Ovulation Calculator

Estimate ovulation and fertile window from cycle length and LMP.

Ovulation Calculator helps you estimate ovulation and fertile window from cycle length and LMP. Enter first day of last period and cycle length, review the breakdown, and use the guidance below to understand what the number means — and what it does not. What the Ovulation calculator does Ovulation Calculator is built for body measurements, calories, and everyday health math. Estimate ovulation and fertile window from cycle length and LMP. You enter first day of last period and cycle 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 First day of last period and your usual Cycle length in days (20–45). Ovulation is estimated at 14 days before the next period, so it falls (cycle length − 14) days after your period started. The fertile window runs from 5 days before to 1 day after ovulation — 7 days in all. It assumes your cycles are regular and the same length each month. The formula: Ovulation ≈ first day of last period + (cycle length − 14 days). Fertile window ≈ 5 days before to 1 day after ovulation.