Estimates resting metabolic rate and daily energy needs from age, sex, height, weight and activity level, using the Mifflin-St Jeor equation.
The equation estimates the energy your body uses at rest, and that figure is multiplied by an activity factor to give a daily total. It is a population equation: it was derived by measuring many people and fitting a line, so it describes an average person with your measurements rather than you. Individual metabolic rates vary substantially around that average — differences of several hundred calories a day between people with identical inputs are well documented. The equation is also not validated for children, which is why ages below fifteen are declined rather than estimated.
How much confidence the number deserves
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It is an estimate from a formula, not a measurement:
actual energy expenditure is measured in a laboratory. Two people with identical inputs can genuinely differ by several hundred calories a day.
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The activity multiplier is the least precise part:
self-reported activity is consistently overestimated, and the step between two adjacent levels is often larger than the difference between them in reality.
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Body composition is not an input:
muscle uses more energy at rest than fat does, and the equation cannot see the difference.
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Any change to how you eat is worth discussing with a professional:
a dietitian or doctor can account for your health, history and circumstances, which a formula cannot.
A population equation, describing an average person like you
The [msm_tool slug="calorie-tdee-calculator"] uses the Mifflin-St Jeor equation, derived by measuring many people and fitting a line through the results — which means it describes an average person sharing your measurements rather than measuring you specifically. Individual metabolic rates vary substantially around that average, and documented differences of several hundred calories a day between people with identical inputs are entirely normal rather than a sign anything is wrong with the formula.
The activity multiplier is the least precise part of the whole estimate
The resting-rate figure is multiplied by an activity factor to reach a daily total, and that multiplier rests on self-reported activity level, which is consistently overestimated in practice. The gap between two adjacent activity levels in the tool is often larger than the real difference between two people who each picked a different one, which makes this step the biggest source of uncertainty in the final number.
Body composition never enters the calculation
Muscle uses more energy at rest than fat does for the same weight, and the equation has no way to see that difference — it works entirely from weight, height, age and sex, with no way to distinguish what that weight is actually made of.
Why ages under fifteen are declined outright
The equation was derived and validated in adults, and children's energy needs are different from an adult's and change continuously with growth in a way the formula was never built to capture. Producing a number for a child anyway would look precise while being genuinely misleading, which is why that age range is refused rather than estimated.
Worth a professional's input before acting on it
Energy needs depend on health conditions, medication, activity and a great deal else that a formula alone cannot see, and a dietitian or doctor can weigh all of that together in a way a single equation cannot. Any real change to how you eat is worth discussing with a professional first, and if your relationship with food or your weight is causing distress, that is worth raising with one too.