Period Calculator
Predict your next expected period start date (and the two cycles after it) from your last period's start date and typical cycle length.
Calculator Inputs
The 4-digit year your previous period started.
The month your previous period started (1-12).
The day your previous period started (1-31).
Your typical cycle length in days (21-35).
- Next Expected Period — Day
- 29
- Next Expected Period — Year
- 2026
- Next Expected Period — Month
- 1
- Following Cycle — Year
- 2026
- Following Cycle — Month
- 2
- Following Cycle — Day
- 26
- Third Cycle — Year
- 2026
- Third Cycle — Month
- 3
- Third Cycle — Day
- 26
About these results
CalcSpectrum calculators produce automated estimates using established formulas and the specific values you enter. Because real-world outcomes depend on input accuracy, mathematical assumptions, and rounding conventions, please verify critical figures with an appropriately qualified financial, tax, medical, legal, or other professional, as relevant.
Health & Fitness Notice: Wellness and body metric calculations provide informational estimates based on generalized population formulas. They do not provide medical diagnosis, clinical advice, or treatment plans. Consult an appropriately qualified medical or healthcare professional for personal guidance.
How It's Calculated
Formula
This calculator projects your next expected period start date by adding your typical cycle length to the start date of your previous period — then repeats the same addition twice more to project the following two cycles. This is a simple calendar-based projection, not a physiological model: it assumes your cycle length stays constant at the value you enter, which real cycles do not always do. Treat these dates as a general planning estimate, not a guarantee — actual period timing can vary from cycle to cycle for many reasons.
Worked Examples
28-day cycle, previous period started January 1
- Next period: Jan 1 + 28 days = Jan 29
- Following cycle: Jan 29 + 28 days = Feb 26
- Third cycle: Feb 26 + 28 days = Mar 26
30-day cycle, previous period started January 1
- Next period: Jan 1 + 30 days = Jan 31
- Following cycle: Jan 31 + 30 days = Mar 2
Frequently Asked Questions
Why does this show three predicted dates instead of just one?
Projecting the next two cycles as well as the immediate next period is a straightforward extension of the same calendar math, useful for short-term planning — it still assumes a constant cycle length, so treat all three as estimates.
Does this calculator track symptoms or store my cycle history?
No. This is a single-calculation date predictor — it does not save any history, track symptoms, or require an account. Enter your previous period's start date and cycle length each time you use it.
How accurate is the predicted date?
It's a calendar-based estimate only, assuming your cycle length stays exactly as entered. Actual period timing can vary cycle to cycle, so treat this as a planning reference rather than a guarantee.