BMR vs TDEE: Which Number Should You Eat To?
At 30 years old, 80 kg and 180 cm, the male Mifflin equation returns 1,780 kcal a day. Multiply by an activity factor of 1.55 and you get 2,759. That gap of nearly a thousand calories is the whole difference between BMR and TDEE, and the second number rests on an assumption you graded yourself. The TDEE calculator prints both. Neither one measures what you personally need to eat.
Resting energy and the Mifflin equation
Basal metabolic rate is energy expenditure measured under strict resting conditions, and resting metabolic rate is the same thing measured under looser ones. Calculator labels use BMR for both. The Mifflin-St Jeor equation was built to predict the resting figure.
The site’s simplified equations are 10 × weight in kg + 6.25 × height in cm − 5 × age, then add 5 for the male equation or subtract 161 for the female one. Those are the sex-specific equations straight out of the research that Mifflin and colleagues published in 1990, and they stop a long way short of a full model of your physiology.
Run the male equation at age 30, 80 kg and 180 cm and the sum is 800 + 1,125 − 150 + 5 = 1,780 kcal/day. That is the calculator’s worked example. The BMR calculator runs that equation next to the revised Harris-Benedict, which returns 1,854 kcal for the same man, and Katch-McArdle, which wants your body fat percentage before it will answer at all.
What TDEE adds
Total daily energy expenditure covers your resting energy, everything you do, and the energy you spend digesting food. This calculator gets there by multiplying resting energy by an activity factor, so 1,780 × 1.55 = 2,759 kcal/day.
That multiplier describes your whole week, training sessions included and everything in between as well. Read the activity multiplier guide before you trust the output, and look at the activity comparison table on the tool to see how hard this single assumption pushes your answer around.
Where the BMR floor myth comes from
During an energy deficit your stored energy contributes to expenditure, which is the point of running one. Eating above your predicted BMR buys no guarantee on its own, because your nutrient needs, your health, your activity, and the size and length of your deficit all carry weight.
Take 500 kcal off the worked example and you land on 2,259 kcal/day. That figure exists for comparison and nothing more. The tool’s low-intake warnings are a safety net, and clearing one says nothing about whether the number above it suits you.
Checking an estimate
Compare your eating patterns, your activity and your weight trend across several weeks, if tracking suits you. Fluid shifts hide the trend inside a shorter window. Recalculate when your weight or your routine changes meaningfully, and resist cutting again on the strength of a few flat readings. The deficit guide explains the limits of predicting weight loss, and the NIDDK Body Weight Planner runs a dynamic model for eligible adults.
Scope and correction note, 13 September 2026: This guide provides general adult education. It has been revised to distinguish estimates from measurements and to remove unsupported guarantees. It has not received independent clinical review. See the editorial policy and medical disclaimer.