Period Calculator — Next Periods & Cycle Regularity Check
Predict your next six periods from your last period, cycle length and period length, and check your recent cycles against the FIGO definitions of a normal menstrual cycle.
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Your Next 6 Periods
Estimated start and end dates if your cycle stays at the same length.
| # | Starts | Ends | In |
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How this calculator works
- Next period = first day of last period + cycle length; each following period adds another cycle length.
- Period end = start + period length − 1 day.
- Ovulation ≈ 14 days before the next period starts.
The regularity check uses the International Federation of Gynecology and Obstetrics (FIGO) 2018 parameters for normal menstruation in non-pregnant adults: a cycle every 24–38 days, bleeding for up to 8 days, and a difference between shortest and longest cycles of no more than 7–9 days (up to 7 days for ages 26–41, up to 9 days for ages 18–25 and 42–45).
By Suhaib Hassan · Reviewed 28 September 2026 against FIGO and ACOG guidance · How CalculatePilot verifies formulas →
A worked example
A last period starting 1 July with a 29-day cycle: the next period is estimated around 30 July, then 28 August. With a 5-day period, the first runs 30 July – 3 August. If the last three cycles were 27, 29 and 33 days, the variation is 6 days — within FIGO's normal limit.
When to talk to a doctor
Cycles often vary by a few days, especially in the first years after periods start and in the years before menopause. It's worth seeing a healthcare provider if your periods come less than 24 or more than 38 days apart, last more than 8 days, are very heavy, stop for 3 months or more when you're not pregnant, or suddenly become irregular. Tracking your start dates over several months makes these conversations easier.
Sources
- Munro MG, Critchley HOD, Fraser IS; FIGO Menstrual Disorders Committee. The two FIGO systems for normal and abnormal uterine bleeding symptoms and classification of causes of abnormal uterine bleeding in the reproductive years: 2018 revisions. Int J Gynaecol Obstet 2018;143:393–408.
- ACOG — Abnormal Uterine Bleeding (FAQ)
Frequently asked questions
How long is a normal menstrual cycle?
Under FIGO's 2018 definitions, a normal adult cycle is 24 to 38 days long, counted from the first day of one period to the first day of the next.
Why did my period come early or late?
Stress, illness, travel, weight changes, intense exercise and some medicines can shift ovulation, which moves your next period. A single early or late period is common; a persistent change is worth discussing with a doctor. If you could be pregnant, take a pregnancy test.
How accurate are period predictions?
They're most accurate for regular cycles. Even then, a few days either way is normal. Using your average from several recent cycles improves the estimate.
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