How to Calculate Your Pregnancy Due Date in the UK: Step-by-Step Guide

Introduction

Health calculators turn guidelines into numbers that fit your body. They're a starting point for a conversation with your GP, not a diagnosis.

This guide explains how to calculate your Pregnancy Due Date in the UK, step by step. Work out your estimated due date and how many weeks pregnant you are. You'll see the formula, a fully worked example and practical tips for UK readers.

What this calculation covers

Work out your estimated due date and how many weeks pregnant you are.

Understanding your figures can help you set sensible fitness goals, track change over time and know when to ask a health professional for advice.

The method and formula

The due date is 280 days (40 weeks) after the first day of your last period. Your dating scan may adjust this.

Every figure on our pregnancy Due Date Calculator follows this method, so you can check the working yourself.

Worked example

Here's a typical example using these figures:

Running them through the method gives:

The headline figure is estimated due date: Sun, 6 December 2026. Change any input and the result will move with it.

How to use the calculator

1. Enter or choose first day of last period.
2. Enter or choose today's date.
3. Read your results. The main figure is shown at the top, with a breakdown underneath.
4. Try different values to compare scenarios side by side.

Tips for UK readers

• The NHS uses BMI as a screening tool, but waist measurement also matters.
• Estimates like BMR and body fat can vary by 5–10% from lab measurements.
• Track trends over weeks rather than day-to-day changes.
• Speak to your GP or midwife about any results that worry you.

Frequently asked questions

What's the quickest way to work out your Pregnancy Due Date?

Use our free pregnancy Due Date Calculator. It applies this method automatically: The due date is 280 days (40 weeks) after the first day of your last period. Your dating scan may adjust this.

Is this medical advice?

No. These tools give general estimates. For medical advice, speak to your GP, pharmacist or NHS 111.

Can I use these for children?

No. Children's BMI and health measures use age- and sex-specific charts. Use the NHS healthy weight calculator for children.

Why use stone and pounds?

Many people in the UK still think in stone and pounds, so we accept those units and show metric too.

Summary

To work out your Pregnancy Due Date, follow the method above: The due date is 280 days (40 weeks) after the first day of your last period. For a quick, accurate answer with your own figures, use the calculator.

Next, read Your Pregnancy Due Date: Common Mistakes, Tips and FAQs.