Basal Metabolic Rate (BMR) is the number of calories your body burns at complete rest to keep vital organs running, typically accounting for 60 to 70 percent of total daily energy use before any movement, digestion, or exercise is added, while Total Daily Energy Expenditure (TDEE) is that resting figure multiplied by an activity factor to estimate everything you burn in a real 24-hour day. The two numbers sit on the same energy ledger: BMR is the floor you cannot go below without losing lean mass, and TDEE is the realistic total that includes walking, training, work, and the thermic effect of food. Because neither can be measured without lab equipment such as a metabolic cart, both are estimated from the same four inputs — sex, age, weight, and height — using published prediction equations. The most widely used formula for BMR is the Mifflin-St Jeor equation from 1990, which modern clinical and dietetic guidelines prefer because it validated within about 10 percent for more people than older alternatives. The revised Harris-Benedict equation (Roza and Shizgal, 1984) is the second most common comparison. To estimate TDEE, you take the BMR result and scale it by an activity multiplier that reflects your typical week.
BMR and TDEE — Two Numbers, One Energy Budget
BMR and TDEE answer different questions but share the same starting point. BMR asks how many calories you would burn if you lay still in a dark room all day, awake, in a comfortable temperature, with no digestion happening. That number powers your heart, brain, lungs, kidneys, the work of maintaining body temperature, and the constant turnover of cells. For most adults it sits between roughly 1,200 and 2,000 calories per day, and it represents the single largest slice of total energy use — usually 60 to 70 percent according to published energy-budget breakdowns.
TDEE asks the more practical question: how many calories do you actually burn in a normal day, including movement, exercise, and the energy cost of digesting food? It is calculated by multiplying BMR by an activity factor that reflects your typical week. A sedentary desk worker with no formal exercise will have a TDEE only slightly above BMR, while a manual laborer or twice-a-day athlete may burn roughly 60 to 90 percent more than their resting number. The gap between BMR and TDEE is the movement and digestion portion of your day, and it is the part that changes most based on lifestyle.
This is why BMR is calculated first and TDEE second. You cannot get a realistic daily calorie target without first knowing your resting baseline, and you cannot know your resting baseline without the four inputs that every published equation requires.
The Four Inputs Every Equation Needs
Every clinically used BMR prediction equation uses the same four variables: biological sex, age in years, body weight in kilograms, and height in centimeters. These were chosen because they are easy to measure and they explain a large share of the variation in measured resting metabolic rate across populations.
- Sex — accounts for average differences in lean body mass. Men typically carry more muscle and less fat than women of the same height and weight, and muscle burns more calories at rest than fat tissue. The sex offset in Mifflin-St Jeor alone is 166 calories (plus 5 for men, minus 161 for women) for an otherwise identical person.
- Age — resting metabolism tends to decline gradually with age, mostly because lean mass erodes over time. The Mifflin-St Jeor equation subtracts 5 calories per year of age; the revised Harris-Benedict subtracts about 5.7 for men and 4.3 for women.
- Weight — heavier bodies burn more calories at rest simply because there is more tissue to maintain. Weight is the largest single driver in both equations, with coefficients of 10 (Mifflin-St Jeor) and 13.4 (revised Harris-Benedict for men) per kilogram.
- Height — taller people have more surface area and typically more internal mass, so height adds a positive contribution: 6.25 calories per centimeter in Mifflin-St Jeor and roughly 4.8 (men) or 3.1 (women) in the revised Harris-Benedict.
Notice what is not in the formula: body-fat percentage, muscle mass, thyroid status, genetics, recent dieting history, or ethnicity. That is why every prediction equation is described as a population-average estimate rather than a personal measurement.
Two Equations, Side by Side
The BMR Calculator runs two of the most widely cited prediction equations at once so you can see how they compare on the same person. The table below summarizes what each formula does and how it is structured.
| Feature | Mifflin-St Jeor (1990) | Harris-Benedict, revised (Roza and Shizgal, 1984) |
|---|---|---|
| Publication year | 1990 | 1984 (re-fit of the original 1919 equation) |
| Formula for men | 10 × kg + 6.25 × cm − 5 × years + 5 | 88.362 + 13.397 × kg + 4.799 × cm − 5.677 × years |
| Formula for women | 10 × kg + 6.25 × cm − 5 × years − 161 | 447.593 + 9.247 × kg + 3.098 × cm − 4.330 × years |
| Weight coefficient | 10 per kg | 13.4 (men) or 9.2 (women) per kg |
| Height coefficient | 6.25 per cm | 4.8 (men) or 3.1 (women) per cm |
| Age coefficient | −5 per year | −5.7 (men) or −4.3 (women) per year |
| Typical clinical preference | Preferred by most modern guidelines | Used as comparison; often returns slightly higher BMR |
| Validation accuracy | Within about 10% for more people than older equations | Within about 10% on average; higher variance |
For a typical adult the revised Harris-Benedict formula returns a slightly higher BMR than Mifflin-St Jeor because its weight coefficient is steeper and its constant term is larger. That gap is a useful visual cue that any single BMR figure should be treated as a range, not an exact number. For the underlying equations themselves, see the Harris-Benedict equation reference and the broader basal metabolic rate overview.
How to Calculate Your BMR in Three Steps
The BMR Calculator applies both equations above from a single set of inputs, so you do not have to do the arithmetic by hand.
- Select your sex and enter your age, weight in kilograms, and height in centimeters.
- Read your Basal Metabolic Rate from the Mifflin-St Jeor equation, shown as the primary result and rounded to the nearest whole calorie.
- Compare it with the revised Harris-Benedict estimate to see your resting-calorie range and how much the two equations disagree on your specific numbers.
That is the complete BMR side of the calculation. Everything happens locally in your browser, so nothing you type is uploaded or stored. The tool intentionally stops at the resting number; it does not multiply by an activity factor or apply a weight-loss or weight-gain goal.
A quick worked example
To see how the two equations behave on the same person, take a 30-year-old man who is 180 cm tall and weighs 80 kg.
Mifflin-St Jeor: 10 × 80 + 6.25 × 180 − 5 × 30 + 5 = 800 + 1,125 − 150 + 5 = 1,780 calories per day.
Revised Harris-Benedict: 88.362 + 13.397 × 80 + 4.799 × 180 − 5.677 × 30 = 88.362 + 1,071.76 + 863.82 − 170.31 = 1,853.6 calories per day, rounded to 1,854 calories per day.
On this profile the two equations disagree by about 74 calories per day. That sits well inside the normal 50 to 100 calorie band the formulas typically produce, and it illustrates why showing both numbers side by side is more honest than reporting a single estimate.
From BMR to TDEE — Adding Your Activity Level
BMR tells you what your body burns doing nothing. TDEE adds the calories you burn moving, digesting, and training. The standard way to bridge the two is an activity multiplier applied to your BMR figure. Sedentary lifestyles (desk work, no structured exercise) use the lowest multiplier; hard physical labor or twice-a-day training uses the highest. Common categorizations break the week into bands such as sedentary, lightly active, moderately active, very active, and extra active, but the exact bands vary by source.
Rather than memorize five multipliers and risk picking the wrong one, the practical workflow is to get your BMR first using the BMR Calculator, then hand that resting figure to a tool that knows the activity bands and the goal logic. The Calorie Calculator does exactly that — it uses Mifflin-St Jeor as its BMR base, multiplies by an activity factor tied to your typical week, then subtracts or adds calories depending on whether your goal is fat loss, maintenance, or muscle gain. This keeps the BMR step honest (no activity assumption baked in) and the TDEE step realistic (no double-counting).
Once you have a TDEE number, the standard guidance is to treat it as a starting reference rather than a precise target. If your weight is not moving in the direction you expect after two or three consistent weeks, the most useful adjustment is usually a small calorie change of about 100 to 200 calories per day, plus a check on whether your logged activity level actually matches your real week.
Reading the Gap Between the Two BMR Estimates
When the Mifflin-St Jeor and revised Harris-Benedict results sit within roughly 50 to 100 calories of each other, that is a normal spread for the methodology and not a sign that anything is wrong. The formulas were built from different samples with different constants, so a small disagreement on the same person is expected.
A gap larger than that often comes down to one of three things: an unusual body size for the assumed population (very light or very heavy inputs), a height or weight entered in the wrong units, or a person whose physiology genuinely sits at one edge of the population distribution. If both equations are telling you something that does not match your real-world results over several weeks, that is the signal to adjust the number you use as your baseline rather than to assume the calculator is broken.
For readers who want to go deeper on how to interpret and validate the two formulas, the companion guide How to Calculate BMR Accurately with Two Proven Formulas walks through the same equations with additional comparison examples.
When the Formula Doesn't Match Real Life
Prediction equations are population averages. Real resting metabolism can differ from these estimates by roughly ±10 percent or more depending on lean mass, thyroid function, genetics, sleep, illness, medications, and adaptation to recent dieting. Highly trained athletes tend to run higher than predicted because of exceptional lean mass, while people who have just finished a long calorie-restricted diet often run lower because the body has down-regulated.
The calculator explicitly assumes a healthy adult and does not adjust for pregnancy, athletic body composition, illness, or medications. It is also a general-information tool, not medical advice — for any health, diet, or medical decision, the right next step is a qualified healthcare professional who can combine the formula output with your actual history and labs. Used as a starting reference, the two-equation approach gives a more honest range than a single number, and a more honest range is the right foundation for any calorie plan.