While counting calories is not how most of us want to spend our day, it is helpful to understand how many calories our bodies need/use just to maintain weight and carry out our normal daily functions. Knowing this helps then to figure out how many calories in a day to lose weight.
There are several factors that affect the amount of calories our bodies use in a day. Gender, age, height, weight and activity level contribute to the number of calories used in a day to maintain weight. Men generally require more calories in a day than women, and people of larger stature and more muscle mass require more calories in a day.
Basal Metabolic Rate (BMR)
Your basal metabolic rate, also known as your BMR, is the minimum rate at which you use calories to stay alive. You’ll want to know your BMR when you design a weight-loss to plan. This allows you to predict the rate at which you can expect to lose weight. You can calculate your BMR using a method called the Harris-Benedict formula. This formula takes your “at rest” BMR and multiplies an activity factor, based on your activity level. While all calculations are an approximation, this formula is one of the most accurate if used correctly.
A traditional BMR calculation does not account for activity level. The only thing the Harris-Benedict formula does not take into account is one’s lean body mass. Therefore, it is possible to underestimate the caloric need of someone with a large percentage of lean muscle mass, and to overestimate the caloric need of someone with low muscle mass and higher body fat percentage. People with more lean muscle have a higher metabolic rate than those with higher fat/lower lean muscle mass.
In this article, you will learn how to calculate your BMR, and use that to determine how many calories in a day you need to lose weight. The rate at wish to lose weight will determine the calorie deficit needed. In order to preserve muscle, most experts advise to lose no more than 1-2 pounds per week.
In order to use the Harris-Benedict formula, you must know how to convert to metric units of measure. Below are the conversions for height and weight:
1 inch = 2.54 cm
Example, if you are 5’1” (or 61”), then your height in cm equals 61 inches x 2.54 cm = 154.94 cm (round to 155 cm)
1 pound = .45 kg (1 kg = 2.2 pounds so 1 pound divided by 2.2 kg equals .45)
The first step you need to do is determine your weight and convert it to kilograms. (weight in lbs x 0.45= weight in kg)
Example, if you are 150 lbs, your weight in kilograms would be 150 x 0.45 = 67.5 kg
The second step is to convert your height to centimeters. (height in inches x 2.54= height in cm)
Example, if you are 5’1” (or 61”), then your height in cm equals 61 inches x 2.54 cm = 154.94 cm (round to 155 cm).
The next step is to determine your BMR, or your resting basal metabolic rate. The formulas are different for men and women. The formulas are as follows:
Men: 66 + (13.7 x weight) + (5 x height) – (6.8 x age in years)
Women: 655 + (9.6 x weight) + (1.8 x height) – (4.7 x age in years)
Example, if you are a 40-year-old woman who is 5’1″ tall (155 cm) and 150 lbs (67.5 kg),
Resting BMR= 655 + (9.6 x 67.5) + (1.8 x 155) – (4.7 x 40)
Resting BMR= 655 + 648 + 279 – 188
Resting BMR= 1394
Finally, to get the most accurate metabolic rate, you must take the resting BMR x an activity factor that best represents your level of activity on average.
Harris Benedict Formula
To determine your total daily calorie needs (to maintain weight), multiply your BMR by the appropriate activity factor, as follows:
- If you are sedentary (little or no exercise) : Daily caloric need = BMR x 1.2
- If you are lightly active (light exercise/sports 1-3 days/week) : Daily caloric need = BMR x 1.375
- If you are moderately active (moderate exercise/sports 3-5 days/week) : Daily caloric need = BMR x 1.55
- If you are very active (hard exercise/sports 6-7 days a week) : Daily caloric need = BMR x 1.725
- If you are extra active (very hard exercise/sports & physical job or 2x training) : Daily caloric need = BMR x 1.9
Using our same example as above, if you are a 40-year-old woman, measure 5’1″ tall, weigh 150 lbs, and are relatively sedentary, your total daily calorie need to maintain weight is:
BMR x 1.2
1394 x 1.2 = 1673 calories
In contrast, if that same woman is moderately active, her BMR would be higher. The activity factor would increase from 1.2 to 1.55. Thus the BMR of a moderately active 40-year-old woman who is 5’1″ tall and weighs 150 lbs would be:
BMR x 1.55
1394 x 1.55 = 2161 calories
How many calories to lose weight
Experts recommend losing no more than 1-2 lbs per week. In order to lose 1 lb per week, you must have a calorie deficit of 3500 calories for the week (500 calories per day). That means you need to consume 500 fewer calories than your BMR each day, OR exercise enough to burn an extra 500 calories per day. That number would need to double in order to lose 2 lbs per week.
For our 40-year-old sedentary woman, to decrease her calorie intake to 1000 fewer calories per day from a BMR of 1673, in order to lose 2 lbs per week, would not be advised. It would be better to decrease calorie intake by 500 calories per day (so a total intake of 1173 calories) and increase activity. Even a walk each day would add to the calorie deficit, improve weight loss, and improve overall health.
So now we have come to the final formula for calculating how many calories in a day to lose weight. In order to lose 1 lb per week:
BMR x activity factor – 500 = daily caloric needs to lose one lb per week.
There are many applications that will calculate this for you, but I believe knowledge is power and it’s important to understand where the numbers come from. I hope you have found this useful in developing your own personal weight loss plan (or weight maintenance plan).
*Note: As you lose weight, you will need to re-calculate your BMR. Your daily caloric needs will change as your weight changes.