Free Period Calculator (+ Guide)

What is a period calculator?

A period calculator predicts when your next periods will start, from the first day of your last period and the length of your cycle.

This one also lists each cycle's likely ovulation day and fertile days, and averages your cycle from past dates.

How it works

The calculator works it out on your device, and your dates stay there.

How long is a normal cycle?

The Office on Women's Health says a typical cycle is 28 days, counted from the first day of 1 period to the first day of the next.

Cycles that come every 24 to 38 days are still regular, and they can change from month to month.

Ovulation and fertile days

ACOG puts ovulation about 14 days before the next period starts, and calls that the most consistent part of the cycle.

An egg lives about 24 hours and sperm about 3 days, sometimes up to 5. So pregnancy is possible from 5 days before ovulation to 1 day after.

A worked example

Here's the example the calculator loads: a 28-day cycle, a 5-day period, and a last period that started 10 days ago.

The next period starts in 18 days. Ovulation is likely 4 days from today, with the fertile days from yesterday to 5 days from now.

Mistakes to avoid

These are the slips that come up most.

• Counting a cycle from the last day of bleeding instead of the first
• Using a calendar estimate as birth control
• Assuming every cycle matches the average
• Ignoring cycles that are often shorter than 24 or longer than 38 days

Who is the period calculator for?

It's for anyone who wants their next few periods on a calendar, and these tools help around it.

• Ovulation calculator – for your fertile days in detail
• Conception calculator – for when a pregnancy began
• Due date calculator – for a due date from your last period
• Water intake calculator – for a day's water

Track a few cycles, and the dates get closer to your own rhythm.