Cycle Length Calculator (From Your History)
Average cycle length and regularity score from your last three cycles — the number every other tool needs.
A large app-data study (600,000+ cycles) found the *average* cycle is 29.3 days — not the textbook 28 — and only ~13% of people actually average 28. Cycle length also drifts down slightly with age through the 30s and 40s. Three cycles is a minimum sample; six paints the honest picture.
Formula
For general information only. Pregnancy and infant care decisions must be guided by your treating clinician or midwife — always defer to their assessment.
Disclaimer: This tool is for general informational and estimation purposes only and is not professional financial, tax, accounting or legal advice. All figures are estimates — verify with a qualified professional before making decisions. Read the full disclaimer.
Average cycle length and regularity score from your last three cycles — the number every other tool needs. The Cycle Length Calculator (From Your History) is a free, private cycle length calculator tool — every result computes instantly in your browser with no sign-up and no data upload.
About Cycle Length Calculator (From Your History)
A large app-data study (600,000+ cycles) found the *average* cycle is 29.3 days — not the textbook 28 — and only ~13% of people actually average 28. It applies average = (c1 + c2 + c3) ÷ 3; count from day 1 of one period to day 1 of the next. Use the Cycle Length Calculator (From Your History) to get an instant, clearly-explained result with the working shown step by step — free, private and with the source method cited.
How to use Cycle Length Calculator (From Your History)
- 1Enter your details in the Cycle Length Calculator (From Your History) input fields above.
- 2The result updates instantly with the working and reference bands shown.
- 3Adjust any value to explore how it changes the outcome — it's free and unlimited.
Why use Cycle Length Calculator (From Your History)?
- ✓Instant cycle length calculator result that recomputes as you type — no waiting, no page reloads
- ✓100% client-side: your health data never leaves your browser
- ✓Shows the actual formula and your numbers substituted in, so you can see exactly how the result is reached
- ✓Based on published, citable sources (Bull JR et al., npj Digit Med 2019)
- ✓Free forever with no sign-up, account or app install
Frequently asked questions
How does the Cycle Length Calculator (From Your History) work?+
A large app-data study (600,000+ cycles) found the *average* cycle is 29.3 days — not the textbook 28 — and only ~13% of people actually average 28. The calculation uses the formula: average = (c1 + c2 + c3) ÷ 3; count from day 1 of one period to day 1 of the next. Everything runs instantly in your browser as you type — your inputs are never uploaded.
Is the Cycle Length Calculator (From Your History) accurate, and what is it based on?+
The method is traceable to authoritative sources: Bull JR et al., npj Digit Med 2019. Results are estimates — individual variation always applies, so treat the output as a well-grounded starting point.
Is the Cycle Length Calculator (From Your History) free and private?+
Yes. It's completely free with no sign-up, and all computation happens locally in your browser, so none of your health data ever leaves your device.
Can I use this for medical decisions?+
This tool is informational only. Always defer to your treating clinician or midwife for pregnancy and infant care decisions.
What can I use the Cycle Length Calculator (From Your History) for?+
It's commonly used for cycle length calculator, average cycle, irregular periods check. Average cycle length and regularity score from your last three cycles — the number every other tool needs.
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