar rule, but they are techniques with their own limits and learning curves — ask a clinician which, if any, suits your goal.
After ovulation, the corpus luteum lives for a fairly consistent ~12–14 days in most people, so the period arrives about two weeks later. The follicular phase before ovulation is the variable part — a 32-day cycle usually means a later ovulation (around day 18), not a longer luteal phase. This calculator applies that rule: ovulation day = cycle length − 14.
No. The calculation is plain calendar arithmetic running entirely in your browser. The date you enter is never transmitted, stored or logged — it exists only on your device while the page is open, which is how cycle information should be handled.
If you just finished with Ovulation Calculator, the natural next steps are Pregnancy Due Date Calculator, Weight Gain in Pregnancy, Age Calculator, or browse every tool in Health Tools.