What Are BMR and TDEE? Mifflin-St Jeor Formula and Activity Factors Explained

"I am eating less but not losing weight." "I am eating plenty but not gaining." A key to understanding why is knowing how much energy you burn in a day. The foundation for that is your basal metabolic rate (BMR) and the total daily energy expenditure (TDEE) derived from it. This article walks through the difference between the two, how to calculate them, and how to put the numbers to use.

What Is Basal Metabolic Rate (BMR)

Basal metabolic rate (BMR) is the energy your body uses just to stay alive while lying still and doing nothing. Keeping your heart beating, breathing, maintaining body temperature, and running your organs—these "calories burned simply by being alive" make up your BMR.

It may be surprising, but of your total daily calorie burn, this basal metabolism—not exercise—accounts for the largest share, typically about 60 to 70 percent of the total. In other words, most of the calories you burn are spent "without doing anything special."

BMR varies with body size, age, and sex. In general, the more muscle you have and the larger your body, the higher your BMR, and it declines with age. Men tend to have a higher BMR than women because they carry more muscle on average. Part of the reason it gets easier to gain weight as we age is this decline in basal metabolism.

The Difference Between TDEE and BMR

If BMR is "the calories you burn just by being alive," then TDEE (Total Daily Energy Expenditure) is "all the calories you actually burn in a day." TDEE is your BMR plus the energy used in daily activity and exercise.

Daily calorie burn is roughly made up of the following components.

• Basal metabolic rate (BMR): energy for sustaining life. The largest share.
• Activity expenditure: energy from moving your body—walking, housework, exercise, and so on.
• Diet-induced thermogenesis (DIT): energy used to digest and absorb food.

What you should really base cutting or bulking on is not BMR alone but this TDEE. "Eat less than your TDEE to lose weight, more to gain it" is the most fundamental principle of weight management. Think of BMR as merely the starting point for finding your TDEE.

If you want to know your BMR and TDEE as actual numbers, the BMR and TDEE Calculator performs the calculations described below all at once once you enter your height, weight, age, and activity level.

Calculating BMR With the Mifflin-St Jeor Formula

There are several formulas for estimating BMR, but the most widely used and considered most accurate today is the Mifflin-St Jeor formula. Published in 1990, it is a relatively new equation that is said to fit modern body types well.

Men: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age + 5
Women: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age − 161

Notice that only the final constant (+5 versus −161) differs between men and women. For example, for a 30-year-old man who is 170 cm tall and weighs 65 kg, you would calculate as follows.

• 10 × 65 = 650
• 6.25 × 170 = 1062.5
• 5 × 30 = 150
• BMR = 650 + 1062.5 − 150 + 5 = about 1567 kcal

So even if this person stayed in bed all day, they would burn about 1567 kcal. There is also the older Harris-Benedict equation, but it tends to read slightly high for modern body types, and the Mifflin-St Jeor formula is increasingly recommended today. Doing this by hand with a calculator is a bit tedious, so the BMR and TDEE Calculator gives you an instant result from just a few inputs.

Getting TDEE With Activity Factors (1.2-1.9)

Once you have your BMR, you estimate TDEE by multiplying it by an activity factor (PAL). You choose the factor from a range of roughly 1.2 to 1.9, depending on how much you move each day.

The math is simple: TDEE = BMR × activity factor. If the person above with a BMR of 1567 kcal chose "moderate exercise (factor 1.55)," their TDEE would be 1567 × 1.55 = about 2429 kcal. That roughly 2429 kcal is the approximate daily energy this person needs to maintain their weight.

When choosing a factor, the trick is not to be greedy and pick too high a value. Telling yourself "I actually move less than I think" and choosing one step lower tends to get you closer to a realistic number.

Using It as a Target for Cutting and Bulking

Once you know your TDEE, your daily calorie target for your goal comes into view. The basic idea is to start from TDEE and subtract to lose weight or add to gain it.

1. For dieting (cutting)

Losing 1 kg of body fat is said to require a deficit of roughly 7200 kcal. Since rapid weight loss tends to cause muscle loss and feeling unwell, a realistic target is TDEE minus about 300 to 500 kcal per day. For the person above with a TDEE of 2429 kcal, about 1900 to 2100 kcal a day would be a cutting target.

2. For bulking (building muscle)

To build muscle, you need to eat more than your TDEE. To add muscle while gaining as little fat as possible, the standard approach is to add about 200 to 400 kcal per day to your TDEE, eat plenty of protein, and train. Beyond total calories, the split of protein, fat, and carbohydrate (the PFC balance) also matters. To work out a concrete split, the Calorie and PFC Balance Calculator makes it easy to derive grams of each macronutrient from your target calories.

In either case, the number you set at first is only a hypothesis. The shortcut to success is to keep it up for two to four weeks, watch how your weight and body change, and adjust your intake little by little.

Caveats: Formulas and Individual Variation

Formulas for BMR and TDEE are very convenient, but you must not forget they are "estimates" derived from statistics. Actual metabolism is affected by many factors that the formulas do not capture.

• Body composition: at the same weight, someone with more muscle and someone with more fat have different BMRs. Because the formula ignores body fat percentage, errors can occur.
• Constitution and hormones: metabolism shifts with thyroid function and hormonal status.
• Daily fluctuation: sleep, stress, temperature, and how active you were that day all change your burn from day to day.

So rather than treating the calculated number as absolute, the right way to use it is as "a starting point you fine-tune by measuring on your own body." If you keep your food and activity the same and your weight does not change, you can take the intake at that point as close to your real TDEE.

Start by grasping your approximate numbers with the BMR and TDEE Calculator, then set your intake to match your goal. Having an objective benchmark in the form of a number is the first step to building your body without relying on feel alone.

Please note that this article is intended to provide general information and is not a substitute for medical or nutritional advice. If you have a medical condition or plan significant dietary restriction, check the latest official information and consult a professional such as a doctor or registered dietitian as needed before deciding.