BMR and TDEE Explained: Mifflin-St Jeor Formula

Two abbreviations carry most of the energy math in an intro nutrition course: BMR and TDEE. They sound like the same thing and they are not, and exam questions are written to see whether you know the difference. This guide defines both, walks the Mifflin-St Jeor equation with a worked example, and covers the parts that trip people up: the activity factors, the 2005 versus 2023 energy equations, and why the "3,500 kcal per pound" rule is only a rough guide.

It follows the energy chapters of Encodr's free Nutrition course. If you want to check your arithmetic as you go, the BMR and TDEE calculator shows the working for every step.

Where BMR and TDEE sit in energy balance

Energy balance compares what you eat with what you spend. When intake is greater than expenditure, the excess is stored and weight rises (positive energy balance). When intake is less than expenditure, the body draws on glycogen and fat stores and weight falls (negative energy balance).

Total energy expenditure (TEE) has three parts:

So TEE = BMR + TEF + physical activity. A typical exam item gives you three of the four numbers and asks for the fourth. If TEE is 2,400 kcal per day, BMR is 1,440 and TEF is 240, then physical activity is 2,400 - 1,440 - 240 = 720 kcal.

TDEE stands for total daily energy expenditure. It is the same idea as TEE, expressed per day. The key point is how it is estimated in practice: you do not measure each component, you estimate BMR with an equation and multiply by an activity factor.

What changes BMR

BMR rises with muscle mass, because muscle burns more energy at rest than fat does. It generally falls with age as muscle mass declines, and it falls with dieting. Those three facts show up as multiple-choice items ("Which change would raise a person's BMR?") more often than you would expect from how short they are.

TEF depends on what you eat. By macronutrient, fat has the lowest thermic effect at about 0-3% of the energy eaten, carbohydrate is about 5-10%, protein is about 20-30% and alcohol is about 10-30%. On a mixed diet the course gives TEF as 5-15% of intake, which at 2,000 kcal is 100 to 300 kcal.

The Mifflin-St Jeor equation

Mifflin-St Jeor predicts resting energy expenditure in kcal per day from four inputs: weight in kilograms, height in centimeters, age in years and sex.

• Men: 10 x kg + 6.25 x cm - 5 x age + 5
• Women: 10 x kg + 6.25 x cm - 5 x age - 161

The only difference between the two is the final constant: +5 for men and -161 for women. That is a 166 kcal gap for two people of the same age, weight and height.

The course teaches Mifflin-St Jeor rather than the older Harris-Benedict equations because the Mifflin-St Jeor authors found Harris-Benedict overestimated measured resting energy expenditure by about 5%.

Worked example (metric)

A man is 30 years old, 80 kg and 180 cm.

1. 10 x 80 = 800
2. 6.25 x 180 = 1,125
3. 5 x 30 = 150
4. BMR = 800 + 1,125 - 150 + 5 = 1,780 kcal per day

Worked example (US units)

A woman weighs 150 lb, is 66 in tall and is 35. Convert first with 1 kg = 2.2046 lb and 1 in = 2.54 cm, then apply the female equation. Round only the final answer, not the converted values. The result is 1,392 kcal per day. Rounding the kilograms or centimeters early is the classic way to land a kcal or two off the keyed answer, and it is a good reason to work from a calculator that keeps full precision until the end. The calculator takes pounds, feet and inches directly.

From BMR to TDEE: the activity factor

TDEE = BMR x activity factor. The commonly quoted factor set is:

Using the 1,780 kcal BMR from above:

• Sedentary: 1,780 x 1.2 = 2,136 kcal
• Moderately active: 1,780 x 1.55 = 2,759 kcal
• Extra active: 1,780 x 1.9 = 3,382 kcal

One thing to hold onto: these factors are a convention, not an official constant. The course is explicit that they have no primary source and are not part of the Mifflin-St Jeor paper. They are fine as rough planning numbers and as exam arithmetic when the question hands you the factor. They are not a measurement of anyone's actual expenditure.

The 2005 EER equations and the 2023 update

A second set of equations appears in the same chapter: the Estimated Energy Requirement (EER) from the 2005 Institute of Medicine report. EER is the energy needed to maintain weight for a person of a given age, sex, weight, height and activity level.

• Men: 662 - 9.53 x age + PA x (15.91 x kg + 539.6 x m)
• Women: 354 - 6.91 x age + PA x (9.36 x kg + 726 x m)

Note that height is in meters here, not centimeters, and that PA is a physical activity coefficient, where 1.00 means sedentary. For a man aged 30, 80 kg and 1.80 m at PA 1.00, EER is 2,620 kcal per day. For a woman aged 25, 60 kg and 1.65 m at PA 1.00, it is 1,941 kcal per day.

In 2023 the National Academies (NASEM) Dietary Reference Intakes for Energy replaced the 2005 EER equations. The physical activity categories were renamed (inactive, low active, active and very active), and the reference population now includes people with overweight and obesity. The 2023 report did not change the AMDR ranges. Many intro courses still teach the 2005 version, and this one does too, so know which edition your instructor is asking about before you pick a formula.

Why the 3,500 kcal rule is only a rough guide

Once you have a TDEE, the tempting next step is to subtract 500 kcal a day and predict one pound a week. The 3,500 kcal rule says about 3,500 kcal is roughly 1 lb (7,700 kcal per kg). A 500 kcal daily deficit gives 500 x 7 / 3,500 = 1.0 lb per week by that rule.

It is a static approximation, and it overestimates loss over time for two reasons: energy expenditure falls as body weight falls, and early loss includes glycogen and water. That makes "the 3,500 kcal rule predicts weight change exactly over long periods" a false statement on an exam.

How to practice this

BMR, TDEE and EER problems are short and mechanical, which makes them good retrieval practice. Do each one from a blank page: write the equation, substitute, compute, then check against the calculator. Setting the problem up correctly is the skill, and it is the step that rereading your notes never trains.

Once your energy math is solid, the natural next step is turning a calorie target into grams of each nutrient, which is covered in how to calculate macros. For the rest of the course, what's in a nutrition course maps every unit and how to study for a nutrition class sets out a plan.

This guide and the calculator are for learning. They give estimates, not medical or dietary advice.