Period Calculator
Predict your next periods — start and end dates for up to 12 cycles, how many days away, your fertile window and ovulation, today’s cycle day and how late a period is — from your last period, or from a list of past start dates to work out your average cycle and its range.
How it works
next period ≈ last period + cycle length (repeated each cycle) · period ends = start + bleeding days − 1 · ovulation ≈ period start + (cycle − luteal phase, 14 by default) · fertile window = ovulation − 5 to + 1 day · from past dates: average cycle = sum of the gaps ÷ number of gaps; likely window = last period + shortest to longest cycle
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 of today. Each predicted period also gets an end date from your usual number of bleeding days, and an estimated ovulation date and fertile window counted back from the next period (the luteal phase, about 14 days, is adjustable under More options). If you track your periods, switch on past dates and paste the start dates of your last few periods: the calculator measures every cycle, uses the average and shows the earliest and latest your next period is likely to come, plus a chart of your cycle lengths so you can see how regular they are. It also tells you which cycle day today is and, if more than a full cycle has passed since the last date you entered, how many days late a period would be. Real cycles vary — 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, 2026, lasts 5 days and your cycle is 28 days, the next periods start June 29 (ending July 3), July 27 and August 24, with ovulation around June 15 and a fertile window of June 10–16. From past dates of June 3, July 1, July 30, August 27 and September 24, the four cycles are 28, 29, 28 and 28 days — an average of 28.3 days, so the next period is expected October 22 and most likely between October 22 and October 23.
Sources: ACOG — Menstruation in girls and adolescents: using the menstrual cycle as a vital sign · Office on Women’s Health — Your menstrual cycle
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. Entering several past start dates gives a better average and shows your usual range.
How do I work out my cycle length?
Count from the first day of one period to the day before the next one starts — or let the calculator do it: switch on past period dates and paste your recent start dates. It averages the gaps and shows the shortest and longest.
How late is late?
A period is usually considered late once it is more than about 5 days past the expected date for a regular cycle. If you could be pregnant, a home test is reliable from the day of the missed period.
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. Using your past dates at least shows the range to expect. If your cycles vary by more than 7–9 days, you skip periods or bleed between them, mention it 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.