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Ovulation Calculator

Your fertile window and ovulation day for the next six cycles.

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The next six cycles

CyclePeriod startsFertile windowOvulation

Chance of conception by day

Why this counts backwards

The luteal phase — from ovulation to the next period — stays fairly consistent at around fourteen days whatever your cycle length. The follicular phase before ovulation is the part that varies. So ovulation is best found by counting back from the next expected period, not forward from the last one. On a 32-day cycle that puts ovulation near day 18, not day 14.

Next ovulation

—

This cycle

Fertile window—
Most fertile days—
Cycle day today—
Next period due—

—

Until the fertile window opens

Calendar prediction is reasonable for regular cycles and poor for irregular ones. Ovulation predictor kits, basal body temperature and cervical mucus tracking all give better evidence. Calendar methods are not reliable contraception. Nothing you enter here is stored or sent anywhere.

Quick Tool Overview

The Ovulation Calculator finds your likely ovulation day and the fertile window around it, then projects the same pattern across the next six cycles. It counts backwards from the expected period, which is more reliable than counting forwards from the last one.

What it does
Predicts ovulation day and the fertile window
You give it
Your last period date and average cycle length
You get back
Ovulation day and fertile window for six cycles ahead
Best for
Trying to conceive and understanding your own cycle
Calculators Free, unlimited use No sign-up Runs in your browser

About the Ovulation Calculator

Work out when you are most likely to ovulate and the fertile window around it, then project the same pattern across the next six cycles. Both a regular and a variable cycle length are supported, since a fixed 28 days is the exception rather than the rule.

The calculation works backwards rather than forwards, which is the part most people get wrong. The luteal phase — from ovulation to the next period — is fairly consistent at around fourteen days regardless of cycle length, whereas the follicular phase before ovulation varies considerably. Counting back from the expected period is therefore more reliable than counting forward from the last one, and the fertile window opens about five days before ovulation because sperm survive that long.

Key features of the Ovulation Calculator

Counts back, not forward

The luteal phase is consistent at around 14 days while the follicular phase varies, so counting back from the next period is the accurate method.

Six cycles projected

The same pattern is mapped forward, so you can plan more than one month at a time.

Adjustable luteal phase

Fourteen days is the default, but if tracking has given you a different figure you can use it.

The full fertile window

Five days before ovulation plus the day itself, because sperm survival is what defines the window.

Clear about its limits

The page states that calendar methods are unreliable for irregular cycles and are not contraception.

How to use the Ovulation Calculator

  1. 1

    Enter your last period

    The first day of bleeding, not the last.

  2. 2

    Set your cycle length

    The average number of days from the first day of one period to the first day of the next.

  3. 3

    Adjust the luteal phase

    Fourteen days is the default. If tracking has given you a different figure, use it.

  4. 4

    Read the calendar

    Ovulation day, the fertile window, and the same projection for the next six cycles.

Frequently asked questions

Roughly 14 days before your next period starts, not 14 days after the last one began. On a 32-day cycle that is around day 18, not day 14.

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