Period Calculator
Predict your next three period start dates and your next ovulation from your last period and cycle length.
How it works
next period ≈ last period + cycle length (repeated each cycle) · ovulation ≈ period start + (cycle − 14)
The predictions add your cycle length to the first day of your last period, then repeat for the following cycles, rolling forward to the next dates still ahead. Real cycles vary by a few days from month to month — stress, illness, travel, medication and weight change all shift them — so these are estimates, not guarantees. Don’t rely on calendar predictions alone for contraception or conception decisions.
Worked example
If your last period began June 1 with a 28-day cycle, the next three starts fall on June 29, July 27 and August 24, with ovulation around June 15.
Frequently asked questions
How accurate are period predictions?
For regular cycles, often within a day or two. But most people’s cycles vary by several days, and factors like stress, travel and illness shift them — so treat the dates as estimates and adjust as your cycle actually unfolds.
Can I use this as birth control?
No. Calendar predictions are not reliable contraception, because the fertile window moves with ovulation, which is hard to predict exactly. Talk to a clinician about effective birth-control options.
My periods are irregular — is this useful?
Less so, because it assumes a steady cycle length. If your cycles vary widely or you skip periods, the predictions won’t be reliable — mention irregular cycles to your doctor.
Related calculators
Estimates based on standard formulas and average cycles — every pregnancy is different. This is not medical advice; your doctor or midwife, with an ultrasound, gives the accurate dates and guidance.