BMR and TDEE are the foundation of any calorie-based plan. Understanding them is the difference between guessing and actually controlling your body composition.
TDEE is not a fixed metabolic property of your body — it is a moving target. As you lose fat, your BMR drops because there is less tissue to maintain. As you gain muscle, BMR creeps up by roughly 6–10 kcal per pound of lean mass. NEAT (non-exercise activity thermogenesis) — fidgeting, walking around, standing — quietly accounts for as much as 500 kcal of daily variation between two people of the same size, and it falls reflexively during prolonged calorie deficits. A 200 lb man calculating TDEE = 2,800 kcal in January will not have the same TDEE in May if he has lost 20 lb of fat and slept less. Recalculate every 8–12 weeks or after any 5% change in body weight.
Reading the formulas in plain English
Mifflin-St Jeor is essentially "ten calories per kilogram of body mass, plus six and a quarter per centimetre of height, minus five per year of age, with a small sex offset". It assumes average body composition. Katch-McArdle ignores sex and age entirely and uses lean body mass (LBM) directly: BMR = 370 + (21.6 × LBM in kg). For lean, muscular individuals Katch-McArdle is markedly more accurate; for higher body-fat percentages Mifflin-St Jeor is the safer bet because LBM is hard to measure without DEXA or hydrostatic weighing.
A worked example
Take a 35-year-old woman, 165 cm, 68 kg, who lifts three times a week and walks 8,000 steps on most days. Mifflin-St Jeor gives BMR = (10 × 68) + (6.25 × 165) − (5 × 35) − 161 = 680 + 1,031 − 175 − 161 = 1,375 kcal. With a "lightly active" multiplier of 1.375 her TDEE is 1,891 kcal. To lose roughly 0.5 kg/week she would aim for about 1,400 kcal/day, prioritising 100–130 g of protein. Two weeks of tracking will reveal whether the multiplier was honest or optimistic.
Common mistakes that wreck the number
Picking the wrong activity multiplier. Three gym sessions a week is "lightly active" for most people, not "moderately active". Add the workouts to a sedentary baseline only if your job is desk-bound.
Confusing weight loss with fat loss. Glycogen and water swings hide a 1–2 kg week-to-week. Use a 7-day rolling average, not a single morning weigh-in.
Eating back exercise calories. Wearable estimates of calorie burn are routinely 20–40% high. If you are using TDEE, your training is already baked into the multiplier — do not add it again.
Crash dieting. Cutting more than 25% below TDEE accelerates muscle loss, drops NEAT, and slashes training quality. The body compensates, and the scale plateaus inside a month.
Reality check. No formula is more accurate than two weeks of disciplined tracking. Treat the number as a starting estimate and let the scale tell you whether to adjust by ±100–200 kcal.
Frequently Asked Questions
Calories burned at complete rest. 60-75% of total daily expenditure.
BMR × activity multiplier. Your maintenance calorie number.
Mifflin-St Jeor for general population; Katch-McArdle if you know body fat %.
Rough estimates. Most people overestimate — start low and adjust.
For maintenance yes; 10-25% below for loss, 5-15% above for gain.