Basal Metabolic Rate (BMR) Calculator

From BMR to TDEE

BMR is not your daily calorie target on its own. Almost no one spends twenty four hours a day motionless. Total daily energy expenditure, or TDEE, multiplies BMR by an activity factor that captures everything else: walking, fidgeting, exercising, doing chores, and the thermic effect of digesting food. The convention is a five-tier ladder. Sedentary, multiply by 1.2: a desk job, no exercise, minimal walking. Lightly active, 1.375: light exercise one to three days a week. Moderately active, 1.55: moderate exercise three to five days. Very active, 1.725: hard exercise six to seven days. Athlete or hard physical job, 1.9. The numbers are population averages and can be off by ten percent in either direction for a given individual, which matters when you are dialing in a precise weight goal.

What changes BMR

Several variables push BMR up or down. Lean body mass is the biggest single lever. Muscle tissue burns more calories at rest than fat tissue does, which is why two people of the same weight and height can have meaningfully different BMRs depending on body composition. Age lowers BMR by roughly one to two percent per decade after twenty, mostly because lean mass declines unless you actively maintain it through resistance training. Sex matters because men tend to carry more lean mass than women at the same body weight, which is why the Mifflin equation has different constants. Hormonal status, including thyroid function, also affects BMR significantly. People with untreated hypothyroidism can have BMRs ten to fifteen percent below the population average, and the reverse is true for hyperthyroidism. Acute illness, fever, and recovery from surgery all raise BMR temporarily.

How to use this number

Three common use cases get the most out of a BMR figure. Fat loss: target a daily deficit of roughly twenty percent below TDEE, which typically lands at five hundred to seven hundred fifty kilocalories per day, producing a sustainable loss of around half a kilogram or one pound per week. Larger deficits work in the short term but rebound for most people through hunger and metabolic adaptation. Muscle gain: target a surplus of ten to fifteen percent above TDEE while doing serious resistance training. Beyond that and most of the extra calories simply add fat. Maintenance: eat near TDEE and weigh yourself weekly to confirm. If your weight drifts up or down over four weeks, adjust intake by about two hundred kilocalories per day to compensate.

Limitations and accuracy

The Mifflin equation was validated in adult populations of mostly European descent. It is reasonably accurate across ethnicities but tends to overestimate BMR slightly in older adults and in people with very high body fat percentages. If accuracy matters more, a calorimetry test at a sports clinic or hospital gives a direct measurement, typically by analyzing the oxygen you consume and the carbon dioxide you exhale over twenty to thirty minutes. The cost is usually fifty to one hundred fifty dollars in the United States. For anyone outside an elite athletic context, the Mifflin estimate is close enough to plan from.

Use the calculator to anchor your starting point, then watch how your body responds over two to four weeks. The right calorie target is the one that produces the trajectory you want when you actually follow it.

How often to recalculate

BMR drifts slowly. Recalculate it whenever your weight changes by more than five kilograms in either direction, when you cross a major age milestone like ten years, or when your training or activity pattern shifts substantially. For most adults that means revisiting the number once or twice a year. A BMR that is many years out of date can mislead you into a calorie target that is too high or too low for your current body, which is the most common reason a previously working diet stops working.