Period Calculator
Last updated: June 2026 · Free · No sign-up required
Enter values above and click Calculate to see your result instantly.
Quick reference
Menstrual cycle phases:
| Phase | Approx. days (28-day cycle) |
|---|---|
| Menstruation | Days 1–5 |
| Follicular | Days 1–13 |
| Ovulation | Around day 14 |
| Luteal | Days 15–28 |
monitor_weight A starting point, not a diagnosis
This calculator gives you the same number a clinician would compute by hand from your inputs. It is informational, never a substitute for a doctor. If a result surprises you, write it down and discuss it at your next appointment — that conversation is the goal, not a verdict from a webpage.
science Built from peer-reviewed formulas
Every health calculator on CalcaTools uses an equation from the WHO, NIH, ACOG, CDC, or a published peer-reviewed paper. The exact citation is in the "Formula & methodology" section. We re-check sources quarterly and update the page when guidance changes — the "Last reviewed" date is at the top.
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Health calculations only help if you can actually use them. We use plain-English labels (no jargon), accept both metric and US units, and show what a "normal" range looks like alongside your result — so you understand what the number means, not just what it is.
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Your weight, height, age, pregnancy details, and any other health input never leave your browser. Calculations run client-side; nothing is logged or stored on our side. We don't want your medical data — we just want the formula to work for you.
Interpretation guide
Cycle facts:
| Item | Typical range |
|---|---|
| Cycle length | 21–35 days |
| Period length | 3–7 days |
| Luteal phase | ~14 days (fairly fixed) |
| Variation | A few days is normal |
Formula & methodology
Formula: Next period = first day of last period + cycle length; repeat for future cycles
The calculator predicts upcoming period start dates by adding your average cycle length to the first day of your last period, then repeating for several future cycles. A typical cycle is 21–35 days with 3–7 days of bleeding. Because the luteal phase is fairly fixed at about 14 days, cycle-length differences mostly come from the follicular phase. Tracking over a few months improves the prediction's accuracy.