For cycles that move around
Irregular period calculator
Periods are usually called irregular when cycle lengths differ by more than 7 to 9 days. A single predicted date won’t fit that, so this calculator uses your last few periods to give ranges wide enough to be useful.
- No sign-up, no email needed
- Nothing you enter is stored
- Ranges that fit real, changing cycles
Your next periods
Enter your dates above and your estimates will show here. Nothing you enter leaves this page.
An estimate, not medical advice or birth control. Don’t use these dates to avoid pregnancy.
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How it handles irregular cycles
Your own pattern, not a textbook one.
We average your three most recent cycles to find the centre of each estimate. Then we widen the range to cover your shortest and longest recent cycles, so a 26-day month and a 35-day month both fit inside it.
When to talk to a doctor
Some variation is normal. See a doctor if your periods suddenly become irregular, come less than 21 days apart, are more than 3 months apart, last longer than 7 days, or if you’re trying to get pregnant and your cycle is hard to predict (NHS).
Why one date doesn’t fit an irregular cycle
If your cycle moves between, say, 26 and 35 days, ovulation and your next period move by over a week too.
| Cycle length | Likely ovulation | Fertile window | Next period starts |
|---|---|---|---|
| 21 days | day 7 | days 2–7 | day 22 |
| 24 days | day 10 | days 5–10 | day 25 |
| 26 days | day 12 | days 7–12 | day 27 |
| 28 days | day 14 | days 9–14 | day 29 |
| 30 days | day 16 | days 11–16 | day 31 |
| 32 days | day 18 | days 13–18 | day 33 |
| 35 days | day 21 | days 16–21 | day 36 |
Days are counted from the first day of your last period (day 1). Real cycles move, so treat each as the middle of a range.
Sources: NHS: Irregular periods · ACOG: Abnormal uterine bleeding
What counts as an irregular period?
Usually cycles that differ by more than about 7 to 9 days from one to the next, or that are shorter than 21 days or longer than 35 in adults. Irregular cycles are common in the first years after periods start and before menopause.
Can you predict an irregular period?
Not to the day, but you can get a useful range. Using the start dates of your last few periods shows how much your cycle moves, and a range that covers that movement is far more reliable than a single date.
How do I know when I ovulate with irregular periods?
A calendar estimate is less reliable when cycles vary, so the fertile window we show is wider. Ovulation tests, cervical mucus changes and basal body temperature can tell you when it’s happening this cycle.
What causes irregular periods?
Common causes include stress, big changes in weight or exercise, starting or stopping hormonal contraception, breastfeeding, PCOS, thyroid problems and perimenopause. A doctor can help find the cause if it’s new or bothering you.
Is my data saved anywhere?
No. The calculator runs in your browser. The dates you enter are never sent to us, never put in the page address and never stored. Close the tab and they’re gone.
Can I use this as birth control?
No. These are estimates from a calendar, and ovulation can move from cycle to cycle. Don’t use them to avoid pregnancy. If you need contraception, talk to a doctor, nurse or pharmacist about methods that are tested for it.