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Ovulation Calculator

Estimate ovulation day, fertile window and next period.

Introduction

Ovulation Calculator — Estimate ovulation day, fertile window and next period. Enter First day of last period, Cycle length (days) to get an instant, accurate result.

Formula

Ovulation ≈ last period start + (cycle length − 14) days. Fertile window is roughly 5 days before to 1 day after ovulation.

Step-by-step

  1. Enter the First day of last period.
  2. Enter the Cycle length (days).
  3. Click Calculate to see your result instantly.

Real-world example

Example: With First day of last period = 2026-10-09, Cycle length (days) = 28, the Ovulation Calculator gives Ovulation Date: 2026-10-23, Fertile Start: 2026-10-18, Fertile End: 2026-10-24.

Frequently Asked Questions

Why subtract 14 days?
The luteal phase (after ovulation) is fairly consistent at about 14 days across most cycle lengths, so ovulation is estimated backward from the next period.
How accurate is this for irregular cycles?
It's an estimate based on average cycle length — irregular cycles will reduce its accuracy.
Is this a reliable method of contraception or a medical diagnosis?
No. This calculator provides an estimate based on average cycle patterns, not a guarantee — actual ovulation timing varies cycle to cycle. It is not a form of contraception and not a medical diagnosis. Consult a doctor for fertility or family-planning concerns.
What inputs does the Ovulation Calculator need?
You'll need to provide: First day of last period, Cycle length (days). All fields use sensible defaults, so you can see a working example immediately and then adjust the values to match your own numbers.
How accurate is the Ovulation Calculator?
The Ovulation Calculator applies the formula exactly as calculated — Ovulation ≈ last period start + (cycle length − 14) days. Fertile window is roughly 5 days before to 1 day after ovulation. — so results are precise for the inputs you provide. Accuracy depends on entering correct, realistic input values.
Is the Ovulation Calculator free to use?
Yes, the Ovulation Calculator is completely free, requires no signup, and runs instantly in your browser.

About the Ovulation Calculator

The Ovulation Calculator uses a real, verifiable formula — Ovulation ≈ last period start + (cycle length − 14) days. Fertile window is roughly 5 days before to 1 day after ovulation. — so results are accurate every time, not an approximation.