How is the next period worked out?
Expected date = first day of your last period + your average cycle length. For example, if your last period started on 1 September 2026, your cycle is 28 days and your period lasts 5 days, your next period is 29 September to 3 October. Ovulation comes about 14 days before it (15 September), and your fertile days are 10 to 15 September.
If your cycle is irregular
Enter the start dates of your last 2 or 3 periods and the calculator works out your average and shows the next date as a range. Example: periods that started on 5 July, 2 August and 1 September are 28 and 30 days apart, an average of 29 days, so the next period is due around 30 September and may come between 29 September and 1 October. If your shortest and longest cycles differ by more than 9 days, the calculator flags your cycle as irregular.
What is normal?
| Item | Normal for adults |
|---|---|
| Cycle length | 21 to 35 days (average 28) |
| Bleeding | 3 to 7 days |
| Variation between cycles | Up to 9 days between the shortest and longest |
| First years after periods start | Irregular cycles are common; they can take up to 3 years to settle |
Is your period late?
- A few days late can happen with stress, travel, weight changes, intense exercise or breastfeeding.
- If you could be pregnant: a home pregnancy test is most accurate from the first day of a missed period. If it is negative and your period still hasn’t come, test again a few days later.
- See a doctor if you have had no period for 90 days or more (and are not pregnant or breastfeeding), if periods come less than 21 or more than 35 days apart, if bleeding lasts more than 7 days, if it is very heavy (soaking a pad every hour or two), if you bleed between periods, if the pain is severe enough to stop your day, or if regular periods suddenly become irregular.
Periods and Ramadan
A woman on her period does not fast in Ramadan and makes up the days afterwards, by agreement of scholars. The calculator checks whether any of your next six expected periods falls in Ramadan (Umm al-Qura calendar) and shows roughly how many days. For example, if your last period started on 1 January 2027, your cycle is 28 days and your period lasts 6 days, the expected period from 26 February to 3 March puts about 6 days in Ramadan 1448. After Ramadan, count your days with the missed fasts calculator.
Frequently Asked Questions
Do I count from the first or the last day of bleeding?
Always from the first day of bleeding. Cycle length means the number of days from the first day of one period to the first day of the next, not the number of days you bleed.
Can I use this to avoid pregnancy?
No. Ovulation can shift in any cycle, so the calendar method is not reliable contraception. If you are trying to conceive, see the ovulation calculator.
Are my dates saved or sent anywhere?
No. Everything is calculated in your browser on your device; no date is sent or stored, even after you close the page.
Why does my period come a few days early or late?
That is normal. Cycle length varies a little from month to month, which is why the calculator shows a range when you add earlier dates.
Sources
- Cleveland Clinic: Menstrual Cycle and Irregular Periods.
- Mayo Clinic: Home pregnancy tests.