# Period Calculator

> Forecast period start and end dates, estimated ovulation, and fertile windows for multiple cycles.

Page: https://askcalculators.com/calculator/period-calculator/
Definition: https://askcalculators.com/calculator/period-calculator/definition.json
JSON Schema: https://askcalculators.com/calculator/period-calculator/schema.json
Version: 2.0.0
Maintained by: AskCalculators (software project; not a credentialed reviewer)
Qualified review: pending. Automated checks do not establish professional approval.
Page status: Live
Review status: Pending

## Use
Open the calculator page and enter values. Query-string parameters can restore and bookmark a calculation.
Example: https://askcalculators.com/calculator/period-calculator/?lastPeriodDate=2026-08-01&periodLength=5&cycleLength=28&lutealPhase=14&cycles=6

## Inputs
- lastPeriodDate: First day of last period; type=date; required; default=2026-08-01
- periodLength: Typical period length; type=number; required; default=5; min=1; max=14
- cycleLength: Average cycle length; type=number; required; default=28; min=20; max=45
- lutealPhase: Luteal phase length; type=number; required; default=14; min=8; max=20
- cycles: Cycles to forecast; type=number; required; default=6; min=1; max=24

## Outputs
- nextPeriod: Estimated next period; format=date; primary
- nextPeriodEnd: Estimated period end; format=date
- nextOvulation: Estimated ovulation; format=date
- fertileWindow: Estimated fertile window; format=text

## Formula
Next period = last period + average cycle length; estimated end = start + period length − 1; ovulation ≈ next period − luteal phase.

## Rules
- Empty optional inputs are undefined, not zero.
- Hidden conditional inputs are ignored.
- Do not invent missing required inputs.

Notice: Calendar predictions are estimates and are not a diagnosis or reliable contraception method.