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.
Period Calculator is a browser-based way to work out your next period and cycle dates. Feed it your first day of your most recent period and average cycle length and it computes your next and following period dates, current cycle day and an ovulation estimate on the spot, keeping your data local.
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.
- Track a few cycles for a more accurate prediction.
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 your average cycle length to the first day of your last period, then shows a likely range of a couple of days on either side to allow for normal variation.
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?
Tracking three or more cycles gives a better average cycle length, which makes the next-period prediction more reliable.
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.