Ovulation calculator: your fertile window, dated honestly

Estimated fertile window from a last period of Thu, Jan 1, 2026worked example

Sat, Jan 10, 2026 - Fri, Jan 16, 2026

Estimated ovulation: Thu, Jan 15, 2026cycle day 14
Six-day conception window: Sat, Jan 10, 2026 to Thu, Jan 15, 2026where Wilcox 1995 traced conceptions
Highest per-day probability: Tue, Jan 13, 2026 to Thu, Jan 15, 2026about 0.33 on the day of ovulation itself
Days in the displayed window: 7 days

Ovulation is estimated at cycle day 14 because it is counted back from your next expected period, not forward from your last one. The window shown is ACOG’s: five days before ovulation through one day after it. None of this is a measurement - a calendar cannot see ovulation.

Realistic range

Mon, Jan 12, 2026 to Thu, Jan 22, 2026

Where ovulation could plausibly fall in this cycle. Bull and colleagues measured a mean luteal phase of 12.4 days with a 95% interval of 7 to 17 days across 612,613 cycles, and 18% of those cycles had a luteal phase under 11 days. A single confident date would be hiding this.

First day of your last periodDay 1 is the first day of full bleeding, not spotting.Average cycle length (days)First day of one period to the first day of the next. Only 13% of tracked cycles were exactly 28 days.

Everything you type here is computed in your browser. Nothing you enter is stored on a server, sent anywhere, or shared — there is no account and no health data to leak.

The next three cycles on the same arithmetic
Period starts	Fertile window	Estimated ovulation
Thu, Jan 29, 2026	Sat, Feb 7, 2026 - Fri, Feb 13, 2026	Thu, Feb 12, 2026
Thu, Feb 26, 2026	Sat, Mar 7, 2026 - Fri, Mar 13, 2026	Thu, Mar 12, 2026
Thu, Mar 26, 2026	Sat, Apr 4, 2026 - Fri, Apr 10, 2026	Thu, Apr 9, 2026

Each row applies the same assumed cycle length again, so the third row carries three cycles' worth of the same assumption. Treat later rows as looser than earlier ones.

This is not a contraceptive method. Fertility awareness methods that work rely on daily observation - basal temperature, cervical mucus or urine hormone tests - plus rules for what to do with the readings. Calendar arithmetic on its own is not one of those methods, and this page is not offering itself as one.

The sources this defers to

ACOG: Fertility Awareness-Based Methods of Family Planning — ovulation about 14 days before the next period; pregnancy possible from 5 days before to 1 day after
NHS: periods and fertility in the menstrual cycle — the NHS puts ovulation 10 to 16 days before the next period, and sperm survival at up to 7 days

Next: the implantation window after the date above, or the next six period dates from the same two inputs.

This ovulation calculator counts back from your next expected period rather than forward from your last one, because the first half of the cycle is what varies. It dates estimated ovulation, ACOG's fertile window of five days before to one day after, and the six-day conception window Wilcox measured in 1995.

Estimated ovulation day, anchored to the luteal phaseconvention — ovulation day of cycle = cycle length - 14. Verified 2026-07-30 against ACOG, Fertility Awareness-Based Methods of Family Planning (patient FAQ) (Counted back from the next expected period rather than forward from the last one, because the follicular phase is what varies. The NHS puts ovulation at 10 to 16 days before the next period, and Bull 2019 measured a mean luteal phase of 12.4 days with a 95% interval of 7 to 17 days, so this is a midpoint of a week-wide window.)
Fertile window (ACOG patient guidance) — fertile window = ovulation - 5 days through ovulation + 1 day. Verified 2026-07-30 against ACOG, Fertility Awareness-Based Methods of Family Planning (patient FAQ) (The window is drawn around an estimated ovulation date, so it is only as good as that estimate. Widening it is the honest response to an uncertain ovulation date; narrowing it is not.)
The six-day conception window — conceptions occurred only from intercourse in the 6 days ending on the day of ovulation. Verified 2026-07-30 against Wilcox, Weinberg and Baird, N Engl J Med 1995 (221 women, 625 cycles. Per-day probability of conception ranged from about 0.10 five days before ovulation to about 0.33 on the day of ovulation itself, so days inside the window are not equivalent to each other.)
Real-world cycle and luteal phase variation — mean cycle 29.3 days; 13% of cycles exactly 28 days; mean luteal phase 12.4 days (95% CI 7-17). Verified 2026-07-30 against Bull et al., npj Digital Medicine 2019 (612,613 cycles from 124,648 users of a fertility app, so the population self-selects towards people tracking closely. The direction of the finding -- that the textbook 28-day cycle is a minority -- is not in doubt.)

Everything you type here is computed in your browser. Nothing you enter is stored on a server, sent anywhere, or shared — there is no account and no health data to leak.

Frequently asked questions

When does ovulation happen in a cycle?

ACOG describes ovulation as happening about 14 days before the start of the next period, so in a 32-day cycle that is day 18, not day 14. The NHS puts it at 10 to 16 days before the next period, which is the width of the real window.

How many days are you fertile?

ACOG states pregnancy can result from sex anywhere from five days before ovulation until one day after it. Wilcox 1995 found conceptions only from intercourse in the six days ending on the day of ovulation itself, with the per-day probability rising from about 0.10 to about 0.33.

Why does this calculator show a range instead of one day?

Because a calendar cannot see ovulation. Wilcox dated it with daily urine hormone measurements; a calculator only knows the numbers you typed. Bull 2019 measured a mean luteal phase of 12.4 days with a 95 percent interval of 7 to 17, so the estimate is a week-wide window.

Can I use this as contraception?

No. Fertility awareness methods that work require daily observation of temperature, cervical mucus or hormone tests, plus training and rules for what to do with the readings. Calendar arithmetic on its own is not one of those methods.

What is the difference between the fertile window and the six-day window?

The fertile window shown here is ACOG's patient-facing one, five days before to one day after ovulation. The six-day window is where Wilcox actually traced conceptions: five days before through the day of ovulation. The extra day exists for uncertainty, not because a conception was recorded there.

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The next number

No body metric means much on its own. Here is what this one feeds into.

Implantation calculatorThe window after ovulation: Wilcox 1999 dated implantation 6 to 12 days out, 84 percent of it on days 8 to 10.Due date calculatorAn ovulation date is a conception date for dating purposes - add 266 days for the due date it implies.

Related calculators

Period calculatorCycle and ovulation Implantation calculatorEarly pregnancy Conception date calculatorEarly pregnancy

Every formula on this page is listed with its source on the methodology page, and changes to any reference value are logged in the data changelog. Canonical URL: https://myhealthmywellness.com/ovulation-calculator