Health & Fitness
Period Calculator
Period — period Calculator performs calendar arithmetic on the dates you enter, accounting for month lengths and leap years. This page runs that same calculation on your numbers, in your browser: no signup, no uploads, nothing stored.
Enter an observed period start and a supplied average cycle length to calculate estimated next and following starts. Any current cycle day assumes repeated cycles of that exact length; the tool does not observe whether another period began.
About this calculator
Period Calculator performs calendar arithmetic on the dates you enter, accounting for month lengths and leap years. That is the whole of the calculation — the mistakes happen in the inputs. It matters when you want a quick screening number before deciding whether a habit, plan or measurement deserves a closer look. Worked example: With a 28-day cycle and a last period starting 1 June, the next is expected around 29 June , and the following near 27 July. Use the calculator with your own figures; it runs entirely on your device.
How to use
- Enter the first day of your most recent period.
- Enter your cycle length in days for the prediction.
- Press Calculate to see your next period date.
- Record actual starts over several cycles and keep observed dates separate from the forecast.
Why use Period Calculator
Fast
Calculations run in your browser. No round trip to a server.
Private
Your inputs never leave your device. Nothing is uploaded.
Free
No signup, no paywall, no ads-in-results, no watermarks.
Educational
Uses common calendar-based estimates for educational purposes only. Confirm medical questions with a qualified professional.
Mobile-ready
Optimised for phones, tablets, and desktops alike.
Transparent
Clear about the method used and its limits, with disclaimers where they matter.
Common uses
Use it for predicting the following period
Use it for a fast next-period estimate
Use it for tracking your cycle length
Use it for estimating your fertile days
Use it for general cycle awareness
Use it when noting irregularities to discuss with a doctor
Technical notes
Period Calculator performs calendar arithmetic on the dates you enter, accounting for month lengths and leap years.
These are general-population estimates and not a substitute for advice from a qualified clinician.
Nothing you type is uploaded — the calculation happens entirely on your device.
FAQ
How is my next period date predicted?
It adds the supplied cycle length to the observed first day of the last period. The displayed two-day range on either side is an illustrative offset only, not a statistically estimated or clinically validated prediction interval.
Why does my period date vary from the prediction?
Cycle length changes with stress, travel, illness, and hormones, so real periods often fall a few days from any calendar estimate.
How many cycles should I track for accuracy?
Several observed start dates let you calculate completed cycle lengths and a descriptive average. Keep their variation visible; more records do not make a fixed-length calendar forecast certain.
Does the Period Calculator send my data to a server?
No. The Period Calculator runs entirely in your browser — your inputs never leave your device, and nothing is uploaded or stored.
Worked example
With a 28-day cycle and a last period starting 1 June, the next is expected around 29 June, and the following near 27 July.