BMR Calculators and the Activity Factor for Weight Loss

Jonathan brought up a good question yesterday about BMR calculators and the activity factor that some BMR calculators add on.

Natures Lumber Jacks

BMR calculators are based on various known assumptions and measurements about your lean body mass and how many calories your lean body mass will burn. These calculators usually need your height and weight and sometimes your age and other things like your wrist or waist measurements.

You can get a decent estimate of your BMR if you used 3-4 calculators and take the average of these.

BUT, there is also an activity factor that some of them suggest. The problem with these activity factors is that they are based on very old research that used lumberjacks as their example for daily activity. Needless to say, a day of lumberjack work is a ton of activity and will burn way more calories than any workout you or I can do after work.

The point is the activity factor that these calculators use is not accurate or realistic to any workout you or I will probably be doing. In fact, assuming that you’re burning less calories in the gym than any calculator suggests is a good bet if weight loss is your goal.

Forget about activity factors and just assume that the total amount of calories you can burn in a day comes exclusively from your BMR. From there just assume any extra calories you burn in the gym will be a bonus.

When it comes to calorie burning and fat loss it will always serve you well to err on the side of less than more. In other words the following 3 assumptions will be the only way to use BMR calculators and food labels to your advantage:

1. Assume that your activity burns less calories than the cardio machines or calculators say

2. Assume your BMR is less than the calculators say

3. Assume that food labels are always underestimating the actual amount of calories in that food

Making these 3 assumptions will be the best way to keep you a step ahead in the calories in vs out game.

John