BMR, TDEE, and why your “maintenance” number is wrong by 200–400 kcal
Calorie calculators are confidence-inducing precisely because they spit out a single number. The reality is messier: any TDEE estimate from a formula has ±15% error baked in, which on a 2 500 kcal day is ±375 kcal — enough to flip a deficit into a surplus, or vice versa. Here is how to use the calculator above without believing it more than you should.
Mifflin–St Jeor vs Harris–Benedict: pick Mifflin
Harris–Benedict (1919, revised 1984) over-estimates BMR by 5–10% in modern populations because baseline activity levels have fallen since the original data was collected. Mifflin et al. (1990) recalibrated against contemporary sedentary adults and is the current gold standard, accurate to about ±10% across most non-athletes. Use Mifflin–St Jeor unless you have a specific reason not to (the calculator on this page does, by default).
Katch–McArdle: better if you know your body fat
BMR = 370 + 21.6 × lean body mass (kg). This drops sex and age from the inputs because lean mass already encodes them. It is the right choice for trained athletes and recreational lifters whose lean mass deviates significantly from population averages, but you need a body-fat estimate first — see the Body Fat Calculator.
The activity multiplier is where the error compounds
Sedentary 1.2, Light 1.375, Moderate 1.55, Active 1.725, Very active 1.9. Self-reported activity is notoriously unreliable — studies consistently show people over-estimate by 20–30%. The practical rule: drop one tier below what you instinctively think you are. “I work out 3 times a week” is usually closer to Light than Moderate once you account for the other 165 hours of the week.
NEAT: the variable nobody puts in the formula
Non-Exercise Activity Thermogenesis — fidgeting, standing desk, taking the stairs, walking the dog — varies by 600–800 kcal/day between two otherwise-identical adults (Levine et al. 1999, Mayo Clinic). The formulas above include nothing about NEAT. If you sit for 12 hours a day, your real TDEE is lower than the calculator suggests, regardless of which formula you picked.
How to actually find your maintenance
Use the calculator as a starting estimate. Eat at that level for 14 days, weigh yourself daily at the same time, average each week. If your weekly average is stable to within ±0.3 kg, that level is your real maintenance. If you’re losing or gaining, adjust by 100–150 kcal and repeat. Two to four weeks of this beats any formula.
Takeaway: Formula-based TDEE estimates are a starting point, not a target. The right TDEE is the one your bathroom scale confirms over two to three weeks of consistent intake. Mifflin–St Jeor plus a dropped activity tier plus scale verification is the honest method. Anything more precise from a calculator alone is theatre.
Sources: Mifflin et al. (1990) Am J Clin Nutr · USDA DRI calculator · Levine et al. (1999) NEAT and obesity.