Ovulation calculator
Ovulation does not happen on day 14 of every cycle. The steadier half is the luteal phase, the stretch from ovulation to the next period, so counting backwards from the expected period beats counting forwards from the last one. Steadier is not fixed, though. Across 612,613 tracked ovulatory cycles the mean luteal phase was 12.4 days with a 95% range of 7 to 17, so the 14 in the field is an average with a fortnight of spread behind it.
A calendar estimate only, and a weak one. In a prospective study of 221 women, the fertile window fell entirely within the textbook days 10 to 17 for only about 30% of them. This is not a contraceptive method and should not be used as one. Ovulation predictor kits and basal body temperature tracking are considerably more reliable.
Ovulation is estimated as one luteal phase before the next period is due, which is cycle length minus about 14 days: day 14 of a 28-day cycle, nearer day 21 of a 35-day one. The fertile window is the six days ending on the day of ovulation. It closes there rather than a day later, because that is where the evidence puts it.
How to find your fertile window
The shape of the fertile window comes from one well-known cohort. Wilcox, Weinberg and Baird followed 221 women trying to conceive, measured urinary oestrogen and progesterone metabolites to pin the day of ovulation, and looked at 625 cycles. Conception followed only from intercourse in a six-day period ending on the estimated day of ovulation. The chance rose across that window, from about one in ten five days out to one in three on the day itself. Nothing after ovulation produced a pregnancy in that cohort, which is why this panel closes the window on the ovulation date rather than the day after it.
The asymmetry has a simple cause. Sperm survive for up to five days in the reproductive tract; an egg is viable for roughly a day. So the window opens long before the event and shuts almost as it happens, and the practical advice that follows is to aim at the days before ovulation rather than at ovulation itself.
The harder finding is what the same group published five years later, and it is the reason this page carries a warning instead of a plan. Using the same cohort, they showed that the fertile window fell entirely inside the textbook days 10 to 17 for only about 30% of women. On every single day between day 6 and day 21 some women were fertile, with a probability of at least 10%. Among women whose period had not yet arrived, 4 to 6% were still potentially fertile in the fifth week of the cycle, because ovulation can simply run late. Their conclusion was that the timing of the fertile window is highly unpredictable even for women whose cycles are usually regular, and a calendar cannot see any of it.
The luteal phase does most of the work in this calculation and it is steadier than the follicular phase, but it is not the constant the folk version claims. In a dataset of 612,613 ovulatory cycles from 124,648 app users, mean cycle length was 29.3 days rather than 28, the mean follicular phase was 16.9 days with a 95% range of 10 to 30, and the mean luteal phase was 12.4 days with a 95% range of 7 to 17. Cycle length also shortens with age, by about 0.18 days a year between 25 and 45. Entering a measured luteal phase instead of the default 14 is the single change that improves this estimate most.
What people use it for
- Planning around a fertile window
- Predicting the next period and the one after it
- Seeing how much a longer or shorter cycle moves ovulation
- Working out a due date if conception happens this cycle
- Deciding when to start using ovulation predictor sticks
Questions
About one luteal phase before your next period, not 14 days after the last one. On a 35-day cycle with a 14-day luteal phase that is day 21.
Six days: the five before ovulation plus the day of ovulation itself. Wilcox and colleagues recorded no conceptions from intercourse after that day in 625 cycles.
The chance climbs through it. In that study it ran from roughly 0.10 five days before ovulation to 0.33 on the day itself, so the two or three days immediately before ovulation carry most of it.
No. Calendar methods have a high failure rate in typical use, and the same research that defines the window shows the window itself moves far more than a calendar can track.
Less often than the tidy arithmetic suggests. In the Wilcox cohort the fertile window lay entirely within days 10 to 17 for only about 30% of women. Every day from 6 to 21 carried at least a 10% chance of being fertile for someone.
The stretch from ovulation to the next period. It is the steadier half of the cycle, but across 612,613 tracked cycles it averaged 12.4 days with a 95% range of 7 to 17.
Yes, if you know yours. A basal temperature chart or a run of positive ovulation tests will give it. The default is a population average, and it is the assumption this whole estimate rests on.
It is the textbook figure rather than the common one. The mean in that large tracking dataset was 29.3 days, and individual cycles vary from month to month even in women who describe themselves as regular.
Because ovulation timing shifts between cycles. Irregular cycles make calendar prediction close to worthless; ovulation tests or temperature tracking work far better.
An ovulation predictor kit picks up the luteinising hormone surge and gives roughly a day and a half of warning. Basal body temperature confirms afterwards that ovulation happened at all. Used together they bracket the event from both sides.
Gradually. In the tracking dataset mean cycle length fell by about 0.18 days per year of age between 25 and 45, almost all of it from a shortening follicular phase.