1. Start from day one
Day one is the first day of proper bleeding in your last period — not spotting.
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Find out when your next period is due, plus your fertile window and likely ovulation day. Free, instant, no sign-up.
Quick period calculator
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Estimates only, not medical advice or contraception.
How the period calculator works
2. Add your cycle length
Your cycle runs from one period's first day to the next. 28 days is the classic average, but anywhere from 21 to 35 days is normal. If yours varies, use your average over the last few months.
3. That's your next period
Day one plus your cycle length gives your expected start date. The calculator repeats this to show your next three periods.
4. Estimating ovulation
Ovulation usually happens about 14 days before your next period. Your fertile window is the five days before ovulation plus the day itself and the day after.
Get more accurate predictions
A calculator uses one cycle. Blossom learns from every period you log, so predictions fit your own pattern — kept private in your own account.
Create your free accountPredictions are estimates, not medical advice, and not a method of contraception. Read our tips.