Period Calculator: Predict Your Next Period Dates
Enter the first day of your last period and your usual cycle length. These dates are estimates only.
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Formula
Next period = first day of last period + cycle length
Ovulation (estimate) = next period - 14 days
Later periods repeat by adding the cycle length again. This is an estimate only.
Worked example
Last period started 10 January 2026 with a 28-day cycle: next period is 7 February 2026, ovulation is about 24 January 2026, then 7 March and 4 April.
Frequently asked questions
How is the next period date worked out?
Add your cycle length to the first day of your last period. A 28-day cycle starting 10 January 2026 gives 7 February 2026.
How is ovulation estimated?
Ovulation usually happens about 14 days before the next period. In the example above, 7 February minus 14 days is 24 January. Real timing varies, so this is not a reliable guide for contraception.
What counts as the cycle length?
The number of days from the first day of one period to the first day of the next. Typical cycles run 21 to 35 days, and this calculator accepts 21 to 45.
Why was my period early or late?
Cycles vary by a few days from stress, illness, travel, sleep, weight changes, hormones or pregnancy. See a doctor if cycles are often irregular or you miss several periods.