Methodology

Every number on this site comes from the formulas below. They are the standard calendar method used by clinical period calendars — and they have known limits, which we state just as clearly.

Core formulas

Inputs: LMP (first day of last period, day 1 = first day of full flow), C = average cycle length in days, D = typical period duration in days. Next period start = LMP + C; period end = start + D − 1; future cycles = LMP + n×C. Estimated ovulation = predicted next period start − 14 days. Fertile window = ovulation − 5 days through ovulation + 1 day. Current cycle day = days since LMP + 1.

Why 14 days

The luteal phase (ovulation to next period) is the most stable part of the cycle, around 14 days for most people (commonly 12–16). Anchoring ovulation backwards from the predicted period is more reliable than counting forward from day 1, which is why we use it.

Irregular-cycle ranges

With shortest cycle S and longest cycle L, the next period window is [LMP + S, LMP + L]; ovulation window is that window minus 14 days; fertile window extends 5 days earlier and 1 day later. For cycle n the window is [LMP + n·S, LMP + n·L] — ranges widen with distance because uncertainty compounds.

Accuracy and limits

For cycles varying by a few days, next-period predictions typically land within 1–2 days. Accuracy degrades as variation grows; with variation over 7–9 days, treat outputs as rough ranges. The method assumes ovulation occurs, and cannot account for stress, illness, medication changes or conditions like PCOS in a given cycle.

These tools provide estimates for personal planning. They are not contraception, not a pregnancy test, and not medical advice. Persistent irregularity, heavy bleeding, severe pain or missed periods warrant a licensed clinician.