Calorie Calculator

Think of your body as a car with two fuel gauges: one for idling and one for driving. The idling gauge is your BMR — the calories your body burns just to keep the engine running while you sleep, breathe, and pump blood. Every adult burns a meaningful amount this way before they take a single step. The driving gauge is TDEE — your actual daily expenditure once you factor in how much you move.

The Mifflin-St Jeor equation calculates BMR using four inputs: weight, height, age, and biological sex. Weight and height determine how much metabolic tissue you carry. Age reduces the estimate slightly because lean muscle mass tends to decline over time, lowering resting burn. Sex accounts for the fact that biological males typically carry more muscle per unit of body weight, which burns more calories at rest. The formula then multiplies BMR by an activity factor between 1.2 and 1.9 — your TDEE.

Your daily calorie target is the number you need to eat to land in the zone you have chosen: maintenance keeps you at TDEE, weight loss subtracts 500 calories, and muscle gain adds 300. The 500-calorie deficit figure comes from the rule that one pound of body fat contains roughly 3,500 calories — a deficit of 500 per day creates a deficit of 3,500 per week, producing approximately one pound of loss. This math holds well over the medium term, though it becomes less precise as body weight drops and metabolism adapts.

This calculator works well for healthy adults who want a reasonable starting point for a calorie target — whether they are losing weight before a life event, building muscle after adding a gym routine, or simply trying to understand why their current intake is not producing the results they expect. It is also useful as a periodic recalibration check: run it again after a major weight change, a shift in your job or lifestyle, or after returning from a period of inactivity.

Do not rely on this calculator as your sole guide if you are recovering from an eating disorder, have been diagnosed with a metabolic condition like hypothyroidism, are pregnant or breastfeeding, have had bariatric surgery, or are an elite athlete training more than 15 hours per week. Each of these situations involves factors the Mifflin-St Jeor formula cannot capture.

Also be cautious about using this result in combination with wearable fitness tracker data without understanding how devices estimate calorie burn. Most consumer devices overestimate activity-based calorie expenditure by 15-30 percent. If your tracker says you burned 600 calories on a run and you eat them all back, you may be eating more than you think. The formula-based TDEE is often more conservative and more accurate as a planning target than device-reported burn data.

The most common mistake is overestimating activity level. Choosing Very active when the reality is Moderately active can inflate your TDEE by 200-350 calories — enough to completely cancel a weight loss deficit. The activity levels describe your typical week across all waking hours, not just your workout intensity. A person who runs five miles three times per week but sits at a desk for eight hours otherwise is Lightly to Moderately active, not Very active.

The second common mistake is treating the result as a fixed target forever. As you lose weight, your BMR drops because you are carrying less mass. A person who starts at 220 lb and loses 30 lb will burn noticeably fewer calories at their new weight, even with identical activity. Recalculate every 10-15 pounds of change to keep the target accurate.

The third mistake is applying a weight loss deficit that is too aggressive and then quitting when progress stalls after a few weeks. A very large deficit triggers metabolic adaptation — the body lowers its resting burn to compensate. A 500-calorie deficit is the largest most people can sustain without triggering significant adaptation. Deficits above 750-1,000 calories per day tend to accelerate muscle loss and eventually slow or reverse progress.

The Mifflin-St Jeor formula for BMR is:

For males: BMR = (10 x weight in kg) + (6.25 x height in cm) - (5 x age) + 5 For females: BMR = (10 x weight in kg) + (6.25 x height in cm) - (5 x age) - 161

The only difference between male and female formulas is the constant at the end: +5 versus -161. This 166-calorie gap reflects average differences in body composition between biological sexes.

TDEE is then: BMR x activity multiplier. Activity multipliers range from 1.2 (sedentary) to 1.9 (extremely active). These multipliers were derived from doubly-labeled water studies, the gold standard for measuring real-world energy expenditure. A moderately active person with a BMR of 1,800 calories would have a TDEE of 2,790 calories — more than 50 percent higher than their resting burn.

BMI is calculated separately as: weight in kg divided by height in meters squared. BMI is shown as a reference figure, not a target. It does not distinguish between muscle mass and fat mass, which limits its usefulness for athletes and anyone with unusual body composition. The calorie result does not change based on BMI — BMI only triggers warnings at the extremes.

The Mifflin-St Jeor formula assumes a fixed relationship between body weight and metabolic rate, but this breaks down at the extremes of body composition. Two people who weigh 185 lb — one with 15 percent body fat and one with 35 percent body fat — will have meaningfully different BMRs because muscle tissue burns roughly three times more calories at rest than fat tissue. The formula gives them the same answer. If you know your body fat percentage, a lean mass-based formula (like the Katch-McArdle) is more accurate. At a BMI above 35 or below 19, the standard multipliers also lose predictive accuracy — metabolic adaptation in both obesity and underweight states makes the formula's assumptions less reliable.