Ovulation calculator: when do I ovulate?

How the ovulation calculator works

The calculator counts backwards from your next period, because that's the part of the cycle that stays most stable. After ovulation, the luteal phase usually lasts about 12 to 14 days. So it takes the first day of your last period, adds your usual cycle length to find your next period, then steps back 14 days to estimate ovulation.

1. Enter the first day of your last period (the first day of real flow, not spotting).
2. Set your usual cycle length. Not sure? Work it out from a few dates with the cycle length calculator.
3. Set how many days you usually bleed, so the calendar colors your period correctly.
4. Read your estimated ovulation date, plus your fertile window and ovulation day for the next four cycles. The sun on the mini calendar marks the estimated ovulation day.

Ovulation day by cycle length

Here's what the 14-days-before rule gives for common cycle lengths. Remember that a normal adult cycle runs from about 21 to 35 days, so day 14 is only right for a 28-day cycle.

How accurate is an ovulation calculator?

It's a good first guess, and it gets less reliable the more your cycles vary. In a 2019 study of more than 600,000 cycles tracked in an app, the average cycle was 29.3 days and the average ovulation day was day 16.9, not day 14. Only about 13% of cycles were exactly 28 days long. Earlier research from the US National Institutes of Health found that most people's fertile window doesn't fall neatly on days 10 to 17, even in regular cycles.

Ovulation can also shift from month to month. Stress, illness, travel, intense training, breastfeeding, PCOS and perimenopause can all delay it. When ovulation moves, the whole second half of your calendar moves with it.

Signs you can check alongside the calculator

• LH tests: urine strips that pick up the hormone surge 24 to 36 hours before ovulation. The most practical at-home check.
• Cervical mucus: clear, slippery, stretchy discharge (like raw egg white) usually shows up in the days before ovulation. More in ovulation discharge.
• Basal body temperature: your resting temperature rises by roughly 0.2 to 0.5°C after ovulation. It tells you ovulation has probably happened, not that it's coming.
• Ovulation pain: a one-sided twinge or ache in the lower belly that some people feel mid-cycle.
• Mood and energy: many people feel more energetic, social and confident in the days around ovulation, when estrogen peaks. See ovulation mood.

Your ovulation estimate gets personal

Lulu predicts your fertile window from your own logged cycles, and can import Apple Watch wrist temperature through Apple Health to add context. Temperature estimates are always retrospective: they suggest ovulation likely happened, they don't confirm it.

When to talk to a doctor