How is ovulation estimated?

Ovulation typically occurs 14 days before the start of the next period, so we estimate it as last period + (cycle length − 14) days.

What is the fertile window?

Sperm can survive up to 5 days inside the body, so the fertile window is roughly 5 days before ovulation through 1 day after.

How is the due date calculated?

It's ovulation date + 266 days (38 weeks), the standard obstetric estimate from conception. Note this differs from last-period + 280 days because it assumes a known ovulation date.

Are these dates guaranteed?

No. Cycles vary month to month, and ovulation can shift due to stress, illness or travel. These are estimates for planning only.

Can I use this for contraception?

No — calendar-based fertility tracking is not reliable for preventing pregnancy. Use this only as informational guidance, and consult a healthcare professional for medical decisions.

Yes. Everything is computed in your browser; nothing is uploaded.

Ovulation Calculator

Estimate your fertile window, ovulation date, next period & due date.

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See how it works

Watch a quick simulation of this tool in action — no file needed.

How to use Ovulation Calculator

Follow these steps — everything happens instantly in your browser.

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Step 1. Enter the first day of your last menstrual period.
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Step 2. Pick your average cycle length (14–45 days).
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Step 3. Read your estimated ovulation date, fertile window, next period and due date.
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Step 4. Review the timeline to see how the dates relate.