I’m going to discuss two equations in this piece, they are both used to calculate daily calories (aka Total Daily Energy Expenditure or TDEE): They are called the Katch-McArdle Formula, and The Mifflin-St Jeor Equation. Most of the automated calorie calculators that you find online use these two methods. But, before we can get to those, we will need to look at the activity level multipliers (ALM) because they apply universally to both methods.
The Activity Level Multipliers (ALM) approximate the number of calories burned through additional daily activity (or the additional energy you need to do your every day tasks: such as walking, talking, working, or exercising).
On the other hand, the two equations below will calculate your basal metabolic rate (the daily calories needed just to run your body in rest, with no additional activity). This base rate value is then multiplied by the ALM that best matches your personal lifestyle to get an approximation of your daily maintenance calories, or TDEE.
The Katch-McArdle Formula
This formula is often considered the best, especially for lean individuals. It functions the same for men and women, unlike other calorie calculations that require a different equation per gender. One of best features of the Katch-McArdle Formula is that it utilizes % Body Fat, which is a great indicator of the fitness condition that an individual is in. This is, unfortunately, also the formula’s biggest downfall, because most people do not know their % Body Fat. However, it can be somewhat estimated by photo comparison (google image search), or you can learn how to measure at home (with a google search), or you can have it measured professionally (local universities often offer these services at their health centers, or you can ask your doctor).
The BMR is then multiplied by the activity level multiplier to get your TDEE, this magic number is the calorie goal to maintain your current weight and activity level.
An Example of the Katch-McArdle Formula
A 200 lb individual with 20% body fat who is lightly active.
First, convert 200 lbs to 90.7 kg (google).
(370+(21.6*(90.7 * (1-0.20))))*1.375
Solving the above equation gets us 2664 Calories. This is how many calories this individual needs to take in each day to maintain their current weight and activity level. If this individual wanted to lose about a pound a week, then they would set a 20% deficit on this number: 2664*0.80= 2131 calories. Alternatively, one could also increase activity level to lose weight, or set a caloric deficit and increase activity level to meet one’s needs.
The Mifflin-St Jeor Equation
This equation is quite popular because most people readily have access to all of the information required, unlike the Katch-Mcardle formula which requires % body fat, the Mifflin-St Jeor equation only requires common factors like weight, height, age, and genetic gender.
Remember your order of operations (PEMDAS).
Note that weight and height are in metric units: if you use the imperial system, then you will have to convert from pounds to kg, and from feet/ inches to cm.
This equation is also calculating you BMR, or resting state calories, so use this to calculate your TDEE then you will need to multiply the final result by the Activity Level Multiplier that best describes your daily activity level.
Let’s use the same person from the example above, but we will need a bit more info.
Weight: 200 lbs or 90.7 kg (google)
Height: 6’2″ or 188 cm (google)
That’s a TDEE of 2650 calories for the individual in this example. If this individual wants to lose weight then they can determine what 80% of their TDEE is (a 20% deficit) and set that as their daily goal: 2650*0.80=2120 Calories per Day.
Which Method Gives the Best Result?
As we can see above, the Mifflin-St Jeor Equation gave us a very similar result to the Katch-McArdle Formula (2650 vs 2664): both of these methods are perfectly capable of approximating your daily caloric needs. Just remember that they are doing precisely that, approximating. There is no method out there to tell you exactly how many calories you need every day, and every day is a little different. Just aim for your goals, and don’t stray too far off the path.
If the calorie goal you calculate doesn’t work for you, then modify your number as needed. There is no guarantee that these methods will give you a TDEE value that matches the reality of your situation. There are so many conditions that can effect your true TDEE: Heart-Disease, Diabetes, Hypo/Hyper-Thyroidism, Fatty-Liver Disease, Pregnancy, Breast-Feeding, Postpartum, Menopausal, Pre-Menopausal, and so many more. So tinker with it, and find what works best for you.
If after reading this you have decided that calculating out your calories by hand just isn’t your thing, then my favorite calculator out there is at tdeecalculator.net. This will calculate your daily calories for you using the methods described above.