Resting energy (Mifflin-St Jeor) = 10 × weight (kg) + 6.25 × height (cm) − 5 × age + 5 for men or − 161 for women. TDEE = resting energy × physical activity level: 1.4 low active, 1.6 moderately active, 1.8 active, 2.0 very active.
The result is an estimate of the calories that would hold your weight steady at your current activity level. It is a starting point, not a measurement, and the useful question is how close it is to your own body.
Mifflin-St Jeor predicts resting energy within 10% of the measured value for most adults, but not all. On a result of 2,500 kcal a day, that range alone spans roughly 2,250 to 2,750, before any error in the activity level you picked.
The most reliable check is your own weight trend. If your weight holds steady over two to three weeks of consistent eating, your real expenditure is close to what you are eating, whatever the calculator said.
Body size is the biggest driver, because a larger body costs more energy to run and to move. Fat-free mass matters most within that, since muscle and organs use more energy at rest than fat tissue does, which is why two people of the same weight can differ.
Resting energy falls slowly with age, partly because muscle mass tends to decline. Daily movement outside planned exercise, such as walking, standing and household work, varies by hundreds of calories between people and is the part of the estimate most often misjudged.
Thyroid hormone sets part of the resting rate, so thyroid status can shift expenditure, which is one reason TSH sits among the related markers. Illness and fever raise energy use for a while, and weight loss itself lowers expenditure, sometimes by more than the lost mass alone would predict.
Recalculate after your weight changes by about 5 kg (around 10 lb), after a lasting change in how active you are, or every few months during a deliberate change in body weight. Each of these moves the inputs enough to change the estimate.
If your weight trend and the estimate keep disagreeing, trust the trend. A persistent gap can come from the activity level chosen, from body composition, or less commonly from a medical factor worth discussing with a clinician.
The equation was built in healthy adults aged 19 to 78. It was not designed for pregnancy, breastfeeding, children or teenagers, and its error is larger in people with obesity and in very muscular people, whose resting energy it tends to underestimate.
The activity multipliers are population averages, and people tend to rate their own activity higher than it is. The estimate also cannot see medical conditions or medicines that change metabolism. If you are changing what you eat because of a health condition, plan it with a clinician or registered dietitian.
Take a 40-year-old man who weighs 180 lb, stands 70 inches and picks moderately active, an activity level of 1.6. The calculator converts units first. 180 lb is 81.6 kg and 70 inches is 177.8 cm. Mifflin-St Jeor then adds 10 × 81.6, which is 816.5, and 6.25 × 177.8, which is 1,111.3, subtracts 5 × 40, which is 200, and adds 5 for men.
That gives a resting energy of 1,733 kcal a day. Multiplied by 1.6, the TDEE is 2,772 kcal a day. The same man would read 2,426 at low active (1.4), 3,119 at active (1.8) and 3,465 at very active (2.0), so the activity choice moves the answer by more than 1,000 kcal. That makes the activity level the single biggest lever in the estimate, larger than any plausible error in the weight or height entered, so it is worth choosing honestly.
Now apply the accuracy limit. Mifflin-St Jeor lands within 10% of measured resting energy for most adults, so his true resting figure most likely falls between about 1,560 and 1,906. Carried through the 1.6 multiplier, that is roughly 2,495 to 3,050 kcal a day. A woman with the same weight, height and age subtracts 161 instead of adding 5, giving 1,567 at rest and 2,507 at 1.6.
Harris-Benedict, published in 1919, is the best known alternative. In the group used to build Mifflin-St Jeor, it overestimated measured resting energy by 5%. A systematic review of the four most used equations, Harris-Benedict, Mifflin-St Jeor, Owen and WHO/FAO/UNU, found Mifflin-St Jeor predicted resting energy within 10% of measured in more people, both with and without obesity, and had the narrowest error range. No individual-level validation was found for the WHO/FAO/UNU equation.
Body composition is the other route. In the same 1990 paper, fat-free mass was the best single predictor of resting energy, with the equation 19.7 × fat-free mass + 413 explaining 64% of the variation, against 56% for weight alone and 71% for the full Mifflin-St Jeor model. That only helps if fat-free mass is measured well, which most people cannot do at home.
Measurement beats any equation. Indirect calorimetry measures resting energy directly, and the review advised measuring when an estimate fails a person in a way that matters. Total daily expenditure can also be measured in research settings, but for everyday planning a weight trend over a few weeks is the practical check.
The Metabolic and insulin panel measures the values this calculator uses. Related: fasting insulin, hemoglobin a1c, tsh.
Resting energy is what your body uses at rest. TDEE adds the energy for daily activity and exercise, calculated here as a multiple of resting energy.
EFSA describes 1.4 as low active or sedentary, 1.6 as moderately active, 1.8 as active and 2.0 as very active.
It is the most reliable of the common equations in a systematic review, predicting resting energy within 10% of measured more often than the others. It is still an estimate.