How Many Calories Should I Eat When Dieting?

Why Do Calories Matter?

Calories are just our way of estimating the amount of energy we get from eating. We also use them as a measure of how much energy the body uses up just existing, as well as what we use during exercise.

Our body uses calories keeping our muscles intact (more muscles = more calories burned), as well as running everything internally, such as the brain, heart, lungs and so on.

Generally speaking, if we eat more calories than we use in a day, we put on weight. The body will use calories for new muscle and glycogen stores within the muscles if you have done the right exercise recently, and will store any excess calories beyond that as fat.

If we eat less calories per day than the body needs to maintain itself, it will start to use up energy it has previously stored. Usually this means burning some fat and cannibalizing some muscle for energy. However, with the right exercise and regular protein, the body is encouraged to keep the muscle and remove just fat instead.

Choosing the right amount to cut

If you reduce your calories too little (say 5%), then the body quickly adapts and you won’t lose any fat. In fact, you can actually get fatter if you do this for long enough.

Reduce your calories too much (40%+) and the body goes into starvation mode, hangs on to as much fat as possible and cannibalises your muscles instead. Your body fat percentage can actually go up!

20-30% deficit is the sweet spot.

There is always an element of muscle loss when in calorie restriction, so the key is to keep that to an absolute minimum through strength training and eating lots of protein

How many Calories should I aim for?

Everyone is different, but we can get a reasonable estimate of your body’s needs by looking at population averages. Firstly we look at the Calories your body needs if you don’t do any exercise at all – if you just lay in bed all day. This is known as the Resting Metabolic Rate (RMR).

There are different ways of working this out, but I recommend the Katch-McArdle Formula:

RMR = 370 + ( 21.6 x Lean Mass in Kilogrammes)

Your lean mass (different to your muscle mass!) is your body weight minus your fat mass. In other words, it’s the weight of everything except the fat in your body.

For example, if you weigh 70kg, with a body fat percentage of 30%, then your lean mass is the remaining 70%, which is 49kg:

Lean Mass = 0.7 x 70 = 49.0kg

If you have 49 kg of lean mass, then your Resting Metabolic Rate will be

370 + (21.6 x 49) = 1,428 Calories per day.

Next we factor in how active you are. All you do is multiply your Resting Metabolic Rate by an activity level:

1.2 – Sedentary: Little or no physical activity.
1.4 – Lightly Active: Light exercise or activity 1-3 days per week.
1.55 – Moderately Active: Moderate exercise 3-5 days per week.
1.7 – Very Active: Hard exercise or activity 6-7 days per week.
1.9 – Extremely Active: Hard daily exercise or activity and physical work

Ideally you will be exercising 5 times per week (moderately active), so we normally use 1.55 as our multiplier.

Continuing the example above, your daily maintenance Calorie level would be

1428 x 1.55 = 2213 Calories per day.

Using that number we can also calculate your Calorie Restriction target for cutting (-20%):

Cutting = 2213 x 0.8 = 1,770 Calories per day

It’s all guesswork

Although we use very precise numbers, we don’t really know how many calories we actually eat, how many are absorbed, how much we use exercising, breathing, thinking etc. Therefore to do a proper cut, you need to adjust your Calorie target over time, depending upon the results you are getting; it is trial and error based upon an educated guess as a starting point.

What if I don’t know my Lean Mass yet? 

If you haven’t weighed yourself, then you can just pick a starting number and try it for a week or two. If you aren’t losing body fat, just reduce it some more!

For women, try 1500 Calories per day, for men, try 1750.

I can’t be bothered to do the maths

That’s fine – get me to do it for you! Either get me to do a weigh in with you, or weigh yourself and send me the numbers.

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