OneTaskBy Tim King · Updated
Period Calculator
Six Upcoming Cycles, Ovulation Day and Fertile Window From Cycle Length
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How the Period Calculator works
This period calculator predicts when your next periods are due from the first day of your last period, your usual cycle length and how many days your period lasts. It lists the next six periods and shows when you are likely to ovulate and be most fertile.
How the period calculator works
A menstrual cycle runs from the first day of one period to the day before the next. The average is about 28 days, but anything from 21 to 35 days is normal for adults. The formula is next period = first day of last period + cycle length, repeated for each later cycle. Ovulation usually happens about 14 days before the next period starts, so it is shown as the next period date minus 14 days, with a fertile window of five days before ovulation and the day after.
First example. A period starting on 20 September 2026 with a 28-day cycle and 5-day period means the next one is due 18 to 22 October 2026, and there are about 13 periods a year.
Second example. With a last period on 10 September 2026, a 32-day cycle and 4-day period, the next is due 12 to 15 October. Ovulation is likely around 30 October and the fertile window runs from 25 to 31 October.
How many periods are there in a year? 365 ÷ cycle length, so 13 with a 28-day cycle, about 11 with a 32-day cycle and 15 with a 24-day cycle.
Cycles vary with stress, illness, travel and contraception, so treat the dates as a guide. Track a few cycles and use your average length for better predictions.
Ovulation day, fertile window and periods per year by cycle length
| Cycle length | Likely ovulation | Fertile days | Periods per year | Range |
|---|---|---|---|---|
| 21 days | Day 7 | Days 2 to 8 | 17.4 | Outside the usual 24 to 38 |
| 22 days | Day 8 | Days 3 to 9 | 16.6 | Outside the usual 24 to 38 |
| 23 days | Day 9 | Days 4 to 10 | 15.9 | Outside the usual 24 to 38 |
| 24 days | Day 10 | Days 5 to 11 | 15.2 | Usual |
| 25 days | Day 11 | Days 6 to 12 | 14.6 | Usual |
| 26 days | Day 12 | Days 7 to 13 | 14.0 | Usual |
| 27 days | Day 13 | Days 8 to 14 | 13.5 | Usual |
| 28 days | Day 14 | Days 9 to 15 | 13.0 | Usual |
| 29 days | Day 15 | Days 10 to 16 | 12.6 | Usual |
| 30 days | Day 16 | Days 11 to 17 | 12.2 | Usual |
| 31 days | Day 17 | Days 12 to 18 | 11.8 | Usual |
| 32 days | Day 18 | Days 13 to 19 | 11.4 | Usual |
| 33 days | Day 19 | Days 14 to 20 | 11.1 | Usual |
| 34 days | Day 20 | Days 15 to 21 | 10.7 | Usual |
| 35 days | Day 21 | Days 16 to 22 | 10.4 | Usual |
| 36 days | Day 22 | Days 17 to 23 | 10.1 | Usual |
| 37 days | Day 23 | Days 18 to 24 | 9.9 | Usual |
| 38 days | Day 24 | Days 19 to 25 | 9.6 | Usual |
| 39 days | Day 25 | Days 20 to 26 | 9.4 | Outside the usual 24 to 38 |
| 40 days | Day 26 | Days 21 to 27 | 9.1 | Outside the usual 24 to 38 |
| 41 days | Day 27 | Days 22 to 28 | 8.9 | Outside the usual 24 to 38 |
| 42 days | Day 28 | Days 23 to 29 | 8.7 | Outside the usual 24 to 38 |
| 43 days | Day 29 | Days 24 to 30 | 8.5 | Outside the usual 24 to 38 |
| 44 days | Day 30 | Days 25 to 31 | 8.3 | Outside the usual 24 to 38 |
| 45 days | Day 31 | Days 26 to 32 | 8.1 | Outside the usual 24 to 38 |
FAQ
How do I work out my cycle length?
Count from the first day of one period to the day before the next one starts. Do this for three to six cycles and take the average, as most people's cycles vary by a few days from month to month.
Is a 35-day cycle normal?
Yes. Adult cycles between 21 and 35 days are considered normal, and teenagers' cycles can be up to 45 days. See a doctor if your cycles are regularly shorter than 21 days or longer than 35.
Why is my period late?
Stress, illness, travel, weight change, heavy exercise, breastfeeding, some medicines and pregnancy can all delay a period. If there is any chance of pregnancy, a home test is the quickest way to check.
Can I use this to avoid pregnancy?
No. Calendar predictions are not reliable contraception because ovulation can shift from cycle to cycle. Sperm also survives several days, so this tool should only be used as a rough planning guide.
Does period length affect my next period date?
No. The next period date depends only on cycle length. Period length just sets how many days each predicted period is likely to last, which helps when planning trips or events around those dates.
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