Period Calculator

Predict the next period start from last period and cycle length.

Period Calculator helps you predict the next period start from last period and cycle length. Enter last period start, cycle length, and period length, review the breakdown, and use the guidance below to understand what the number means — and what it does not. What the Period calculator does Period Calculator is built for body measurements, calories, and everyday health math. Predict the next period start from last period and cycle length. You enter last period start, cycle length, and period length, then Simple Calculators computes the result instantly. These tools use widely published formulas. They are educational screens, not a diagnosis. How the math works Enter the Last period start date, your usual Cycle length (20–45 days) and Period length (up to 10 days). The next period is the start date + cycle length, and it ends (period length − 1) days later, because the first day counts as day 1. The period after that is one more cycle on. Ovulation is estimated at 14 days before the next period, with the fertile window from 5 days before to 1 day after it. The formula: Next period = last period start + cycle length. Assumes your cycle stays the same length each month.