Understanding your weight and BMI

Body Mass Index (BMI) is one way of looking at whether your weight is in a healthy range. But it's not the full picture.

This article explains how BMI works, why it's used in the NHS, and what other factors matter when it comes to your health.

What is BMI?

BMI is a number that compares your weight to your height. It’s calculated by dividing your weight in kilograms by the square of your height in metres.

You don’t need to do this yourself; Aide or your healthcare team can help track this for you.

Here’s how the NHS classifies BMI for most adults:

• Below 18.5 – underweight
• 18.5 to 24.9 – healthy weight
• 25 to 29.9 – overweight
• 30 or more – obese

What’s good about BMI?

BMI is a simple and widely used tool. It gives a quick overview of possible health risks, especially when combined with other measures like blood pressure or blood sugar levels.

The NHS uses BMI to decide when someone might benefit from extra support or treatment. For example, access to weight management services or certain medicines.

What are the limits of BMI?

BMI doesn’t tell the whole story.

It doesn’t measure fat directly or show where it's stored in your body. It doesn’t reflect your fitness, muscle mass, or how healthy your eating and activity habits are. For example:

• Athletes or people with high muscle mass may have a high BMI but low body fat.
• People with a “normal” BMI can still carry harmful fat around the stomach.
• BMI doesn’t account for age, gender, or ethnicity.

For some ethnic groups, health risks can start at a lower BMI. For example, people from South Asian, Black, or Chinese backgrounds may be at risk of diabetes or heart disease from a BMI of 27.5.

Your healthcare team may consider this when planning the right care for you.

Waist size and other health checks

Waist measurements can be a useful extra tool. Carrying more weight around your middle (called visceral fat) is linked to a higher risk of conditions like type 2 diabetes and heart disease.

A healthy waist size is generally:

• Less than 94cm (37 inches) for men
• Less than 80cm (31.5 inches) for women

Your clinician may also consider your blood pressure, cholesterol, HbA1c (blood sugar levels), and other health markers.

What does this mean for you?

BMI is just one measure. It doesn’t define you, and it’s not the only way to understand your health.

You don’t need to aim for a “perfect” number. Even small changes in weight, waist size, or daily habits can make a big difference to your long-term health.