31 Day Cycle Ovulation Calculator: When Do I Ovulate?

How it works

With a 31-day cycle, you most likely ovulate around cycle day 17 (day 1 is the first day of your period). Your fertile window is roughly days 12 to 18, and your best chance of conceiving is from day 15 to day 17. Enter your last period above to turn that into dates.

When do you ovulate with a 31-day cycle?

Ovulation usually happens about 14 days before your next period, not 14 days after the last one. The part of the cycle after ovulation, the luteal phase, is fairly steady at 12 to 14 days for most people, while the part before it stretches or shrinks. So with a 31-day cycle you count back 14 days from day 32 (the first day of your next period) and land on day 17.

Cycle day	What usually happens in a 31-day cycle
1 to 5	Your period (most people bleed 3 to 7 days)
6 to 11	Follicular phase: estrogen rises, energy and mood usually lift
12 to 16	Fertile window opens: sperm can survive up to 5 days
17	Estimated ovulation, the most fertile day along with the 2 days before
18 to 31	Luteal phase: progesterone rises; PMS often shows up in the last week

Your fertile window with a 31-day cycle

An egg lives for about 12 to 24 hours after ovulation, but sperm can survive up to five days. That makes days 12 to 18 your fertile window, with the highest chances on days 15, 16 and 17. In the classic study of couples trying to conceive, sex on the two days before ovulation and on ovulation day itself carried the highest chance, about 27 to 33% per cycle.

Testing for ovulation

With a 31-day cycle, start daily ovulation (LH) tests on cycle day 15, about 17 days before your next period is due, and keep going until you get a positive. Ovulation usually follows 24 to 36 hours later. See our ovulation test calculator.

Is a 31-day cycle normal?

A 31-day cycle is longer than average but still within the normal 21 to 35 days. Longer cycles usually mean ovulation comes later, not that something is wrong: the extra days are in the first half of your cycle.

Cycles also vary from month to month. If yours swing by several days, use the dates as a starting point and check signs of ovulation such as stretchy, clear discharge or a positive LH test. Our ovulation calculator for irregular periods works from your last few cycles instead of a single number.

31-day cycle compared with other cycle lengths

Cycle length	Likely ovulation day	Fertile window
27 days	day 13	days 8 to 14
29 days	day 15	days 10 to 16
31 days (yours)	day 17	days 12 to 18
33 days	day 19	days 14 to 20
35 days	day 21	days 16 to 22

Calendar dates are estimates and are not a method of contraception. See a doctor if your cycles are often shorter than 21 or longer than 35 days, if periods stop for 3 months, or if you've been trying to conceive for 12 months (6 months if you're 35 or older).

Frequently asked questions

What day do I ovulate on a 31-day cycle?

Most likely around day 17, counting the first day of your period as day 1. It can move a day or two either way from cycle to cycle.

When is the best time to try to conceive with a 31-day cycle?

Have sex every 1 to 2 days from about day 12 to day 18, with the most fertile days being 15 to 17.

Can I get pregnant right after my period with a 31-day cycle?

It's less likely but possible: your fertile window usually opens around day 12, and sperm can survive up to five days, so an early ovulation or a long period could overlap.

When should I take a pregnancy test with a 31-day cycle?

From the day your period is due, day 32, for an accurate result. Use our pregnancy test calculator for the exact date.

Sources

This page is for information only and isn’t medical advice. If something worries you, please talk to a doctor.

Ovulation calculator for a 31-day cycle