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Ovulation Calculator

Estimate ovulation window from cycle length

Updated

What is the Ovulation Calculator?

Estimate an ovulation date and fertile window from your average cycle length and, optionally, the first day of your last period. You can also view an estimated next period and due date.

This is a calendar-based estimate, not a way to confirm ovulation. Cycle timing can change from month to month, even when periods are usually regular, so the dates may not match your actual fertile days. The estimate is less useful for irregular cycles.

Use the results as general information, not as a way to prevent pregnancy. All calculations run in your browser.

How it works

This calendar estimate uses the cycle length and luteal-phase length you enter. It cannot confirm ovulation or account for changes in an individual cycle.

  1. Enter the number of days in your average cycle.
  2. Enter a luteal-phase length if you know it; the default is 14 days.
  3. The tool estimates the ovulation day as cycle length - luteal-phase length.
  4. It marks an estimated fertile window from five days before that date through one day after it.
  5. If you enter the first day of your last period, the estimated cycle days are shown as calendar dates. The optional due-date estimate adds 280 days to that date.

These dates are approximate. Cycle length and ovulation timing can change, so use the estimate for general planning only, not for pregnancy prevention or medical decisions.

Examples

Typical 28-day cycle

Calculate ovulation day and fertile window for a regular cycle with standard luteal phase.

Cycle length: 28 days, Luteal phase: 14 days, Last period start: 2026-07-01
 
Estimated ovulation day: 14, Fertile window: days 9–15, Ovulation date: Tue, Jul 14, 2026, Estimated next period: Wed, Jul 29, 2026

32-day cycle with known luteal phase

Estimate fertile days and due date when the luteal phase is tracked.

Cycle length: 32 days, Luteal phase: 12 days, Last period start: 2026-06-15
 
Ovulation day: 20, Fertile window: days 15–21, Ovulation date: Sat, Jul 4, 2026, Estimated due date: Mon, Mar 22, 2027

Frequently asked questions

How accurate is the ovulation calculator?

It estimates ovulation from cycle length and an assumed luteal-phase length. Ovulation timing can vary between cycles, so the result cannot identify the exact day you ovulate or confirm a fertile window.

What if my cycles are irregular?

Calendar estimates are less reliable when cycle lengths vary. An average cycle length can provide context, but it cannot account for when ovulation happens in a particular cycle. A healthcare professional can discuss other ways to track ovulation if needed.

What is the luteal phase?

The luteal phase is the time between ovulation and the start of the next period. Its length varies between people and can vary from cycle to cycle; this calculator uses 14 days by default unless you enter another estimate.

Can the calculator predict a due date?

If you enter a last-period date and turn on the due-date option, the tool adds 280 days to estimate a due date. It is a calendar estimate, not a substitute for pregnancy dating from a healthcare professional.

How do I find my average cycle length?

Count from the first day of one period to the first day of the next. Record several cycles and calculate their average. Averages can help describe past cycles, but they do not predict exactly when ovulation will occur next.

Can I use this calculator to avoid pregnancy?

No. Do not rely on this calculator to prevent pregnancy. Calendar predictions cannot reliably identify all fertile days. Ask a healthcare professional about contraception options.

Is my cycle information stored or sent anywhere?

The calculator's date estimates run in your browser and are not sent to a calculator service.

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