Skip to content
Health in Plain

Next Period Calculator

To predict your next period, add your cycle length to the first day of your last period. Enter this information below to see the predicted date, along with your fertile window and ovulation for the current cycle.

Next period

—

Fill in the fields above to calculate.

The math

Next period = first day of last period + cycle length. If your cycle lasts 28 days and your last period started on day 1, the next one arrives around day 29. Longer or shorter cycles shift the date proportionally.

Worked example

Last period started June 3, 30-day cycle:

  • Predicted next period: July 3 (June 3 + 30 days)
  • Ovulation for this cycle: around June 19

How much variation is normal

The prediction assumes the next cycle will run as long as the last ones, and cycles rarely oblige exactly. The FIGO criteria StatPearls cites define regular cycles as those where the difference between your shortest and longest cycle is 7 days or less (ages 26 to 41), or 9 days or less at ages 18 to 25 and 42 to 45. Variation past that is common rather than rare: in a pooled analysis of 581 U.S. women followed for up to a year, 43.0% varied more than 7 days.

Separately, the Office on Women’s Health treats 24 to 38 days as normal cycle frequency and suggests talking to a clinician about cycles outside it — or about 90 days with no period at all, when pregnancy and breastfeeding aren’t the explanation.

When the prediction is off

Stress, weight changes, travel, and hormonal shifts can move a period earlier or later. If your cycles vary a lot, check the ovulation calculator and the fertile window calculator to better understand your cycle.

This calculator is informational and gives an estimate. Persistent changes in your cycle deserve evaluation by a gynecologist.

Frequently asked questions

How do you predict the next period?

Add your average cycle length to the first day of your last period. In a 28-day cycle, the next period comes about 28 days later.

Does the prediction work with an irregular cycle?

It becomes less accurate. With cycles that vary a lot, the predicted date is only an approximate reference.

How many days of variation are normal?

Cycles count as regular when the gap between your shortest and longest is 7 days or less at ages 26 to 41, and 9 days or less at ages 18 to 25 and 42 to 45, per the FIGO criteria StatPearls uses.

When is a period late enough to worry?

The Office on Women's Health says to talk to a clinician about cycles longer than 38 days or shorter than 24 — and to check in if you go 90 days without a period and aren't pregnant or breastfeeding.

Sources

  1. Your menstrual cycle — Office on Women's Health, U.S. Department of Health and Human Services
  2. Physiology, Menstrual Cycle — StatPearls (NCBI Bookshelf, U.S. National Library of Medicine)
  3. Najmabadi S et al. Menstrual bleeding, cycle length, and follicular and luteal phase lengths in women without known subfertility. Paediatr Perinat Epidemiol. 2020 (PMC8495765)

Written by Lucas Silva · Reviewed by Lucas Silva · Updated on August 14, 2026