How Do Scales Calculate BMI?

As obesity becomes an increasingly bigger problem in the UK, many steps are being taken to encourage people to take their health into their own hands.

BMI scales are a simple and easy way to see your body's overall health, but how do they calculate BMI?

What is BMI?

BMI stands for Body Mass Index and is used to determine if you’re a healthy weight for your height. It’s one of the most common screening tools as you only need your height and weight to calculate it.

The formula for calculating BMI is a person’s weight in kilograms divided by the square of their height in metres. Or use our free BMI chart to identify your BMI without having to calculate anything.

It’s worth noting that BMI is not completely reliable, however. It’s easy to calculate, which is why it’s so widely used, but it does not account for body fat, age, sex, ethnicity, pregnancy or muscle mass.

What Is a Healthy BMI?

A healthy BMI ranges between 18.5 and 24.9 for the majority of adults.

If your BMI is below 18.5 then you’re classed as underweight. A BMI between 25 and 29.9 is considered overweight, 30 to 39.9 is the obese range and anything above 40 is severely obese.

The NHS has an online BMI calculator to help you work out your BMI and find out if you’re in the healthy range.

How Do Scales Calculate BMI?

As with the standard BMI calculation, BMI scales take your body weight and height into account when calculating your BMI.

This can be done by manually inputting your height, using an automatic height measure or letting the scale do all the work for you.

The scale then takes the information provided and calculates your BMI for you.

Are BMI Scales Accurate?

Some scales utilise Bioelectrical Impedance Analysis (BIA) to calculate your BMI, like body fat scales.

But, this is not very accurate for a floor scale.

BIA is a weak electrical charge that is sent around your body to test its resistance. Then, using complicated formulas, the scale calculates various variables based on your body’s resistance to the electrical charge. They may not be able to correctly work out the height of your body without you inputting it, which skews the results from the start.

With Marsden’s BMI scales, you enter height manually using a numerical keyboard on the indicator. Our accurate Class III approved medical scales take your weight, while the attached height measures let you accurately work out your height.

This way you get accurate, reliable BMI results.

Benefits of Using BMI Scales

The main benefit of using BMI scales is to track your health. Knowing your Body Mass Index can help you work out if you’re putting yourself at risk of health conditions, such as diabetes and high blood pressure, because of your weight.

Another benefit is that you can use these scales in the comfort of your own home. That allows for a more regular check than waiting to visit your GP.

One final benefit is that by having a BMI scale to use at home, you can easily check the effectiveness of your training regime or diet.

Disadvantages of Using BMI Scales

There are a number of ‘BMI scales’ on the market that do not accurately check BMI. They can provide inaccurate results, which negates the point of having a BMI scale as you will not actually know if you’re in an unhealthy range.

Additionally, as mentioned above, BMI is not a completely trustworthy calculation. There are a number of factors it doesn’t take into account, which can affect the final result.

What Is Better: Body Fat or BMI?

One of the most common questions is whether to calculate body fat or BMI.

BMI is seen as one of the easiest methods for determining overall body health. However, because it doesn’t take body fat and muscle mass into account it can be misleading. Muscle mass is heavier than body fat and, therefore, you can have an overweight BMI but actually be healthy.

There are a number of methods for calculating your body fat, but the best value for money is a body fat scale. Marsden’s body composition scales calculate body fat as well as a wide range of other readings to give you a complete image of body health.

Marsden BMI Scales

When choosing a BMI scale, it’s important to choose a scale that provides accurate results every time. All of the scales listed below are Class III Approved and therefore suitable for use in a medical environment. This means you can count on them to be reliable and accurate.

Marsden M-550

The Marsden M-550 is our lowest priced floor scale with a BMI feature. It’s Class III Approved, can weigh up to 160kg and has graduations of 200g up to 100kg. It’s highly reliable and is a favourite of GPs across the UK because of its robust aluminium structure.

In addition to BMI, this scale also features hold and tare functions and is covered by the Marsden 4 Year Warranty. This scale is battery powered with up to 100 hours’ continuous use.

Marsden M-430

At £175, the Marsden M-430 Floor Scale is a bit more expensive than the M-550. It has a higher capacity of 220kg, but graduations are still precise at 200g. It’s lightweight and robust and is used by medical professionals around the world.

As with the M-550 it features Hold, Tare and BMI functions and is also Class III and MDD Approved.

Marsden M-125

For a complete package when calculating BMI, choose the Marsden M-126 Column Scale with Manual Height Measure. This scale has the largest capacity out of the scales mentioned and can weigh up to 250kg; it’s also more precise with graduation of just 100g for an exact result.

It includes all the features of the previous scales, as well as a Body Surface Area (BSA) feature. The attached height measure has 1mm increments and can measure up to a height of 2m.

By choosing a scale with a height measure attached, you are ensuring that your BMI calculations are as accurate as possible.

As with all the scales in this blog post, the M-125 is Class III and MDD Approved and is included in the Marsden 4 Year Warranty.

Further Reading

Do you want to start calculating body fat instead of BMI? Use this helpful guide to work out how to measure body fat and find out the healthy body fat ranges.

We briefly body composition scales in this blog post. Find out how accurate they are in this blog post.

Obesity currently affects one in three people in Britain, but it’s expected to affect half of Britons by 2045.