Period Calculator
Project several period starts and bleeding windows from an entered first day and typical cycle length. Sensitive inputs stay in your browser.
Projection uses a 28-day cycle measured from first day to first day.
Context matters.
- Cycles vary and this is not contraception or a diagnosis. Pregnancy, postpartum changes, perimenopause, illness and medicines can shift timing.
Sources & scope
The calculation guide and this source trail show what the result means, where the rule came from and when it was last reviewed. This is source-and-calculation QA, not a claim of medical, legal, tax or other professional review; time-sensitive decisions still belong with the linked authority.
A visible path from input to answer.
Project several period starts and bleeding windows from an entered first day and typical cycle length. The page keeps the calculation scope and its limits beside the result so the output can be checked, copied or revisited.
- Read the record. The calculator uses First day of last period, Typical cycle length (days), Typical bleeding length (days) and Cycles to project supplied in the form.
- Apply the scope. It applies the Menstrual-cycle self-tracking — educational projection, not contraception or diagnosis reference and the explicit date, unit and counting conventions shown in the result.
- Expose the checkpoints. Intermediate rows, warnings and timeline events stay visible; the emphasized row identifies the primary checkpoint when one exists.
Projection uses a 28-day cycle measured from first day to first day.
- Next estimated startSep 6, 2026
Calculation purpose: Project several period starts and bleeding windows from an entered first day and typical cycle length. This is a high-sensitivity reference output; the linked authority or qualified professional controls the final decision.
Calendar inputs use plain year-month-day semantics. Leap days, month ends, date-pair order and unsupported dates are checked before the result is shown.
Rule set 1.1.0 · Last reviewed 2026-08-22 · Next review due 2027-02-22. The source panel below contains the linked evidence.
What to know before using the result.
How is the next period date estimated?
The entered average cycle length is added to the first day of the last period, with an optional variability range around the estimate.
Can this calculator confirm ovulation or pregnancy?
No. Cycle dates vary and a calendar estimate cannot confirm ovulation, contraception effectiveness or pregnancy.
When should cycle changes be discussed with a clinician?
Seek professional guidance for concerning symptoms, possible pregnancy or a persistent change from the person's usual pattern.