From the date of your last period and your cycle length: your fertile days and next period.
How it works
Ovulation usually happens about 14 days before your next period. The fertile window is the 5 days before ovulation plus the day itself, because sperm can survive up to 5 days.
It’s an estimate: cycles vary from month to month. It isn’t a reliable method of contraception.
Frequently asked questions
Does it work with an irregular cycle?
It’s less accurate. Ovulation tests or tracking basal body temperature help narrow it down.
More health and fitness calculators
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Daily calories — Basal metabolic rate and calories to maintain, lose or gain weight.
Ideal weight — Ideal weight for your height with four formulas and the healthy range.
Body fat — Body fat percentage with a tape measure (US Navy method).
Daily water — How much water to drink a day based on weight, exercise and heat.