Period Calculator
Enter your last period start date and cycle details to predict your next three periods and your estimated ovulation day.
Last updated: June 20, 2026 · Medically reviewed by TG we-Calculate Editorial TeamLast period start date
days
days
Thu, Jan 29, 2026 — Mon, Feb 2, 2026
How do you calculate your next period?
Add your average cycle length to the start date of your last period to find the next start date, then repeat for each future cycle. The period lasts your typical number of bleeding days. Ovulation is estimated about 14 days before the next period begins.
Formula
How this is calculated
The calculator takes three inputs: the calendar date your last period started, your average cycle length in whole days (the gap from the start of one period to the start of the next, typically 21–35), and your usual bleeding duration in whole days. Cycle length and duration are truncated to whole numbers, and the start date is read as a local calendar date so month and leap-year rollovers are handled exactly.
To project each upcoming period it adds the cycle length to the last start date once per future cycle: the next start is lastStart + cycleLength, the one after is lastStart + 2×cycleLength, and so on for three cycles. Each period’s end date is its start plus (duration − 1) days, because day one is the first bleeding day. Ovulation is estimated as the next period start minus 14 days, the assumed length of the luteal phase, which stays roughly constant even when cycle length differs.
This calendar method assumes regular, predictable cycles. It cannot account for cycle variability, stress, illness, travel, breastfeeding, perimenopause or hormonal contraception, so the dates are estimates only and must not be used as contraception.
Examples
| Input | Result |
|---|---|
| Last period 2026-01-01, 28-day cycle, 5-day period | Next period 2026-01-29 to 2026-02-02; ovulation around 2026-01-15 |
| Last period 2026-01-01, 30-day cycle | Next period starts 2026-01-31 |
| Last period 2026-02-10, 21-day cycle | Next period starts 2026-03-03 |
About this calculator
The period calculator uses the calendar method to estimate when your future periods are likely to arrive. Starting from the first day of your most recent period, it adds your average cycle length once for each upcoming cycle to project the next three start dates, and applies your usual bleeding duration to estimate each end date.
It also estimates ovulation by subtracting 14 days from the next predicted period start, because the luteal phase (the time between ovulation and the next period) stays close to 14 days for most people even when cycle length varies. These predictions are estimates: actual cycles can shift with stress, illness, travel, breastfeeding, perimenopause and hormonal contraception, so treat the dates as a guide rather than a guarantee and never as a form of contraception.
Frequently asked questions
It is most accurate when your cycles are regular. If your cycle length varies a lot, the predicted dates are rough estimates and may be off by several days.
Ovulation is estimated as roughly 14 days before your next period starts, since the luteal phase tends to last about 14 days regardless of total cycle length.
No. Calendar predictions are not reliable contraception because ovulation timing can shift. Use a proven contraceptive method if you want to avoid pregnancy.
Also known as
Sources & references
TG we-Calculate Editorial Team. (2026). Period Calculator [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/period-calculator
TG we-Calculate Editorial Team. "Period Calculator." TG we-Calculate. 2026. https://we-calculate.com/calculator/period-calculator.
TG we-Calculate Editorial Team, "Period Calculator," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/period-calculator
@misc{wecalculate_period_calculator, title = {Period Calculator}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/period-calculator}}, year = {2026}, note = {TG we-Calculate} }
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