Cycle Length Calculator: Average Cycle & Regularity Check
Average several cycles and see how regular you are.
Period Calculator: Next Period Date & Cycle Forecast
See your next period and the six after it.
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About Period Calculator: Next Period Date & Cycle Forecast
Work out your next period start date and forecast six cycles ahead from your last period, cycle length and period length.
How is the next period calculated?
Your next period is estimated as your last period start plus one full cycle length. Subsequent cycles add the same cycle length each time. The window shows the start date and the end date of each predicted period, based on your typical period length.
Why do my predicted dates keep moving?
The forecast assumes a steady cycle length. Real cycles shift with stress, travel, illness, sleep and hormonal changes, so treat the dates as a window rather than a fixed day. Track your actual dates over a few months for a more reliable average.
📐 Methodology & Reference Accuracy:
Calculations, formulas, and conversions execute locally in your web browser. Output models are benchmarked against official reference standards and updated for 2026.
🔒 This tool runs entirely in your browser. Nothing is uploaded or stored.
Background reading: Period Calculator on Wikipedia. More: all women's health.
Frequently Asked Questions about Period Calculator: Next Period Date & Cycle Forecast
How do I find the first day of my last period?
Use the day your bleeding actually started, not the day it ended or the day you noticed it. That first full day of flow is the standard reference point for cycle maths.
What cycle length should I use?
Use your average cycle length, which is the mean of the last few cycles. Typical cycles fall between 21 and 35 days; if yours falls outside that, it is worth speaking to a clinician.
Can I use this with birth control?
Pill, implant and injection cycles are not natural menstrual cycles, so these predictions will not be meaningful while you are using hormonal contraception. Use a dedicated birth control reminder instead.
Can I use this with irregular periods?
Only roughly: the forecast needs one stable cycle length, so use your shortest recent cycle for the earliest date and your longest for the latest. For a proper average worked out from your real dates, run the cycle length calculator first and bring that number back here.