Ovulation Calculator
Enter the first day of your last period and your cycle length to see your estimated ovulation day, fertile window and next expected period.
The time from the first day of one period to the first day of the next. Leave 28 if unsure.
What You Should Know About Ovulation Calculation
How is the ovulation day calculated?
The calculation rests on the fact that the interval between ovulation and the next period varies little between women — in most it is about 14 days. Ovulation is therefore estimated by counting back 14 days from the expected period.
The time from the start of a period to ovulation, by contrast, is variable. In a woman with a 35-day cycle ovulation occurs around day 21 rather than day 14, which is why entering your cycle length correctly matters.
Why does the fertile window last several days?
Sperm can survive for several days in the female reproductive tract, while an egg can only be fertilised for a short time after release. The overlap of these two periods is the fertile window.
Pregnancy can therefore occur when intercourse takes place a few days before ovulation. The window opens a few days before ovulation and closes shortly after.
Can this tool be used for contraception?
It cannot. The calendar method is not a reliable form of contraception. Ovulation can happen earlier or later than expected because of illness, stress, sleep disruption, travel or weight change.
If you wish to prevent pregnancy, a method of proven effectiveness should be chosen. Suitability is assessed at consultation.
If your cycles are irregular
If your cycle length varies noticeably from month to month, calendar calculation is not reliable. Polycystic ovary syndrome, thyroid dysfunction, significant weight loss or intense exercise can all disturb ovulation.
In such cases whether and when ovulation occurs is established more reliably with medical follow-up.
What are the signs of ovulation?
Some women notice vaginal discharge becoming clear and stretchy, mild pelvic discomfort, breast tenderness or increased libido around ovulation.
These signs do not occur in everyone and do not on their own prove that ovulation has happened. Urine LH tests or ultrasound follow-up give more definite information.
When should you see a doctor?
If you are under 35 and pregnancy has not occurred after a year of regular unprotected intercourse, or after six months if you are over 35, assessment is advised.
If your periods are very irregular or absent, or you have a known gynaecological condition, it is better not to wait for these intervals.
Important Warning: This tool is for information only, gives an estimate and does not replace a medical examination. It cannot be used as a method of contraception.
Frequently Asked Questions
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