Ovulation calculator
When do you ovulate, and which are your fertile days? Calculate from your last period, see a 3-cycle calendar and save your cycles.
Enter the first day of your last period and your fertile days will appear here.
Don't use this calculation to avoid pregnancy: ovulation can shift from cycle to cycle.
Based on the NHS page on periods and fertility and the 1995 and 1999 NEJM studies by Wilcox and colleagues. Open the NHS page
This tool is for information only. It is not a diagnostic tool and does not replace a medical examination.
About this tool
What does this tool do?
From the first day of your last period and your cycle length it works out your most likely ovulation day, your fertile window, your next period and the next three cycles. It is based on three sources. According to the NHS, most women ovulate around 10 to 16 days before their next period. The 1995 study by Wilcox and colleagues in the New England Journal of Medicine found that pregnancy almost only follows sex during the 6-day period ending on the day of ovulation. The same team's 1999 study found that in most pregnancies the fertilised egg implants 8 to 10 days after ovulation. When signed in you can save the start of each period; the tool works out your average cycle from your entries and shows your next fertile window in your dashboard.
How do I use it?
If your cycle is regular, enter the first day of your last period and choose your cycle length; a cycle is the number of days from the first day of one period to the first day of the next. If your cycle changes from month to month, choose “Irregular cycle” and enter your shortest and longest cycle of recent months; the tool then shows a wider possible range instead of a single day. The result updates as you type.
How should I read the result?
The fertile window is the 6 days when pregnancy is possible; the chance rises towards ovulation day (in Wilcox's data about 10 percent for sex 5 days before ovulation and about 33 percent on the day of ovulation). In the day strip, light days are your fertile window, the dark day is your most likely ovulation day and the outlined day is today. You can take a pregnancy test from the first day of a missed period; most tests work from that day.
Limits of this tool
A calendar calculation is an estimate; ovulation can shift from cycle to cycle even in the same woman. The tool should therefore not be used as a method of contraception. Ovulation tests or medical monitoring show ovulation more precisely. The tool does not help choose the baby's sex: Wilcox's 1995 study showed that the timing of sex in relation to ovulation does not influence the baby's sex. If you have been trying for a long time without getting pregnant, or your periods are very irregular, talk to your doctor.
