Ovulation Calculator — Predict Your Fertile Window

Ovulation Calculator

Find your most fertile days in seconds — enter the first day of your last period and your average cycle length to get your estimated ovulation day, your 6-day fertile window, and your next expected period on a clear calendar.

Your cycle

Results update automatically as you fill in the fields. Estimates assume ovulation happens about 14 days before your next period.

First day of your last period

Pick the date from the calendar popup, or type it as MM/DD/YYYY.

Average cycle length (days)

Count from the first day of one period to the first day of the next. Most cycles run 21 to 35 days.

–

Estimated ovulation day

–

Fertile window (6 days)

–

Next period expected

Please note: this is an estimate based on the standard calendar method — real ovulation can shift from month to month, especially with irregular cycles, stress, illness, or hormonal birth control. Do not use this tool as contraception. For a medical-grade read, confirm with ovulation predictor kits or your doctor.

100% private

Runs entirely in your browser. Nothing you enter is uploaded anywhere.

No signup

Use it instantly. No accounts, no email, no waiting.

Free forever

No trial limits, no locked features.

How to use the Ovulation Calculator

1Enter your last period start
Pick the first day of your most recent period from the calendar, or type it as MM/DD/YYYY.
2Enter your average cycle length
The number of days from the first day of one period to the first day of the next — 28 is the average, but enter yours.
3Read your results
Your estimated ovulation day, the 6-day fertile window around it, and your next expected period appear instantly, with a highlighted calendar you can screenshot.

Features

Ovulation day estimate

Calculated as last period + (cycle length − 14), the standard calendar-method formula.

6-day fertile window

Shows the five days before ovulation plus ovulation day — the days pregnancy is possible.

Next period prediction

Estimates your next period start so you can plan ahead.

Visual calendar

A real month calendar with fertile days, ovulation day, and period days color-coded at a glance.

Any cycle length

Works for short, average, and long cycles from 20 to 45 days — not just the textbook 28.

Totally private

Your cycle dates never leave your device — no accounts, no tracking.

Frequently asked questions

Is this ovulation calculator free and private?

Yes. It is free, requires no signup, and every calculation happens in your browser — your cycle dates are never sent to a server.

How is my ovulation day calculated?

Using the standard calendar method: ovulation is estimated as your last period start + (your cycle length − 14) days. For a 28-day cycle that lands on day 14; for a 32-day cycle it lands on day 18.

What is the fertile window?

The 6 days when pregnancy is possible: the 5 days before ovulation plus ovulation day itself. Sperm can survive up to 5 days, so the highest chances are the 2–3 days before ovulation.

How accurate is an ovulation calculator?

It is an estimate. Ovulation can shift by a few days month to month due to stress, illness, travel, or hormonal changes, and the calendar method assumes you ovulate 14 days before your next period. For better accuracy, combine it with ovulation predictor kits or basal body temperature tracking.

Can I use it with irregular periods?

You can enter any cycle length from 20 to 45 days, but with irregular cycles the estimate is less reliable — use your longest and shortest recent cycles to see the possible range, and talk to your doctor if cycles vary widely.

Can this be used as birth control?

No. Do not rely on a calendar estimate to avoid pregnancy — ovulation timing varies and sperm survives for days. This tool is for planning and information only.

What does a 21-day or 35-day cycle change?

With a 21-day cycle, ovulation is estimated around day 7; with a 35-day cycle, around day 21. That is why entering your actual average — not the textbook 28 — matters.

What should I do if my period is late?

A late period with unprotected sex in your fertile window warrants a home pregnancy test, taken at least the first day of your missed period for accuracy. If cycles stay irregular, check with your doctor.