🌸 Ovulation Calculator
Enter the first day of your last period and your average cycle length to estimate your ovulation day, fertile window and next period. For reference only — not for birth control.
Your next 3–4 cycles (estimates only)
How to use the ovulation calculator
Enter the day your last period started, your average cycle length (from the first day of one period to the day before the next one starts) and how many days your period usually lasts. You'll see your next estimated ovulation day and fertile window, and the table maps out your period, fertile window and ovulation for the current cycle and the next three (about 3–4 months). Use the average of your last 3–6 cycles for the best estimate.
How ovulation is estimated
A menstrual cycle has two phases: the follicular phase before ovulation and the luteal phase after it. The follicular phase is the part that varies from person to person; the luteal phase is fairly constant at about 14 days. So ovulation is estimated by counting 14 days back from your next expected period — not 14 days forward from your last one.
- Next period
first day of last period + cycle length - Ovulation day
next period − 14 days(day 14 of a 28-day cycle, day 18 of a 32-day cycle) - Fertile window
5 days before ovulation through 1 day after
The fertile window starts several days before ovulation because sperm can survive in the reproductive tract for up to about 5 days, while an egg can be fertilized for only about 12–24 hours after it's released.
Ovulation by cycle length
| Cycle length | Ovulation (cycle day) | Fertile window (cycle days) |
|---|---|---|
| 21 days | Day 7 | Days 2–8 |
| 28 days | Day 14 | Days 9–15 |
| 30 days | Day 16 | Days 11–17 |
| 35 days | Day 21 | Days 16–22 |
| 40 days | Day 26 | Days 21–27 |
Getting a more accurate picture
Calendar estimates assume your cycle is regular, but stress, sleep, illness, breastfeeding, weight changes and travel can all shift ovulation. Ovulation predictor kits (LH tests), tracking basal body temperature or cervical mucus, and ultrasound follicle monitoring show when you actually ovulate. If you're trying to conceive and it hasn't happened after 12 months (6 months if you're 35 or older), or your cycles are very irregular, talk to a doctor.
Do not use this for birth control. Calendar-based ovulation and fertile-window dates are statistical estimates, and there is no guaranteed "safe day". This calculator does not replace medical advice; see a healthcare provider about irregular or painful periods.