Ovulation Calculator: Fertile Window by Cycle Day

Ovulation calculator: your fertile window by cycle day

Short answer

Counting day 1 as the first day of your period, ovulation is usually estimated as your cycle length minus 14 days, so day 14 of a 28-day cycle. The fertile window is the 6 days ending on ovulation day: days 9–14. Real cycles vary a lot, so this is an estimate and not a form of contraception.

Last reviewed 28 October 20262 published sourcesNot medical advice

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Ovulation calculator

Day 1 is the first day of your period. If your cycles vary, add your shortest and longest to see a wider window.

Enter your usual cycle length in days.

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Estimated ovulation and fertile window by cycle length
Cycle length	Ovulation	Fertile window
24 days	Day 10	Days 5–10
26 days	Day 12	Days 7–12
28 days	Day 14	Days 9–14
30 days	Day 16	Days 11–16
32 days	Day 18	Days 13–18
35 days	Day 21	Days 16–21

Cycle length

Ovulation

Fertile window

24 days

Day 10

Days 5–10

26 days

Day 12

Days 7–12

28 days

Day 14

Days 9–14

30 days

Day 16

Days 11–16

32 days

Day 18

Days 13–18

35 days

Day 21

Days 16–21

What the research found

Wilcox, Weinberg and Baird followed 221 healthy women who were trying to conceive, using daily urine hormone tests to pin down ovulation in 625 cycles. Conception happened only when sex took place in a 6-day window ending on the day of ovulation, with the chance rising from 0.10 five days before to 0.33 on ovulation day (Wilcox, Weinberg & Baird, 1995).

Looking at when that window fell, the same team found that clinical guidelines, which put the fertile days of a 28-day cycle between days 10 and 17, matched only about 30% of women. On every day from day 6 to day 21, women had at least a 10% chance of being in their fertile window, and timing was unpredictable even in regular cycles (Wilcox, Dunson & Baird, 2000).

Making the estimate more useful

Entering your shortest and longest recent cycles widens the window to cover them. Ovulation tests or a clinician can narrow it further. If you are planning a pregnancy, see the folate intake calculator for the folic acid advice.

Common questions

When do I ovulate?

The usual estimate is your cycle length minus 14 days, the clinical rule of thumb: day 14 of a 28-day cycle, day 18 of a 32-day cycle. In a large study, though, ovulation timing varied widely even in women with regular cycles.

How long is the fertile window?

About 6 days. In a study of 221 women trying to conceive, pregnancies came only from sex during the six days ending on ovulation day; the chance per day rose from about 10% five days before to about 33% on ovulation day.

Can I use this to avoid pregnancy?

No. It is not contraception. Only about 30% of women in the same study had their fertile window entirely within the days the standard rule predicts, and every day from day 6 to day 21 carried at least a 10% chance of being fertile. Talk to a clinician about contraception.

Sources

Figures on this page were checked against these publications on 28 October 2026.

Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. Effects on the probability of conception, survival of the pregnancy, and sex of the baby. New England Journal of Medicine. 1995;333(23):1517–1521. doi.org/10.1056/NEJM199512073332301

Wilcox AJ, Dunson D, Baird DD. The timing of the "fertile window" in the menstrual cycle: day specific estimates from a prospective study. BMJ. 2000;321(7271):1259–1262. doi.org/10.1136/bmj.321.7271.1259

Ovulation calculator: your fertile window by cycle day