How to Use This Tool
Almost every TDEE calculator online asks you to pick between "lightly active", "moderately active" and "very active", and almost everybody picks one level too high. Those adjectives are aspirational, they are not descriptions, and the resulting overestimate is the single most common reason a carefully planned deficit produces no weight change. This calculator replaces the adjectives with descriptions of what a week actually looks like.
The inputs
Enter sex, age, height and weight. The tool calculates basal metabolic rate using the Mifflin-St Jeor equation, which has been the standard prediction equation since 1990 and is more accurate for contemporary populations than the older Harris-Benedict formula. Both figures are shown so you can see the disagreement between them, which is typically 3–7% and is a useful reminder of how approximate all of this is.
Choosing an activity level honestly
The multipliers range from 1.2 to about 1.9. Read the descriptions rather than the labels. A desk worker who trains hard four times a week but otherwise sits down is usually around 1.5, not 1.725 — because an hour in the gym is a small fraction of a day, and the sedentary remainder dominates. Conversely, a warehouse worker who never trains can legitimately be above 1.6 through daily movement alone.
The component doing most of the work here is NEAT: non-exercise activity thermogenesis, meaning walking, standing, fidgeting and general moving about. It varies by up to 2,000 calories a day between individuals of similar size, dwarfing the difference made by three gym sessions. It also drops during a diet, which is a significant reason why weight loss slows down before the maths says it should.
Using the goal planner
Four scenarios appear beneath the result: aggressive and moderate deficits, maintenance, and a lean surplus. Each shows a daily target and a projected weekly weight change based on roughly 7,700 calories per kilogram of body tissue. Two honest caveats apply. First, that conversion figure assumes fat loss; early weeks include water and glycogen changes that make the scale move faster than the maths predicts. Second, a deficit larger than about 25% of your TDEE reliably costs muscle mass and adherence, which is why the aggressive option carries a warning rather than a recommendation.
What to do with the number
Treat it as a hypothesis. Eat at the calculated maintenance figure for two to three weeks, weigh yourself daily at the same time and average each week. If the weekly average is stable, your real TDEE is close to the estimate. If it drifts, adjust by 100–150 calories and repeat. That measured figure will beat any equation, because it accounts for your actual NEAT, your actual food logging accuracy, and your actual metabolism rather than the population average of all three.
