Menstrual Cycle Calculator
Estimates the next period date and an approximate ovulation date from the first day of your last period and your cycle length.
What this calculator does
The Menstrual Cycle Calculator estimates your result instantly from the values you enter, using standard health & fitness formulas. Estimates the next period date and an approximate ovulation date from the first day of your last period and your cycle length. Calculations run in your browser, so nothing you type is sent to a server, and the output is an estimate rather than a certified or official figure.
Result
About Menstrual Cycle Calculator
The Menstrual Cycle Calculator uses the first day of your last period and your usual cycle length. It adds one full cycle to estimate your next period, and subtracts 14 days from the cycle length to estimate ovulation.
Cycle length must be at least 20 days; both dates are returned in year-month-day form.
Formula & Mathematical Calculation
Next Period = f(First Day of Last Period, Cycle Length (days))
Calculated according to standard health formulas implemented in the calculator core.
Calculation Example
With a last period starting 1 January 2026 and a 28-day cycle, the next period is 1 January + 28 days = 29 January 2026.
Ovulation is estimated at 28 − 14 = 14 days after the start, so 15 January 2026. The tool reports Next Period 2026-01-29 and Estimated Ovulation 2026-01-15.
Key Benefits
- Estimates your next period date from one cycle length
- Adds an approximate ovulation date
- Handles the calendar arithmetic across months
- Works for cycle lengths from 20 days upward
How to Use this Calculator
-
Enter your last period start
Pick the first day of your last period in the date field. -
Enter cycle length
Type your usual cycle length in days (at least 20). -
Calculate
Select Calculate to see your next period and estimated ovulation dates.
Assumptions & Limitations
What the calculation assumes
- Every value you enter is treated as an exact, known amount.
- Any rate or interval you enter is assumed to hold for the whole period unless the form asks for it separately.
- Results use standard published formulas and account only for the values entered above.
- Rounding is applied for display; the underlying calculation uses your full precision.
- Results are returned in the same units you enter, unless the tool states the output unit.
What it does not cover
- Menstrual Cycle Calculator produces a calculated result for reference and learning, not a certified or official measurement.
- Real-world outcomes differ when tolerances, rounding rules or local standards apply.
- Definitions and standards vary by region and change over time; verify the convention that applies to your case.
- Figures are only as accurate as the inputs you supply — check them before acting on the result.
- For work where the result carries real consequences, confirm it against an authoritative source.
Frequently Asked Questions
Free to use, no account needed. Calculations run in your browser, so nothing you type is sent to a server. Results are estimates — how we build and check our calculators.