About Online Period Calculator
We build free cycle tools that respect the person using them.
Why this site exists
Period prediction tools have been around for decades, yet most ask you to hand intimate health data to an app account before doing thirty seconds of arithmetic. We think that trade is unnecessary. Every calculator on this site runs entirely in your browser: no sign-up, no server storage, no data selling — ever.
What we build
A growing family of cycle tools — period prediction, irregular-cycle ranges, late-period checks and more — each paired with plain-language explanations written to a published editorial standard and checked against sources like ACOG, Mayo Clinic, Cleveland Clinic and the NHS.
How we work
Small team, explicit process: research, draft, independent medical review, publish, and re-review on a schedule. Our editorial policy and methodology pages describe exactly how content and calculations are produced. Questions and corrections are welcome at contact@onlineperiodcalculator.com.