The Math of Weight Loss

These weight loss tips are from my Sports Nutrition class at University of Alberta:

Before deciding on weight loss & an appropriate goal (given appropriate time) check your BMI. The general formula is weight (kg) / (height (m))^2
Normal range for a healthy BMI is 18.5 to 24.9
Keep in mind that this calculation is validated on the general population; elite endurance or strength athletes with lots of lean muscle will not get accurate BMI results.


First of, understand the energy balance needed for weight loss:

Calories out (ie. exercise) needs to be greater than energy in (ie. food energy). So while physical activity is essential for weight loss, nutrition in my opinion plays an even larger role.

To lose 1lb of body weight, you must have a deficit of 3500 calories. (Ie. energy out is greater than energy in by 3500 calories over 1 week.) Over one week, 1 lb lost would be a 400-500 calorie deficit per day. To lose 2 lbs/week, a deficit of 800-1000 calories is required.

Losing 1-2lbs per week is a fantastic weight loss goal that is sustainable & achievable over the long term. The ACSM strongly recommends not to have calorie deficit of greater than -1000 calories per day. At the very minimum, ACSM recommends consuming at least 1200 calories /day.

Energy in

But, to know what your "deficit" or energy out should be to lose 1 lb/week, you need to know what your energy in must be to support your body & activities. To estimate your resting metabolic rate (energy your body needs to function) we use the Mifflin equation:

RMR = (10 x weight) + (6.25 x height) - (5 x age) - Gender factor
(Females subtract 161, males subtract 5)

This RMR number is the calories needed to function per day, without physical activity. If you exercise, or are on your feet all day, it may be helpful to multiply your RMR by an activity factor.

Rest = 1.0
Very light activity = 1.5
Light activity (eg. slow walk) = 2.5
Moderate activity (eg. brisk walk) = 5.0
Heavy activity (eg. running) = 7.0

To be even more specific, think of a typical day and multiply each applicable activity factor by the number of hours you spend doing that.
For example, I may sleep 9 hours, work at a desk 8 hours, run for 2 hours, and do light housework for 5 hours.
9+8 = rest time = 17hrs x 1 = 17
2 hrs = heavy activity = 2hrs x 7 = 14
5 hrs = very light activity = 5hrs x 1.5 = 7.5
Sum of activity factors (for 24 hours) = 17+14+7.5 = 38.5
Divide activity factor sum by 24 = 38.5/24 = Avg. activity factor of 1.6

Use your average activity factor to multiply to your RMR number. This final number is calories in required to maintain body weight at your activity level.
To lose weight, subtract 400-500 calories. You should be losing 1lb/week at this rate.

Energy out

To be at a 400-500 calorie deficit, you can increase energy out (increase physical activity) or decrease energy in (decrease food calories).
Tip for physical activity for fat loss: while low intensity long duration exercise is known to burn more fat than carbohydrate (sugars) in proportion, the total calories burned is less than at high intensity , shorter duration exercise. So up the intensity (best at 70% of capacity) & work out until you're exhausted and know you burned more calories than a slow 2 hour jog.

Remember, losing more than 2lbs/wk of body weight can be detrimental to your health! Take it slow & but work hard.

Best of luck!