Health
Ovulation Calculator
Enter the first day of your last period and your average cycle length to estimate your next ovulation date, your six-day fertile window, and when your next period is likely to start.
Quick answer: Ovulation typically happens about 14 days before your next period starts — day 14 of a 28-day cycle, day 21 of a 35-day cycle. The fertile window is the five days before ovulation plus the day after.
- Fertile window—
- Next period starts—
- Following ovulation—
A calendar estimate only — ovulation is assumed 14 days before the next period, but cycles vary from month to month. Ovulation predictor kits or basal body temperature tracking are more reliable. Do not use this as a contraception method.
How it works
1. Count from your last period
Enter the first day of your last menstrual period and your average cycle length. The luteal phase after ovulation is fairly constant at ~14 days, so ovulation is estimated at cycle length minus 14 days after your period started.
2. Find the fertile window
Sperm survive up to five days and the egg about one, so the fertile window runs from five days before ovulation through the day after — the days with a realistic chance of conception.
3. Plan the next cycles
The calculator also projects your next period and the following cycle's ovulation, so you can plan a month or two ahead. Track a few real cycles to refine your average length.
Frequently asked questions
When do you ovulate?
About 14 days before your next period — not 14 days after the last one, unless your cycle is exactly 28 days. With a 32-day cycle, ovulation lands around day 18.
How long is the fertile window?
About six days: the five days before ovulation plus roughly 24 hours after, because sperm live up to five days but the egg only about one. The two days before ovulation are the most fertile.
How accurate is a calendar estimate?
It's a starting point. Cycles vary — stress, travel, and illness shift ovulation — so ovulation predictor kits, basal body temperature, or cervical-mucus tracking pin it down more reliably. Don't rely on the calendar method as contraception.