BMI, BMR, TDEE: explained without the myths

BMI is weight divided by height squared. The formula has no biological insight, it is a 19th-century population statistic that the WHO adopted in the 1990s as a quick screening tool. The categories (underweight <18.5, normal 18.5-24.9, overweight 25-29.9, obese 30+) are population thresholds where mortality and disease risk start to climb on average.

Where BMI works: across a population, a meaningful percentage of people in the 'obese' BMI bracket also carry excess body fat with associated metabolic risks. Public health policy and large-scale epidemiology use BMI productively because they care about averages, not individuals.

Where BMI breaks: it cannot distinguish muscle from fat. A 6'1" lifter at 220 lb with 12% body fat has a BMI of 29 (overweight) and excellent health markers. A 5'4" sedentary office worker at 135 lb with 32% body fat has a BMI of 23.2 (normal) and poor health markers. Same BMI category lies in opposite directions.

It also breaks at the extremes of height (people 6'4"+ score systematically higher; people under 5'2" score systematically lower for the same body composition), and shows ethnic variation. Asian populations show metabolic risks at lower BMI than the WHO categories suggest, leading some Asian-specific health guidelines to lower the overweight threshold to 23.

BMR is the calorie cost of staying alive while doing nothing. Heart pumping, lungs breathing, brain consuming roughly 20% of the total, kidneys filtering, cells maintaining themselves. For an adult, BMR is typically 1,200-2,000 calories per day depending on size, sex, age, and lean mass.

The most accurate everyday formula is Mifflin-St Jeor (1990):

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

Older formulas (Harris-Benedict, original or revised) overestimate BMR by 5-7% on average. Katch-McArdle is more accurate if you know your body fat percentage from a DEXA scan or hydrostatic weighing — but most people don't have those numbers handy.

BMR matters because it is the floor that any deficit must stay above. Eat below your BMR for an extended period and your body downregulates everything: hormones, body temperature, energy levels, and importantly, the metabolism itself. The Minnesota Starvation Experiment showed BMR can drop 40% in chronic semi-starvation. Sustainable cutting respects the BMR floor.

TDEE = BMR × activity multiplier. Standard multipliers:

• 1.2: sedentary (desk job, no exercise)
• 1.375: lightly active (light exercise 1-3 days/week)
• 1.55: moderately active (moderate exercise 3-5 days/week)
• 1.725: very active (hard exercise 6-7 days/week)
• 1.9: extra active (hard exercise + physical job)

These are estimates with real variance. A 'sedentary' office worker who walks the dog 30 minutes daily and parks far from buildings runs closer to 1.3 than 1.2. Two people with the same gym routine can differ by 200 calories in non-exercise activity thermogenesis (NEAT), the unconscious fidgeting and movement of daily life.

TDEE includes the thermic effect of food (TEF), the calories burned digesting and processing what you eat. TEF is roughly 10% of total intake, with protein highest (20-30% of protein calories), carbs in the middle (5-10%), and fat lowest (0-3%). Higher-protein diets have meaningfully higher TEF, which is one reason high-protein eating tends to feel less restrictive at the same total calories.

For a sane weight plan: estimate TDEE, log intake honestly for 2-3 weeks while weighing daily, then check whether your actual weight trend matches the estimate. If your TDEE estimate said 2,500 calories and you ate 2,500 calories and lost 0.5 lb in two weeks, your real TDEE is closer to 2,750. Adjust and continue.

Energy balance is the only mechanism by which body fat changes. Calories in vs calories out, with hormones modulating the rate but not breaking the rule. Anyone selling you 'metabolic confusion' or 'hormonal weight loss' that violates conservation of energy is selling fiction.

Cutting (fat loss). Target deficit: 300-500 calories below TDEE. A 500-calorie daily deficit yields about 1 lb of fat loss per week (3,500 calories per pound of fat is a useful approximation). Larger deficits work but accelerate muscle loss, hunger, and metabolic slowdown. Sustainable cuts run 12-20 weeks, not lifetime.

Bulking (muscle gain). Target surplus: 200-500 calories above TDEE. A 250-calorie daily surplus yields about 0.5 lb/week, with about 50-70% muscle and 30-50% fat depending on training quality, protein adequacy, and age. Aggressive surpluses produce mostly fat, there is a ceiling on muscle protein synthesis that calories cannot push past.

Maintenance. Eating at TDEE keeps weight stable to within ±2 lb of natural fluctuation (water, glycogen, gut contents). Most people fluctuate ±3-5 lb across a normal week from these factors alone, which is why scale weight should be averaged over 7+ days for any meaningful trend.

Protein adequacy. During a cut, target 0.8-1.2 g/lb of body weight in protein. This preserves muscle while in deficit. Below 0.6 g/lb you lose materially more lean mass; above 1.2 g/lb the marginal benefit drops fast. Carbs and fat fill the remaining calories based on preference and performance needs.

Activity multiplier inflation. People consistently overestimate their activity level. The 'moderately active' description fits about 30% of people who pick it. Most desk workers with a few weekly gym sessions are actually 'lightly active' (1.375). Estimating one tier lower than your gut says is usually closer to truth.

Tracking error in food logs. People underreport calories in by 20-40% when self-tracking. Restaurant meals are notorious, 'medium fries' is anything from 320 to 510 calories at different chains. Use a food scale at home for the first 3-4 weeks of any cut to calibrate.

Online calculator inconsistency. Different sites use different formulas (Mifflin-St Jeor vs Harris-Benedict vs Katch-McArdle) and different activity multiplier assumptions. Numbers can vary by 200-400 calories across calculators. Pick one (we use Mifflin-St Jeor on the BMR/TDEE calculator) and stick with it for trend tracking.

Body weight scale variance. Daily weight fluctuates 2-5 lb from water and glycogen alone. Looking at a single day's weight tells you nothing — average over 7-14 days for a real trend signal.

Use BMI for a quick demographic check, especially with people you do not know well. 'Where do I roughly fall?' is what BMI answers. Do not use BMI to set a weight goal or judge body composition, it is the wrong tool.

Use BMR to set the floor of your eating plan. Never eat chronically below BMR, it crashes the metabolism and is psychologically miserable. This is the number that prevents the worst self-inflicted damage from aggressive dieting.

Use TDEE to set the actual eating target, at TDEE for maintenance, 300-500 below for cutting, 200-500 above for bulking. Recalculate every 4-6 weeks during active cuts because the number drifts as weight changes.

Use body composition tools (DEXA, BodPod, calipers) when BMI gives a confusing answer and you actually want to know your fat-to-muscle ratio. These cost $50-150 per session at clinics or fitness centers and give numbers BMI cannot produce.