Many people come to Bodyscan to discover their resting metabolic rate (RMR) and, from there, their maintenance calories and a daily calorie target to lose fat (or gain muscle).

Of course, there are dozens of calorie calculators online, like this one, that use a variety of formulae to arrive at an estimate of your RMR.

How accurate are most calorie calculators?

The problem with all of them is that they are restrained by having to use your total weight as one of the inputs. Most of them use age, sex, weight and height.

By using overall weight and ignoring body composition, the calculator has to assume that you have a ‘typical’ amount of fat and muscle for your age. But if you have low muscle mass, the RMR result will be out by quite some margin.

That’s because most of your calorie-burn is performed by your lean body mass – muscles and vital organs – so if you have low lean mass you will burn fewer calories than someone of the same weight but with more muscle. The calculator doesn’t know how much lean mass you have, it just assumes an amount based on your total weight.

A DEXA scan offers a more accurate calorie calculator

With a DEXA scan, we can use a much better formula, because we know exactly how much of you is lean mass, fat and bone.

Whichever calculator you use, though, don’t make the classic mistake of treating the RMR or maintenance result as sacrosanct or immutable. The number is just a guide – usually a very good one – but is not calorie-perfect because we are all unique.

Many people return for a DEXA progress scan after a few months, having lived according to the calculator result, but without any change in weight on the scale (which they can track at home) or in body composition (which we measure with DEXA).

If you are not losing weight in a deficit then you are not actually in a deficit! Therefore either the calculator result is wrong or you are consuming more calories than you think or you are less active than you think. Or all three! So don’t make your lifestyle fit the number; instead, live your lifestyle and see if the number is right.

Assuming our DEXA calorie calculator is on target (and it almost always is) we can prove the calculator is working and giving the right result by precisely tracking changes in body fat between each scan. The video below explains the advantages of using DEXA to calculate your RMR and maintenance calculator.

As a real-life example, our calorie calculator estimates a Bodyscan customer, John, to have an RMR of 2055 calories and maintenance of 2826 calories a day based on ‘light activity’.

It also says (below) that to lose 10kg in 20 weeks (half a kilo a week), John should be in a deficit of 550 calories a day, meaning a daily target of 2276 calories (2826 minus 550).

John decides to return for a progress scan in 12 weeks and has lost 4kg of body fat. So, is he on target?

Clearly not! At a pace of half a kilo per week, he should have lost 6kg of fat. The 4kg fat loss equates to a daily deficit of just 367 calories (two-thirds of the 550).

The DEXA calorie calculator is working, it’s usually you who is not!

Now, as we suggest above, if we assume the calculation of RMR is ‘on the money’, then John has not been sticking to his calorie plan. While obviously in a deficit (because of his weight loss and fat loss), his deficit is only two-thirds of what it should be. This is usually due to inconsistency and over-consumption at weekends.

The other very likely factor is that he is not as active as he believes. Bodyscan customers often talk about activities they will do, intend to do, should do, often do, plan to do, try to do, etc, and not what they actually and consistently do. There are 168 hours in a week, so if your only activity is three one-hour gym/exercise sessions then face up to it – you are sedentary.

With Bodyscan’s DEXA calorie calculator, we are very confident that it works for more than 95% of the population. When customers’ results aren’t as forecast or intended, it is almost certainly because they have not consumed the target number of calories or maintained the level of activity they claimed.

If John had lost 6kg of body fat in 12 weeks then we would feel sure of two things – that our calculator was accurate and that John had followed the programme correctly.

To get a very good idea of your own RMR and a fat-loss schedule, book your first DEXA scan here. We have two great locations, in Marylebone and the City.