Period Calculator

How the estimate is produced

The calculation adds your average cycle length to the first day of your last period, then repeats forward. Cycle length is counted from the first day of one period to the day before the next, not between the end of one and the start of the next, which is the most common counting error and produces estimates roughly a week off.

Cycles vary, and that is normal

The textbook 28-day cycle is an average, not a standard. Anything from 21 to 35 days is considered within normal range for adults, and 21 to 45 days for adolescents in the first years after menarche. Variation of a few days between cycles is entirely typical. A prediction based on an average will therefore be approximate for most people and unreliable for anyone with genuinely irregular cycles.

Illness, significant stress, travel across time zones, substantial weight change, intense exercise and starting or stopping hormonal contraception all shift timing. A single unexpected cycle is usually one of these rather than anything concerning.

This is not contraception

Calendar-based prediction must not be relied on to avoid pregnancy. Ovulation timing varies between cycles even in people with regular periods, sperm can survive several days in the reproductive tract, and the fertile window shifts accordingly. Calendar methods have substantially higher failure rates than modern contraception. If avoiding pregnancy is the goal, speak to a healthcare professional about appropriate methods.

When to seek medical advice

Worth raising with a clinician: cycles consistently shorter than 21 or longer than 35 days, bleeding lasting more than seven days, soaking through protection hourly, bleeding between periods, periods stopping for three months or more without pregnancy, or pain severe enough to interrupt normal activity. These are common and usually treatable, and none of them is something to simply tolerate.

Your data stays on your device

The calculation runs entirely in your browser. No dates are stored, transmitted or logged. Cycle data is sensitive personal information, and several tracking applications have been found sharing it with advertisers and data brokers, so keeping it local is a deliberate design choice rather than a technicality.

This tool provides general information and is not medical advice.

Related tools

• User Agent Parser — Parse a User-Agent string into browser, engine, operating system and device. Explains why UA strings are unreliable and what to use instead.
• QR Code Generator — Generate QR codes for URLs, text, Wi-Fi and contact details. Adjustable error correction and size, produced entirely in your browser.
• Credit Card Validator — Validate a card number with the Luhn algorithm and identify the issuing network from its prefix. Runs locally, nothing is transmitted.
• Text Case Converter — Convert text between camelCase, PascalCase, snake_case, kebab-case, CONSTANT_CASE, Title Case and sentence case. Runs entirely in your browser.