Your next period is due on the first day of your last period plus your cycle length. If your last period started on 1 September 2026 and your cycle is 30 days, the next one is due on 1 October 2026 and the one after on 31 October 2026.
Enter your last period, your average cycle length and how many days your period usually lasts. The calculator lists your next periods for 3, 6 or 12 cycles, with the likely fertile days in each cycle.
How to calculate your next period
- Write down the first day of bleeding of your last period. This is cycle day 1. Spotting before a period does not count.
- Work out your cycle length: the number of days from the first day of one period to the first day of the next. If you have noted a few, take the average.
- Add the cycle length to the first day of your last period. Keep adding it for later months.
Worked example
Last period started on 1 September 2026, cycle 30 days, period lasts 5 days.
| Cycle | Period starts | Period ends |
|---|---|---|
| Last period | 1 September 2026 | 5 September 2026 |
| Next | 1 October 2026 | 5 October 2026 |
| Then | 31 October 2026 | 4 November 2026 |
| Then | 30 November 2026 | 4 December 2026 |
The dates drift through the month because a 30-day cycle is shorter than most calendar months.
What is a normal cycle
ACOG treats a cycle of 21 to 35 days as usual for adults, and 21 to 45 days in the first few years after periods start. The NHS says bleeding usually lasts 2 to 7 days. It is normal for your cycle to vary by a few days from month to month.
The calculator warns you if your cycle is outside 21 to 35 days. Predictions for very short or long cycles are less reliable.
When to see a doctor
See a doctor if:
- your periods stop, or you miss periods and a pregnancy test is negative;
- your cycle is regularly shorter than 21 days or longer than 35 days;
- bleeding is very heavy, lasts more than 7 days, or you pass large clots;
- you bleed between periods, after sex, or after the menopause;
- period pain stops you from going about your day.
If your period is late and you could be pregnant, take a home pregnancy test. If it is positive, the pregnancy calculator shows your due date and how far along you are.
Periods and fertile days
Each cycle in the result also shows the likely fertile window, the 5 days before ovulation plus ovulation day. For more detail, including ovulation dates and how to confirm them with test strips, use the ovulation calculator. Calendar dates alone are not reliable contraception.
This calculator gives an estimate from a published formula. It does not diagnose any condition and is not a substitute for a doctor. Talk to a doctor before making decisions about your health, diet or treatment.
Period questions
How do I calculate my period cycle?
Count the days from the first day of one period to the day before the next period starts, including both ends of that count. For example, if one period starts on 1 September and the next on 29 September, your cycle is 28 days. Track three to six cycles and use the average for the best prediction.
How do I calculate my next period date?
Add your cycle length to the first day of your last period. With a last period on 10 September 2026 and a 28-day cycle, the next is due on 8 October 2026. For a later period, add the cycle length again. The calculator does this for up to 12 cycles.
Is a 35-day cycle normal?
Yes. For adults, ACOG counts 21 to 35 days as the usual range, so a steady 35-day cycle is at the long end of normal. If your cycles are often longer than 35 days, or suddenly change, mention it to a doctor. A long cycle also means you ovulate later, around day 21.
Why is my period late?
The most common reason in a sexually active woman is pregnancy, so take a test first. Stress, illness, travel, weight change, heavy exercise, breastfeeding and some medicines can also delay a period. If you miss three periods in a row and are not pregnant, see a doctor.
Can I use a period calculator with irregular periods?
It gives only a rough guide. If your cycle length varies by more than 7 to 9 days, the calculator cannot tell which length applies this time. Keep a record of each start date for a few months and show it to a doctor, who can check the cause.
Related calculators
Last reviewed 30 September 2026. Results are estimates; official notices always take precedence.