Likely fertile window and ovulation date from your last period, by the calendar method
Runs in your browser. Nothing you type is sent anywhere. Updated on 24 Oct 2026.Ovulation Calculator puts ovulation about 14 days before your next expected period and the fertile window on the five days before it plus the day itself. For a 28-day cycle starting 1 October, ovulation is around 15 October and the window 10 to 15 October. It is a calendar estimate and not contraception.
Enter the "First day of your last period".
Set your "Usual cycle length", from the first day of one period to the first day of the next.
If your cycles vary, tick "My cycles vary" and enter the shortest and longest of your last six.
Read the estimated ovulation date, the fertile window and the most likely days; open "Next six cycles" for later months.
Press "Share result" to copy a link; the dates stay in your browser.
Enter the first day of your last period and your usual cycle length to see when you probably ovulate, your fertile window, its most likely days, and the same dates for the next six cycles.
How it is worked out. The second half of the cycle, from ovulation to the next period (the luteal phase), is the steadier half and usually lasts about 14 days. So ovulation is placed 14 days before your next expected period: day 15 of a 28-day cycle, day 19 of a 32-day cycle. If you know your luteal phase is shorter or longer, you can change the 14. The fertile window follows Wilcox, Weinberg and Baird, "Timing of sexual intercourse in relation to ovulation" (New England Journal of Medicine, 1995): conception came only from intercourse in the six days ending on the day of ovulation, and was most likely in the last three of them.
If your cycles vary. Tick "My cycles vary" and give the shortest and longest of your last six or more; the page then also shows the calendar rhythm rule taught in fertility-awareness methods: the fertile time runs from day (shortest − 18) to day (longest − 11). For cycles of 26 to 32 days, that is days 8 to 21.
The limits, plainly. This is the calendar method, and the calendar cannot see ovulation. A prospective study of 221 women (Wilcox, Dunson and Baird, "The timing of the 'fertile window' in the menstrual cycle", BMJ, 2000) found that the fertile window fell on unpredictable days even for women with regular cycles: only about 30% had it entirely within days 10 to 17. Stress, illness, breastfeeding, recent hormonal contraception and PCOS all move ovulation. Basal temperature, cervical mucus and LH tests track it as it happens.
Not contraception. No day on this page is "safe". If you want to avoid pregnancy, speak to a doctor, nurse or family-planning service about a reliable method. After a year of trying to conceive without success, or sooner if you are 35 or over, see a doctor.
This page gives general information and is not medical advice. The dates you enter stay in your browser. Once a pregnancy is confirmed, the pregnancy due date calculator dates it from the same period.
Ovulation is usually about 14 days before the next period, not a fixed number of days after the last one. In a 28-day cycle that is around day 15, counting the first day of bleeding as day 1; in a 32-day cycle around day 19; in a 24-day cycle around day 11.
About six days: the five days before ovulation and the day of ovulation itself. That comes from a 1995 study in the New England Journal of Medicine by Wilcox, Weinberg and Baird, which also found the last three of those days the most likely to lead to conception.
The result is an estimate from dates. A 2000 BMJ study by Wilcox, Dunson and Baird found the fertile window fell on unpredictable days even in regular cycles, and only about 30% of women had it entirely within days 10 to 17. Ovulation tests, basal temperature and cervical mucus track the real day more closely.
Yes, but expect a wider window. Tick 'My cycles vary' and enter your shortest and longest recent cycles; the calendar rhythm rule then puts the fertile time from day (shortest − 18) to day (longest − 11). Very irregular cycles are worth mentioning to a doctor.
No. The calendar method alone is not reliable contraception, because ovulation moves from cycle to cycle. No day this page shows is safe. Speak to a doctor, nurse or family-planning service about a method that suits you.
From the day your next period is due, shown as 'Next period' for each cycle. Testing earlier can give a negative result even if you are pregnant, because the hormone the test detects may not yet be high enough.
Formula only: no rates or limits are read from an official source.
Still stuck? Ask about Ovulation Calculator and anybody who knows can answer. No account needed.
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