Period Predictor
Predict the start of your next period from the first day of your last period and your usual cycle length. A menstrual cycle is counted from the first day of one period to the first day of the next, so the calculator simply projects one cycle forward — and shows the next few cycles too, which is handy for planning ahead. It is most accurate for regular cycles; irregular cycles vary from month to month.
Calculate
Default result: Jan 29, 2026
Period Predictor · Result
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Next period
2026-01-01 × 28
This calculator provides an estimate for general information only and is not medical advice. Due dates, gestational age, and fertility windows are estimates — babies rarely arrive exactly on the estimated due date, and individual cycles and pregnancies vary. Always confirm dates and any health decisions with your healthcare provider or OB-GYN.
Reviewed by the calculators.dev team · Last updated 2026-06-24
Formula reviewed against Menstrual cycle length and variation (ACOG; reproductive health references)
How to calculate
Enter the first day of your last period and your average cycle length. The calculator adds one cycle to find your next period, then projects two more cycles so you can see the pattern over the coming months.
next period = first day of last period + cycle length. Each following period adds another full cycle, so the third upcoming period is the last period plus three times the cycle length. A 28-day cycle starting January 1 gives January 29, February 26, and March 26.
Example calculation
From a last period on January 1, 2026 with a 28-day cycle, the next period is expected one cycle later, on January 29, 2026. Projecting forward, the following periods fall on February 26 and March 26, 2026.
- nextPeriodDay
- January 29, 2026
Assumptions
- A cycle is measured from the first day of one period to the first day of the next, so the next period is simply one cycle length after the last.
- The projection assumes your cycle length stays the same each month. Real cycles vary, especially if they are irregular.
- Cycle length must be between 20 and 45 days; values outside that range are reported as an error rather than a misleading date.
- Stress, illness, travel, and other factors can shift a period earlier or later than the prediction.
- A late period is not predicted here — if a period is overdue and pregnancy is possible, take a test or speak with your provider.
Common mistakes
- Counting the cycle from the end of a period instead of the first day. A cycle is measured from the first day of one period to the first day of the next.
- Expecting the prediction to be exact for irregular cycles. The estimate assumes a steady cycle length and will drift if yours varies.
- Treating a missed predicted period as a problem on its own. Many things shift a cycle; if you might be pregnant, test or consult your provider.
Frequently asked questions
When is my next period?
It is one cycle length after the first day of your last period. For a 28-day cycle that started January 1, the next period is expected on January 29.
How do I count my cycle length?
Count from the first day of one period to the first day of the next. The average over a few months is the most reliable figure to enter.
Why might my period not arrive on the predicted day?
Cycle length naturally varies, and stress, illness, travel, and other factors can shift it. The prediction assumes a steady cycle, so treat it as a guide.
Can a missed period mean I am pregnant?
It can, but many other things also delay a period. If your period is late and pregnancy is possible, take a pregnancy test or speak with your provider.