Free Ovulation Calculator (+ Guide)

What is an ovulation calculator?

An ovulation calculator estimates when an ovary releases an egg, and the days around it when pregnancy is most likely.

It works from the first day of your last period and how long your cycles usually are.

How ovulation timing works

ACOG's patient guide explains that ovulation comes about 14 days before the next period, and that the days between ovulation and the next period are the most consistent part of the cycle.

So the calculator counts back 14 days from your next expected period. The fertile window runs from 5 days before ovulation to 1 day after.

A worked example

Say a period started on October 1 and the cycle is 28 days.

The next period is due around October 29, so ovulation is around October 15. The fertile window is October 10 to October 16.

What the dates can't tell you

The Office on Women's Health notes that the time from ovulation to the next period can range from 7 to 19 days, so a calendar is only an estimate.

• Using these dates as birth control
• Assuming every cycle is the same length
• Ignoring signs like changes in cervical mucus or a rise in morning temperature

Who is the ovulation calculator for?

It's for anyone tracking their cycle or planning a pregnancy, and these tools help around it.

• Due date calculator – for your due date and week
• Sleep calculator – for bedtimes that end a full cycle
• Water intake calculator – for how much to drink a day

Track a few cycles, and talk to a doctor if they're irregular.