Period Calculator

Last period start

Cycle day 1 is the first day of bleeding.

As of date

Leave blank to use today (UTC).

Average cycle length (days)

Typical range is 21–35 days.

Typical period length (days)

Usually 3–7 days.

Luteal phase (days)

Default 14; ovulation ≈ next period − luteal.

These estimates are for general guidance only and are not medical advice.

Period Calculator – Quick Cheat Sheet

Cycle day: Day 1 is the first day of bleeding.
Next period: Predicted as Last start + (n × cycle length) at/after “As of”.
Window: We show ±2 days around the predicted start to reflect natural variability.
Bleed end: Approx. Next start + period length − 1 days.
Ovulation: Estimated as Next period − luteal (default 14 days).
Fertile window: Ovulation −5 to Ovulation +1 days.
Validation: “Last start” must not be after “As of”. Inputs are clamped to reasonable ranges.

Notes & cautions

These are estimates, not a diagnosis or a guarantee. Irregular cycles may reduce accuracy.
Do not use for contraception. For health concerns or irregularities, consult a clinician.
All calculations use UTC and strict YYYY-MM-DD parsing to avoid timezone issues.