Period Calculator
Next periods and fertile window, on a calendar.
Period Calculator predicts the next few periods and the fertile window from your last period date and your usual cycle and period length, and draws them on a calendar of the coming months. It is arithmetic on the dates you enter, held in your browser and stored nowhere unless you ask.
Loading Period Calculator…
Runs entirely on your device. No file or text you enter here is uploaded. How to verify it.
This is a calculation tool, not medical advice. The result is an estimate from a published formula and cannot account for your individual circumstances. Talk to a healthcare professional about anything that matters.
How to use Period Calculator
- Enter the first day of your last period.
- Set your usual cycle length and how many days the bleed lasts.
- Read the predicted dates and the calendar of the coming months.
Questions about Period Calculator
Can I use this as contraception?
No, and it is the one thing this page asks you not to do. A predicted fertile window is a guess made from arithmetic, sperm can survive around five days inside the body, and ovulation moves with stress, illness, travel and sleep. Fertility awareness methods that do work rely on daily temperature or cervical mucus observations and proper training, not on a calendar.
Why do the later predictions drift?
Because every prediction assumes each future cycle will be exactly the length you typed. Most people's cycles vary by a few days from one to the next, which is ordinary, so the fourth prediction has inherited three cycles of error. Treat the first prediction as a reasonable guide and the later ones as increasingly rough.
How is the fertile window worked out?
Ovulation is counted backwards from the predicted next period by the luteal phase, which defaults to 14 days, and the window runs from five days before it to one day after. Counting forwards from the period instead is the common mistake: the luteal half is the stable one at roughly 12 to 16 days, while the half before ovulation is the part that stretches.
Is any of this stored?
Only if you tick the box. Left alone, the dates live in the tab and vanish on reload. Tick it and they are written to this browser's own storage on this device — not to a server and not to an account, because there is no account, but anyone using this browser profile could open the page and read them. Unticking erases them.
What if my cycles are very irregular?
Then a calendar prediction is close to guesswork, and this page will say so by drifting. Cycles shorter than 21 days or longer than 35, bleeding between periods, or a pattern that changes suddenly are all worth raising with a clinician rather than modelling — a calculator cannot see thyroid function, PCOS, perimenopause or contraception.
Is Period Calculator really free, with no limits?
Yes. There is no account, no daily cap, no file-size tier and no watermark, because there is no server cost to recover. The site is funded by clearly disclosed partner recommendations on the Partners page, never by metering the tools.
Are my files or text uploaded when I use Period Calculator?
No. Everything is processed by JavaScript running in your browser, on your device. You can verify it: open your browser's developer tools, switch to the Network tab, and use the tool. You will see no request carrying your data — because there is nowhere for it to go.