Zerdly

Period Calculator

Predict your next period, cycle length and fertile days with this free period calculator that works entirely in your browser.

100% FreeRuns in your browserBasic
How to use this calculator

These dates stay in this browser tab. Nothing is saved, put in the page address or sent anywhere.

If you choose, these entries can be kept in this browser on this device only (local storage) so they are here next time. They are never uploaded. Anyone using this browser could see them.

Nothing is saved.

Upcoming periods (estimates)

Enter the first day of your last period.

Estimate only — not medical advice. Cycles, ovulation and due dates vary between people and from cycle to cycle. Confirm with a healthcare professional.

How the method works

Cycle variation widens every estimate
Illustrative method
Cycle variation widens every estimateA cycle-day axis from day 1 to day 32. For cycles of 26 to 30 days and a 14 day luteal phase, ovulation is estimated between cycle days 13 and 17, the fertile-window estimate spans days 8 to 17, and the next period between days 27 and 31.Day 1 = period start (entered date)Fertile-window estimatedays 8–17Ovulation estimatedays 13–17Next period estimatedays 27–311714212832cycle day
Cycles of 26–30 days with a 14-day luteal assumption: next period on cycle day 27–31, ovulation on cycle day 13–17 (next start − 14), fertile-window estimate days 8–17 (ovulation − 5 to ovulation). These are ranges of estimates, not certainties or safe days.

Irregular cycles, recent hormonal contraception, postpartum and breastfeeding weaken every calendar estimate. Fixed example values for illustration — not your result.

Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.

How to use this calculator

  1. Enter the first day of your last period, your cycle length, your period length and how many cycles ahead to show.
  2. Optionally add past start dates to see your own intervals, and choose whether to forecast with the length you entered or your mean or median interval.
  3. Optionally enter a shortest and longest cycle for a range. Save on this device only if you want the entries kept in this browser.

How it works

Each start is k × cycle length after the last period; the period ends (period length − 1) days after it starts.

Past start dates must be in order with no duplicates and none in the future; the tool shows each interval and the mean, median and range, and says which one drives the forecast.

A range widens each cycle — k × shortest to k × longest — and is a what-if envelope, not a statistical confidence interval.

Nothing is saved unless you choose Save on this device; Clear all data removes it. A late period on the forecast is not a sign of pregnancy or of any condition.

Worked example

28-day cycle from 1 Jan 2026. Next periods 29 Jan, 26 Feb and 26 Mar; a 5-day period starting 29 Jan ends 2 Feb. Past starts of 1 Jan, 29 Jan and 28 Feb give intervals of 28 and 30 days, mean 29.

Common mistakes

  • Expecting an irregular cycle to follow the average.
  • Entering past dates out of order.
  • Reading a late forecast date as a medical signal.

Tips

  • Estimate only — not medical advice. Dates and fertile windows vary between people and cycles; confirm with a healthcare professional.

Frequently asked questions

What if my cycle is irregular?

Add your recent start dates and a shortest/longest range: the forecast then shows how wide the possibilities are instead of one date.

Privacy?

Runs entirely in your browser. Your inputs are not saved or transmitted.

Related tools

Explore more in Family, Parenting & Custody.

Cite this tool

Zerdly. (2026). Period Calculator. Retrieved from https://www.zerdly.com/tools/period-calculator/

Tip: Enter any known values to calculate the remaining results.

Designed to run locally in your browser. Tool inputs are not intentionally stored by Zerdly unless stated otherwise.