Whether you're trying to lose weight, training for fitness or simply keeping track of your physical health, the trusty bathroom scales can help you monitor your progress. But weight is only one factor in determining your health. It's more important to know your body composition – your lean muscle mass and how much fat you're carrying.

Body fat scales can be a better indicator of overall health than their conventional weighing scale cousins.

How do body fat scales work?

Body fat bathroom scales work by sending a very low electrical current through your body via your feet. Tissue containing a lot of water, such as muscle, lets the current through easily, but fat contains comparatively little water, so it resists the current – the higher the impedance, the more fat there is in your body.

The scales use that data, together with personal data you enter such as your height, age, sex and fitness level, to calculate your body-fat percentage.

CHOICE tested 12 body fat scales and found that all the scales tended to under-read body fat percentage, which could make you think you're much healthier than you really are.

Who shouldn't use body fat scales?

Most models' instructions warn they aren't suitable for people with pacemakers as the scales' electrical current could interfere with the pacemaker. 

Many also warn the readings can be unreliable for children, athletes and bodybuilders, people with metal plates or screws in their bodies, and pregnant women.

What is a healthy body fat percentage?

A healthy body fat range will differ based on age and gender, so when you pre-set your details for your body fat scale, you'll want to get them right.

Recommended body fat percentages
Gender Age Low Recommended High Very high
Female 20 – 39 5 – 20% 21 – 33% 34 – 38% > 38%
40 – 59 5 – 22% 23 – 34% 35 – 40% > 40%
60 – 79 5 – 23% 24 – 36% 37 – 41% > 41%
Male 20 – 39 5 – 7% 8 – 20% 21 – 25% > 24%
40 – 59 5 – 7% 11 – 21% 22 – 27% > 27%
60 – 79 5 – 12% 13 – 25% 26 – 30% > 30%

How to use them

As with conventional scales, it's best to measure yourself regularly and record the results rather than rely on one-off or occasional measurements.

  • Make sure your feet are bare and clean for good contact with the scales' sensor pads.
  • Put the scales on a hard, level floor.
  • Don't take a reading immediately after waking, after a meal, or for 24 hours after excessive exercise or alcohol intake. Your body's water content could be uneven or atypical, which will make the reading unreliable.
  • Measure yourself at the same time of day under the same conditions.
  • Check with your doctor before using a body fat scale if you have a pacemaker as the electrical charge sent through the scales could have a negative impact.

Don't take the displayed values as absolute truth on any of the tested models. The best ones have a difference of about 3-4%, so, if they say your body fat percentage is 25%, the real number is likely to actually be +/- 3-4%. Nevertheless, they're still good for tracking physical changes over time.

What to look for

Display

This should be big and clear so it's easy to read when standing on the scales. Some have a wireless display unit that can be mounted on the wall for easier viewing.

Displayed data

This should be easy to understand and stay on screen long enough to read easily. Some models flash the information past quickly, so it's hard to follow. Ideally, you should be able to redisplay the data without needing to weigh yourself again.

Controls

These should be clearly labelled and easy to use. Programming the scales should be straightforward.

User profiles

There should be enough profiles (saved sets of individual information) for all household members who will regularly use the scales. A "guest" profile can be handy for visitors who want to use the scales but don't need their profile stored permanently.

Instructions

These should be easy to understand, with useful diagrams and advice on how to interpret your results.

Weight only

If you want the scales to behave like simple weight scales and only display your weight, a "weight only" default setting is handy. Most of the scales have this option.

Usable on carpet

Useful if you don't have hard floors. Scales don't generally measure weight effectively when placed on carpet.

Other features

Many of the models on test display other information, such as BMI and body composition information (muscle mass, bone mass and water), which could be useful if you're trying to build up muscle, for example. Some also display goals that are based on your personal information (age, height and sex) and measured weight and body fat (such as a suggested goal weight or daily kilocalorie intake).

Body Mass Index (BMI) is often used by health professionals to assess whether a person is underweight, normal weight, overweight or obese. While BMI is a better indicator of health than weight alone, it's far from perfect. The BMI categories don't allow for very muscular individuals; a person with lots of lean muscle and low body fat will be heavy for their height and could be classed as overweight or obese. Similarly, some people may have a BMI that indicates they're healthy when in fact they have too much body fat and little lean tissue.

You should only use BMI as a general guide to your state of health. Accurate analysis of your body fat percentage, measured over time, is more useful.

Cost

Body fat bathroom scales range from around $25 to $250.