Period Calculator

Methodology

Each period starts a whole number of cycle lengths after the first day of the last one, and ends period length − 1 days later. The estimated ovulation day is the next period’s start minus 14 days. All dates are counted on the calendar, so a daylight-saving change cannot move them. Cycle length is a whole number from 21 to 45 days and period length from 1 to 10 days; both ranges are limits of the tool, not medical definitions.

Frequently asked questions

How do I calculate when my next period is due?

Add your cycle length to the first day of your last period. From Jan 5, 2026, a 26-day cycle gives Jan 31, a 28-day cycle gives Feb 2, a 30-day cycle gives Feb 4 and a 35-day cycle gives Feb 9, 2026. Count the cycle from the first day of one period to the first day of the next.

How is the ovulation day estimated?

As the start of the next period minus 14 days. For a period that began Jan 5, 2026 that is Jan 19 with a 28-day cycle and Jan 26 with a 35-day cycle. The 14 days are an assumption. The NHS says ovulation happens around 10 to 16 days before the next period in most women, and the US Office on Women’s Health gives a span of 7 to 19 days, so the real day can be several days either side.

Can I use these dates to avoid pregnancy?

No. The page shows one estimated day from a fixed rule and cannot tell you when you can or cannot get pregnant. The NHS notes that it is difficult to pinpoint when ovulation happens and that sperm can survive for up to 7 days. For contraception, talk to a doctor, nurse or pharmacist.

What cycle length should I enter?

The number of days from the first day of one period to the first day of the next, as a whole number from 21 to 45. If your cycles vary, the dates will be off by the difference each cycle: with a 29-day cycle instead of 28, the sixth period after Jan 5, 2026 starts on May 30 and not May 25.

Why is my period late compared with the date shown?

The calculator repeats the cycle length you entered and knows nothing about why a cycle runs long or short. The US Office on Women’s Health advises talking to a doctor or nurse if your cycles are longer than 38 days or shorter than 24 days, or if you are worried about your cycle. The calculator accepts 21 to 45 days and shows no note for 21 to 23 or 39 to 45, so compare your number with those limits yourself.

Can I see a whole year of periods or print them?

Yes. Show 12 months lists every period that starts within 365 days of the first one: whole days of 365 ÷ cycle length, plus one. That is 14 periods at 28 days (the last starting Jan 4, 2027 for a Jan 5, 2026 start), 13 at 30 days and 11 at 35 days. Print 12-month list prints only that table, and the month calendar steps up to 12 months ahead.