Free Conception Calculator (+ Guide)

What is a conception calculator?

A conception calculator works backward to the day a pregnancy most likely began, from a due date or the last period.

This one gives the likely day and the window of possible days around it.

How it works

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

From a due date

ACOG Committee Opinion 700 sets the due date 280 days after the first day of the last period, and conception is assumed about 2 weeks after that day.

So conception is about 266 days before the due date. A due date from an early ultrasound gives the closest answer.

From your last period

ACOG puts ovulation about 14 days before the next period starts. In a 28-day cycle that's day 14, and in a 32-day cycle day 18.

Sperm can live up to 5 days and an egg about 24 hours, so the window runs from 5 days before ovulation to 1 day after.

A worked example

Here's the example the calculator loads: a due date 150 days from today.

Conception most likely happened 116 days ago, in a window from 121 to 115 days ago, and the last period started 130 days ago.

Mistakes to avoid

These are the slips that come up most.

• Treating the likely day as exact, when the window spans 7 days
• Using a 28-day cycle when yours is longer or shorter
• Using the date of a positive test, which comes days after conception
• Trusting a period-based date over an early ultrasound

Who is the conception calculator for?

It's for anyone wondering when a pregnancy began, and these tools help around it.

• Due date calculator – for your due date and milestones
• Pregnancy week calculator – for how far along you are
• Ovulation calculator – for your fertile days
• Period calculator – for your next periods

Take the window to your doctor or midwife, and let their dating be the one you go by.