Ovulation calculator
Enter the first day of your last period and your usual cycle length. We'll show your predicted ovulation day and fertile window for the next three cycles.
Leave at 14 unless you've measured yours (days from ovulation to your next period).
Enter your last period date.
How ovulation is predicted
The follicular phase (period to ovulation) varies a lot between people and cycles; the luteal phase (ovulation to next period) is steadier, typically 12–14 days. So the calculator counts back 14 days from when your next period is expected. With a 28-day cycle that puts ovulation on day 14; with a 32-day cycle, day 18.
Once you have a positive test, work out your due date with the due date calculator.
Frequently asked questions
When am I most fertile?
In the two days before ovulation and on ovulation day itself. Wilcox et al. (NEJM, 1995) found conception chances were highest when intercourse happened 1–2 days before ovulation, and close to zero the day after.
How accurate is an ovulation calculator?
It is a prediction based on the average luteal phase of 14 days. Real ovulation can shift by several days from cycle to cycle, even in people with regular periods. Ovulation predictor kits (LH tests), cervical mucus changes or basal body temperature tracking can confirm it.
What if my cycles are irregular?
Enter your average cycle length, but treat the result as a rough guide. With irregular cycles, LH test strips and tracking cervical mucus give more reliable timing. If your cycles are often shorter than 21 or longer than 35 days, it's worth talking to a doctor.
Can I get pregnant outside the fertile window?
It's unlikely but possible, because ovulation doesn't always happen when predicted. Calendar methods are not reliable contraception on their own.