Period Calculator

Predict your next 3 period dates and fertile window. Privacy-first: no accounts, no tracking, your dates never leave your device.

First day of your last menstrual period (LMP)

The first day of bleeding from your most recent period.

Average cycle length (days)

Days from one period's start to the next. Leave at 28 if unsure.

Average period length (days)

How many days your bleeding usually lasts.

Use a different reference date

Useful for retrospective calculations (e.g., entering past cycles). Leave blank to use today's date.

Reference date (optional, defaults to today)

Next predicted period starts

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Next ovulation: — Full fertile-window view →

Period	Start	End

Predictions get less reliable past the first cycle. Calendar-method studies report roughly 6 days of mean error for the next cycle, and about 42% of people whose cycles look "regular" still have enough variability to break a multi-cycle prediction.

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How this is calculated

Formula (ACOG additive convention):
Period start[k] = LMP + k × cycle length, for k = 1, 2, 3.
Period end[k] = Period start[k] + (period length − 1) (inclusive last day).
Ovulation = LMP + (cycle length − 14), using the standard 14-day fixed luteal phase.

This is the canonical method used by every major incumbent (Flo, Calculator.net, Omni, American Pregnancy Association) and documented in ACOG's guidance on fertility-awareness-based family planning.

Prediction accuracy: Symul L et al. (HROpen 2020;2020(2):hoaa011) analyzed large-scale cycle data and found ~42.5% of people with apparently regular cycles have ≥7 days of intracycle variability. A review of calendar-method studies (PMC3658377) reports mean error around 6 days for the next bleed. This is why predictions degrade past cycle 1 and why this tool shows a caveat.

How we choose and check our formulas