Period Predictor
Enter the first day of your last period and your average cycle length to see your next several period start dates — each with its weekday, bleeding-end date, estimated ovulation day and 6-day fertile window. Runs entirely in your browser, nothing is uploaded. Estimates only, not medical advice or a contraceptive method.
About this tool
The Period Predictor projects your upcoming menstrual cycles from two things you already know: the first day of your last period and your average cycle length (the number of days from the start of one period to the start of the next — 28 days for many people, but anywhere from about 20 to 45 is common). From there it lists your next several period start dates, and for each cycle it also shows:
- the weekday the period is expected to start on,
- the bleeding-end date (period start + your period length − 1),
- the estimated ovulation day — roughly your luteal-phase length before that period, since the luteal phase stays fairly constant even when cycle length changes, and
- the 6-day fertile window — the five days before ovulation through ovulation day, when conception is most likely.
Everything runs locally in your browser — your dates are never uploaded to a
server, there is no sign-up, and the tool is free. Adjust the cycle length,
period length, luteal phase and how many cycles to project with the sliders, or
deep-link a prediction by putting the values straight in the URL
(?last_period=2026-07-01&cycle_length=28&cycles=6).
These predictions are estimates. Real cycles drift by a few days with stress, illness, travel, sleep and other factors, and irregular cycles are inherently harder to predict. This tool is not a contraceptive method and is not medical advice — talk to a healthcare professional about anything concerning.
FAQ
How does the predictor calculate my next period?
It adds your cycle length to the first day of your last period, and repeats
that for as many cycles as you ask for. With a last period of 2026-07-01 and a
28-day cycle, the next start is 2026-07-29, then 2026-08-26, 2026-09-23,
and so on. Each date is labelled with its weekday and its bleeding-end date
(start + period length − 1).
How are ovulation and the fertile window estimated?
Ovulation is estimated as the predicted period start minus your luteal-phase length (default 14 days), because the luteal phase — the time between ovulation and the next period — stays fairly constant even when cycle length varies. The fertile window is the six days ending on ovulation day: the five days before it (sperm can survive several days) plus ovulation day itself.
What if my cycles are irregular?
Predictions are only as steady as your cycles. If your cycle length varies a lot month to month, use your average and treat every date as a rough guide — the real day can land a few days either side. For very irregular cycles the fertile window and ovulation estimates are especially approximate. This tool is an estimate, not a contraceptive method and not medical advice.
Is my data private?
Yes. All the math runs in your browser using local WebAssembly — the dates you enter are never sent to a server, there is no account, and nothing is stored after you close the page. You can even use the tool offline once it has loaded.
Developer & Automation Access
Run it from the terminal
Same engine as this page, headless — via the gizza CLI:
gizza tool period-predictor "2026-07-01"New to the CLI? Get gizza →
Open it by URL
Pre-fill and auto-run this tool with query parameters — the names match the API/CLI:
https://gizza.ai/tools/period-predictor/?last_period=2026-07-01&cycle_length=28&period_length=5&luteal_phase=14&cycles=6Machine-readable descriptor: tool.json — title + parameters JSON Schema for agents.