The Ultimate Guide to Calculating BMI

With over 90 years’ experience in providing accurate and reliable weighing scales, including 50 years of supplying UK medical weighing scales, Marsden is here to answer all your questions regarding Body Mass Index with our ultimate guide to calculating BMI.

Read on to find out more.



What Is BMI?

BMI stands for Body Mass Index and is a numerical measure that uses both your height and weight to calculate whether your weight is healthy.

How is your BMI calculated?

Your Body Mass Index can be calculated by dividing your weight in kilograms by your height in metres squared. Alternatively, many people find it quicker, easier and more accurate if you use the BMI calculation function available on many Marsden medical scales and fitness scales.

What are the BMI ranges?

According to the NHS, a BMI ranging between 18.5 and 24.9 is ideal for the majority of adults.

If your Body Mass Index is below 18.5, then you’re in the underweight BMI range. As mentioned, anywhere between 18.5 and 24.9 is a healthy weight, and between 25 and 29.9 is considered overweight. If your BMI is between 30 and 39.9, then you’re in the obese range and over 40 means you’re severely obese.

For children and teenagers between 2 and 18 years old, the BMI calculation also uses age and gender as contributing factors.

How Accurate are BMI calculations?

Calculating your BMI is an accurate way to work out if you’re a healthy weight, for the majority of adults. Different BMI ranges allow for natural body shape variations to be taken into account.

However, it’s likely your GP or healthcare professional will consider other factors too when making an assessment on your weight.

As muscle is denser than fat, it means very muscular people can have a high Body Mass Index without carrying much body fat. Some heavyweight athletes and weight trainers may, therefore, have a high BMI in the obese range, despite being at a healthy weight.

Your ethnic group can also affect the accuracy of your BMI calculation. For Asian adults, a BMI below 25 may be a health concern and mean they are at a higher risk of developing problems related to being underweight, although 25 to 29.9 is considered the overweight range.

Pregnant women should also avoid BMI as a calculation for their weight and instead speak to their healthcare professional if they have any concerns.

Is calculating your BMI the same as working out your body fat percentage?

Your BMI and your body fat percentage are not the same. Body Mass Index is the ratio of your weight to height to calculate if your weight is healthy in terms of the BMI ranges. Whereas, your body fat is the amount of fat you have without including the weight of muscle, bones, water content and some organs.

As we know, two people can have the same BMI as they are the same weight and height, but actually one may have more muscle and the other more body fat.

For a more accurate reflection of your health in terms of weight, it is possible to calculate your body fat using several calculation techniques. Body callipers can be used to measure body fat from different parts of the body which can then be compared using a body fat percentage chart.

Our Body Composition Scales provide numerous readings to help give you a clearer image of how healthy your body is, such as body fat percentage, fat mass, fat-free mass and basal metabolic rate. A full guide to the readings provided by our body composition scale can be viewed here.

This involves a small, safe electrical current that flows through the body from the base of the Marsden scale. As electrical current flows through water, fat and muscle at different speeds, it is then possible for the scale to measure those speeds and generate an accurate calculation of how much of each element your body is composed of.

Body fat percentage measuring scales, underwater density tests and the Dexa Scan, as well as manual formulas below, can also be used.

Women’s Body Fat Manual Formula

(All measurements at the fullest point)

Factor 1 (Total body weight x 0.732) + 8.987
Factor 2 Wrist measurement / 3.140
Factor 3 Waist measurement (at naval) x 0.157
Factor 4 Hip measurement x 0.249
Factor 5 Forearm measurement x 0.434
Lean Body Mass Factor 1 + F2 - F3 - F4 + F5
Body Fat Weight Total body weight - Lean Body Mass
Body Fat % (Body Fat Weight x 100) / Total body weight

Men’s Body Fat Manual Formula

Factor 1 (Total body weight x 1.082) + 94.42
Factor 2 Waist measurement (at naval) x 4.15
Lean Body Mass Factor 1 - F2
Body Fat Weight Total body weight - Lean Body Mass
Body Fat % (Body Fat Weight x 100) / Total body weight

Medical Weighing & BMI Scales from Marsden

At Marsden, we supply a wide range of medical scales for the professional healthcare industry and environments such as hospitals, local GP surgeries, care homes and clinics.

We strive for accuracy and reliability, and after rigorous testing, all Marsden Professional medical weighing scales are Class III approved in accordance with the director of NAWI (Non-Automatic Weighing Instruments) and both UK and EU regulations. Many of our scales are also approved by the MDD (The Medical Devices Directive) who cover medical equipment intended to be used to treat, monitor or diagnose patients.

To discover more about our BMI calculating weighing scales, body composition scales and other medical weighing scales from Marsden, browse our complete medical scales range or call 01709 364296 to discuss your requirements with our friendly and informed sales team.

Image Credits - jcomp

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