Ovulation Calculator
One date — your last period — and see your ovulation day and fertile window on a calendar.
Estimates only. Do not use this as birth control — calendar methods fail often. For medical decisions, talk to a doctor or midwife.
The first day of actual bleeding — not the day it ended.
From the first day of one period to the first day of the next. Most cycles run 21–45 days; 28 is the default.
Your cycle calendar
- Period start
- Fertile window
- Ovulation
- Today
Next 3 cycles
Calculated in your browser. Your dates stay on this device and are never sent to a server.
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How is the ovulation day calculated?
This ovulation calculator uses the standard luteal-phase method: your next period date (last period + cycle length) minus 14 days. The luteal phase — from ovulation to the next period — is the most stable part of the menstrual cycle at about 14 days, while the first half varies. So with a 28-day cycle ovulation lands near day 14, and with a 32-day cycle it is closer to day 18.
When exactly is the fertile window?
From 5 days before ovulation to 1 day after — a 7-day window. Sperm can survive up to about 5 days in fertile cervical mucus, and the egg lives roughly 12–24 hours after release, so the days just before ovulation matter most. This fertile window calculator marks all 7 days, but the highest-chance days are the two or three before ovulation plus the day itself.
Can I use this as birth control?
No. Calendar-based (rhythm) methods have a high failure rate — with typical use, roughly 12–24 of every 100 women get pregnant within a year — because ovulation shifts with stress, illness, travel and sleep. This fertility calculator is for planning and awareness only. For contraception, ask a doctor about a proven method.
What if my menstrual cycle is irregular?
Use your average length over the last 3–6 cycles and treat the result as a rough guide. If your cycles vary by more than a few days, or fall outside 21–45 days, a period predictor alone cannot pin down ovulation — ovulation test kits (LH strips), basal body temperature or cervical mucus tracking are more reliable. Ongoing irregularity is worth raising with a doctor.
Is my period data sent to a server?
No. Every date is calculated in your browser with JavaScript. Your last period date and cycle length are saved only in this browser's local storage so the form is ready next time, and "Clear saved dates" wipes them immediately. There is no account, no upload and no tracking of what you enter.