What's the best way to measure body fat?

An apple and pear wrapped in a measuring tape

Being overweight or obese increases your risk of many health problems. But it’s not just the amount of fat: where it is matters too.

Fat around the abdomen (belly) is linked to even higher risks of diabetes, heart and circulatory disease and cancer.

So, how exactly do you measure body fat? There are numerous methods, some better than others. Here’s a guide from our Senior Dietitian Tracy Parker.

1. Weight

A man in jeans stands on the bathroom scales

This is a measure of your overall body mass – including bones, blood, organs and fat. For it to be accurate, you need reliable scales.

If you’re tracking your weight over time, weigh yourself at the same time of day, under the same conditions and on the same of scales. In the morning, after emptying your bladder, is a good time.

The plus side:

Quick and easy with minimal cost.

The down side:

It only measures total body weight – it doesn’t take into account changes in body fat or muscle, and it doesn’t tell you where the fat is. For body fat, you need to use other body composition methods such as skinfolds or smart scales.

2. Body Mass Index (BMI)

How to work out your BMI

BMI is used to work out if you are a healthy weight. It is calculated by taking a person’s weight in kg and dividing it by their height squared. The higher the figure, the more overweight you are and the greater your health risks.

The plus side:

Quick and easy and with minimal cost. And it matters: for most adults, there is a clear correlation between higher BMI and negative health consequences.  As with any weight measure, you need reliable scales,  plus you’ll need a tape measure for height.

The down side:

It can’t differentiate between fat and lean muscle weight. It isn’t very accurate for people who are elderly, pregnant, or very muscular.

3. Waist circumference

A yellow measuring tape

This is a measurement of your waist to check if you are carrying too much fat around your abdomen (belly). You can have a healthy BMI and still have excess abdominal fat, meaning you are still at risk of heart disease, type 2 diabetes and stroke.

The plus side:

All you need is a tape measure. It’s a good way to measure fat round your abdomen.

When measured properly, its accuracy is typically within 5 per cent of the body fat value measured using underwater weighing, which is one of the most accurate ways of measuring body composition. 

The down side:

These are measurements of excess body fat, not a precise measurement of body composition. For an accurate reading you need to know where to place the measuring tape.

Wrap a tape measure around the waist midpoint between the bottom of your ribs and top of your hips. For most people this is just above the belly button.

4. Waist: hip ratio

Apple and pear body shapes

This is the ratio of waist circumference to the hip circumference. The higher the ratio, the more fat is stored around the waist or abdomen – in other words,  an “apple shape”. This shape poses a greater health risk than fat stored elsewhere in the body (a “pear shape”).

The plus side:

All you need is a tape measure and a simple calculation: waist measurement divided by hip measurement. You can use any units as it is only the ratio that is important. High risk is defined as a waist-hip ratio above 0.90 for males and above 0.85 for females.

The down side:

You need to know where to place the measuring tape – measure the circumference of your hips at the widest point of your buttocks. For your waist circumference you need to measure around the waist, midway between the bottom of your ribs and top of your hips.

5. Weight to height ratio - the “string challenge”

A bundle of string, thread and a measuring tape 

This is another way of looking at how much abdominal (belly) fat you have. Measure your height with a piece of string, then fold the length of string that matches your height in half and check to see if it fits around your waist. If it doesn’t, it means you are at increased risk of type 2 diabetes and heart and circulatory disease.

The plus side:

You only need a piece of string (a tape measure will also work). It works for any race, age or gender.

6. Skinfold measurements – callipers or “the pinch test”

A pair of callipers

Skinfold calipers measure the thickness of your subcutaneous fat – the fat underneath the skin – at certain body locations.

Calipers are the cheapest, easiest and most portable method to measure body fat in specific areas.

Using at least three spots on your body – chest, abs and thigh are often used – pinch the skin, pulling the muscle away from the fat and measure the fold with the calipers.

Always test on the same side. It is recommended to take the average of two measurements at each place. You then put those numbers into an online calculator which will give you your body fat percentage.

However in practice it is more useful to use the measurements as a way to monitor body fat over time, rather than having to calculate your percent body fat each time. If your skinfold thickness is going down, then you are probably losing fat.

The plus side:

Calipers are the cheapest, easiest and most portable method to measure body fat in specific areas. You can pick up a set of calipers for £5-£10 online. Professional assessment can be around £50. If it’s body fat you’re concerned about, these probably give the best trade-off between cost, convenience and accuracy of all of these tests.

The down side:

Simple when you know how, but the accuracy depends on the skill of the person taking the measurements. It is best to have the same person take the measurements each time. If you are uncomfortable stripping down in front of the tester, it may not be the test for you. It can be difficult to get reliable measurements if you are obese.

7. Bio impedance – “smart scales”

A set of smart scales with smart phone

Smart scales don't just give your weight, but also a host of body composition stats including your body fat percentage. They can look like normal scales with foot plates, or have additional hand plates.

They work by sending tiny electrical impulses through the body and measuring how quickly they return. This works because the current flows more easily through the parts of the body that are mostly made up of water, such as muscle and blood, than through fat or bone.

The plus side:

Simple and quick. The measurement can be taken as easily as standing on scales. The percentage of body fat is given instantly. Some will also connect to a fitness app so you can track your progress.

The down side:

The reliability of the results can vary – if you’re dehydrated then the amount of body fat will be overestimated. You also need to take the measurements in similar conditions to get reliable and accurate results. They are not suitable for people with pacemakers. Some are relatively inexpensive (£15-50), but some models can be pricey.

8. Hydrostatic weighing (underwater weighing)

A woman floats in water 

Underwater weighing measures your density, which is then used to estimate body fat. Fat is less dense than bone and muscle, so a person with a higher percentage of fat will weigh less underwater, relative to the amount of water they displace, and be more buoyant.

You will need to sit on an underwater mounted chair and scale. Once you have expelled all the air from your lungs, you will be lowered into a tank of water until fully submerged and remain motionless while the underwater weight is measured. 

The plus side:

It is a very accurate technique. Consistent results means that it is a reliable method to measure progress.

The down side:

Most tanks are located at universities or research institutions and may not be open to the public. At around £40 a go, it’s not cheap. It needs considerable commitment, as it requires you to be fully submerged for 5-7 seconds and repeated 2-3 times, so it’s not suitable for the elderly or children.

This method is also not the best for assessing athletes, as they have denser bones. It does not identify the exact parts of the body where the fat is located. It is less common since the introduction of air displacement assessment, which is easier to do.

9. Air displacement - Bod Pod

A woman stands up inside a bod pod

The Bod Pod technology is similar to underwater weighing but uses air instead of water.You are weighed before sitting in a small ‘pod’ machine. By measuring how much air is displaced whilst in the ‘pod’, body density is measured from which body fat can be calculated.

The plus side:

It is safe, non-invasive and fast. It is very accurate with excellent reliability for repeat tests. Unlike underwater weighing, the Bod Pod does not require getting wet. The pod accommodates people of any age, shape and size and is accurate for groups including children, obese, elderly, and disabled people.

The down side:

It is unlikely to be found in your local gym, mainly in research and academic institutions. It can be costly - from £40-60. Some people may find it claustrophobic. The test also does not identify the exact parts of the body where the fat is located.

10. DEXA (Dual Energy X-ray Absorptiometry)

A man sits under a DEXA bone density machine

DEXA is more commonly used to assess bone density, butcan also be used to measure body composition. You lie still on a table while a machine arm passes over your entire body, which emits a high- and a low-energy X-ray beam.

It is considered the gold standard for measuring body composition.

By measuring the absorption of each beam into parts of the body, technicians can get readings for bone mineral density, lean body mass and fat mass.

The plus side:

This method is safe, precise, reliable and quick - around 4 minutes. It is considered the gold standard for measuring body composition. It tells you not just how much fat you’re carrying, but how it’s distributed around your body.

The down side:

The high level of accuracy comes with a price tag. One scan can cost more than £100.

11. The “thigh gap” test

A woman's thigh gap

This is based on the premise if you can stand with your legs together and see a gap between your thighs – ‘a thigh gap’ – you're slim. For some it is seen as a desirable shape and something to aim for, but it is not necessarily healthy or normal.

The plus side:

None. There is no health benefit to having a thigh gap. Whether you have a thigh gap is due to your bone structure and body shape, not if you are thin or not. If your hips are wider in relation to your knees, you will have more curves compared to those with narrower hips, even if you are the same weight.

Also, most women's bodies accumulate either muscle or fat in the thigh area, which can also cause a closure of any gap.

The down side:

Having goals that are purely based on aesthetics rather than health or performance can leave you frustrated and knock your body-image. And it might distract you from using a more sensible measurement (like BMI or waist measurement) instead.

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12. Circumference body fat calculator

A man uses a tape measure to measure the circumference of his tummy

This method to estimate body fat works by entering a number of body circumference measurements, such as waist, hip, forearm, as well as your gender height and weight, into a calculator based on the US Navy body fat formula. There are various websites that will calculate this for you, using the measurements you take.

The plus side:

You only need a measuring tape.

The down side:

For accurate readings you need to know where to place the measuring tape. It is only an estimate of body fat.

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The bottom line:

Expensive, in-a-lab methods are the most accurate; calipers probably give the best trade-off between cost, convenience and accuracy. But if you are just looking to monitor your progress, noticing how your clothes fit is as good as anything too.

 

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