Mifflin-St Jeor is the best-validated of the common equations, but it's a population average — individual metabolisms run ±10% or more around it. Use the number as a two-week experiment: eat at the target, watch the scale's weekly trend, and adjust by 100–200 kcal at a time based on what actually happens.

Almost everyone picks too high. The multiplier covers your whole life, not just workouts — a desk job plus four gym sessions is 'moderate', not 'very active'. If you're between two levels, pick the lower one; it's easier to add calories later than to explain a stalled cut.

It's the standard for losing about half a kilo a week, and it's sustainable for most people most of the time. But if the target lands below your BMR — common for smaller or lighter people — the calculator flags it: living below basal burn is a medical conversation, not a diet plan.

Losing fat tolerates a bigger deficit than building muscle tolerates a surplus. A +300 kcal surplus supports lean gaining for most people; eat much beyond it and the extra shows up as fat rather than faster muscle. The asymmetry is biology, not caution.