Use Mathematics to Lose Weight the Scientific Way


This article deals with the mathematics of weight management, and in particular, how to make use of calculations to accurately plan and achieve weight loss. It does not deal with fad diets, magic diet pills or specific exercise regimes. How you choose to achieve your target calorie intake through diet and/or exercise is up to you. This guide will show you how to calculate and monitor your weight management calorie goals.

The first step in preparing to lose weight should be understanding your body and it’s current state. Measure your height and weight and consider how you would describe your current level of activity. Be completely honest with yourself. All of these factors will be used to quantify your calorie needs and plan for losing weight over the next few weeks.

How many calories does your body currently burn in a day?

Your Basal Metabolic Rate (BMR) is a measure of how many calories your body uses up while completely at rest. It will be our first calculation and it involves your weight, height and age.

BMR formula for women

– Metric measurements, kg and cm
BMR = 655.1 + (9.563 x weight) + (1.850 x height) – (4.676 x age)

– Imperial measurements, pounds and inches
BMR = 655 + (4.35 x weight) + (4.7 x height) – (4.7 x age)

BMR formula for men

– Metric measurements, kg and cm
BMR = 66.5 + (13.75 x weight) + (5.003 x height) – (6.755 x age)

– Imperial measurements, pounds and inches
BMR = 66 + (6.2 x weight) + (12.7 x height) – (6.76 x age)

BMR does not, in itself, take into account your individual level of physical activity. So it cannot be used purely on its own to determine the number of calories required to maintain your current weight. People with the same BMR but different levels of daily activity will have differing daily calorie requirements to maintain current weight.

To calculate daily calorie requirements a multipler is applied to BMR in accordance with the Harris Benedict Formula, as follows:

Little/no exercise: BMR * 1.2 = Daily Calories
Light exercise: BMR * 1.375 = Daily Calories
Moderate exercise (3-5 days per week): BMR * 1.55 = Daily Calories
Heavy exercise (6-7 days per week): BMR * 1.725 = Daily Calories
Very heavy exercise (very active, every day): BMR * 1.9 = Daily Calories

By now, you should have a number written down, or on your calculator, which will be your BMR after it has been adjusted by the Harris Benedict Formula. This figure is the number of calories you need to consume (based on your current activity level) on a daily basis to maintain your current weight.

So in other words, if you consume that number of calories and maintain your current level of physical activity, averaged over a period of time, you should be exactly the same weight at the end as you were at the beginning.

How do I lose Weight?

If we look at the calculations above for maintaining current weight, there are two avenues for those looking to lose weight.  This is best demonstrated with an example, but essentially it comes down to creating a calorie deficit from the number of calories required to maintain your body weight.

Jane is a 30 year old office worker.  She drives to work and does not do any exercise when she gets home from work (Little/no exercise).  She currently weighs 120.9Kg (266 pounds or 19 stone) and is 180cm (71 inches or 5 foot 11 inches) tall.

Jane’s BMR is 655.1 + (9.563 x 120.9) + (1.850 x 180) – (4.676 x 30) = 2004

Jane’s Harris Benedict adjusted BMR is 2004 * 1.2 = 2405

To maintain her current weight, Jane can consume 2405 calories on a daily basis, without changing her level of activity.

Jane can lose weight in one of the following ways:

– By monitoring the food she eats, Jane reduces the numbers of calories she consumes on a daily basis.  Her activity levels will remain the same, but she decides to try to consume 400 less calories on a daily basis.  By reducing her daily calorie intake from 2405 to 2005 and leaving her activity level as it was, she will have a calorie deficit of 400 and Jane will lose weight.

– By monitoring the food she eats, Jane decides that she does not want to eat any less, but has time in the evenings to go for walks, cycle or go to the gym a couple of times a week.  Jane is still consuming 2405 calories, but now that she is increasing her level of activity, her Harris Benedict adjusted BMR (the amount of calories she needs to consume to maintain her current weight) has increased from 2405 to 2004 * 1.375 = 2755.  Due to her body’s new requirements for an additional 350 calories to maintain her current weight, Jane will now lose weight as she is consuming less calories than the new Harris Benedict adjusted BMR, she has a calorie deficit of 350.

– Alternatively, Jane can combine elements of the two options above by reducing her daily calorie intake slightly by maybe only 100 calories and also increasing her level of activity by going for walks, cycling, etc.  By doing this, Jane will have a calorie deficit of 2755 – 2305 = 450 and she will lose weight.

How much weight will I lose and how quickly will I lose it?

To work out a rate of weight loss, you need to convert the calorie deficit into a figure in pounds or kilograms.

There are 3500 calories in 1 pound and 7700 calories in 1 kilogram of body fat.

If you are creating a calorie deficit of 450 calories per day, this works out at 3150 calories per week.

Converting this to weight, 3150 calories corresponds to 0.9 pounds or 0.41 kilograms per week.

I’ve been doing this for a while, but it’s stopped working

A common complaint is that initially after following a plan like this, people see the weight coming off, but after a few weeks or months, the weight is coming off more slowly compared to the start of the plan.  They think the diet and exercise plan has stopped working.  But it is important after a while to revisit the calculations and see if the calorie goals are still relevant.  Let’s look at the numbers again with the help of an example.

Jane has been doing really well.  She now weighs 95.5Kg (210 pounds or 15 stone).

Jane’s BMR is now 655.1 + (9.563 x 95.5) + (1.850 x 180) – (4.676 x 30) = 1761

Jane’s Harris Benedict adjusted BMR is now 1608 * 1.375 = 2421

Jane was using the 3rd diet and exercise plan above and consuming 2305 calories on a daily basis.  Now that Jane has lost a lot of weight, the amount of calories required to maintain her new weight has reduced from 2755 to just 2421.  In other words, her calorie deficit is now just 2421 – 2305 = 116.  So it is little wonder that with a calorie deficit of just 116 that the weight is now coming off so slowly compared to her original calorie deficit of 450 at the start of the weight loss plan.

If Jane wants to lose weight more quickly again, she needs to revisit her eating and exercise plan to increase her calorie deficit.  She can do this by further reducing calorie intake from food, or increasing her level of physical activity again.




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One Comment:

  1. this is really informative thanks. I am curious about a program. there is a workout routine a lot of people have been using. i have looked around a lot places to determine if this really works as others point out it will. everybody’s got good things to say on it and also i have seen it working as now i’ve been using it for 2 weeks as of now. what is your opinion of it? should i remain on this program as it’s doing incredibly well.

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