Period predictor
See your next 3 period start dates. Calendar math, free, runs in your browser. Honest about where the prediction is reliable and where it is not.
Tell us about your cycle
30 seconds. We do not store anything on a server.
How the prediction works
The math is simple. Last period start plus your typical cycle length equals next period start. Plus another cycle length equals the period after that. The calculator shows the next 3 dates so you can plan ahead.
What makes the prediction useful or not is how stable your cycle length is. The math assumes you keep cycling at the same length. Real cycles drift by a few days even for women considered regular.
How accurate calendar prediction is
- Cycles varying less than 3 days: prediction is typically within 1 to 2 days of actual.
- Cycles varying 3 to 5 days: prediction is within 2 to 4 days of actual.
- Cycles varying 5 to 7 days: prediction can be off by a week. Use as a rough planning window only.
- Cycles varying more than 7 days: calendar prediction is not reliable. Track 3 to 6 cycles to find your actual range, or consult a clinician about the underlying cause of variability.
When the prediction is wrong, what to think about
Several factors shift cycle timing in ways the calendar cannot see:
- Stress. Cortisol can delay ovulation, which delays the period.
- Sleep disruption or travel. Shift work and significant time zone changes can shift cycles by days.
- Significant weight change. Both gains and losses, especially over short periods, affect ovulation timing.
- Intense exercise. Especially with low energy availability, can delay or skip ovulation entirely.
- Illness or medication change. Acute illness, antibiotics, and many medications can affect a single cycle.
- Post-pill recovery. Cycles often take 3 to 12 months to fully regularize after stopping hormonal birth control.
- Perimenopause. Starting in the late 30s to mid 40s, cycle length variability increases as ovarian reserve declines. Predictions become less reliable.
- PCOS or PMOS. The condition formerly called PCOS, now PMOS as of May 2026, often involves anovulatory or irregular cycles. Calendar prediction is not reliable. See PCOS renamed to PMOS for context.
Frequently asked questions
How does the period predictor work?
It adds your cycle length to your last period start to estimate the next period. It then adds another cycle length to estimate the period after that, and so on. The math is straightforward; the accuracy depends entirely on how consistent your cycle length actually is.
How accurate is calendar-based period prediction?
For women with regular cycles (varying by less than 5 days month to month), calendar prediction is accurate within 2 to 3 days. For more variable cycles, accuracy drops proportionally. Cycles often vary more than people remember, so treat predictions as a window, not a fixed date.
What makes a cycle 'regular' or 'irregular'?
A typical regular cycle is 21 to 35 days, with month-to-month variation less than 7 days. Cycles longer than 35 days, shorter than 21 days, or varying more than 7 days month to month are considered irregular and may warrant medical evaluation, especially for fertility planning or if symptoms accompany them.
What can make my period come earlier or later than predicted?
Stress, significant weight change, intense exercise, sleep disruption, illness, travel across time zones, and hormonal changes (including peri-menopause and post-pill recovery) all affect cycle timing. Even regular cycles drift by a few days. Use the prediction as a planning window, not a commitment.
Should I track my period for fertility or contraception?
For fertility planning, period tracking is useful as a starting frame but should be paired with ovulation prediction kits or basal body temperature for precision timing. For contraception, calendar-based tracking alone has a typical-use failure rate too high to rely on; use sympto-thermal method or another method.