What should you actually eat to hit your goal?

For resting metabolic rate we use Mifflin–St Jeor: 10·kg + 6.25·cm − 5·age, then +5 for men or −161 for women. The 1990 validation reported R²=0.71, and it remains the most reliable common prediction equation for healthy adults — outperforming the older Harris–Benedict. Accuracy drops at very high BMI or in very muscular people, which is exactly why the Calibrate tab exists.

The textbook 1.2–1.9 activity factors trace to early-20th-century cohorts and tend to overshoot the needs of modern, mostly-seated people by roughly 15–25%. We deliberately use lower multipliers as a starting point. This isn't the tool being stingy — it's the tool being honest about the data.

Constrained-energy-expenditure research (Pontzer and colleagues) shows the body adapts at higher activity, so TDEE does not keep climbing in a straight line as you train more. That's a second reason very high 'very active' multipliers mislead — and why calibrating against your own weight trend beats any formula.

Energy balance says: change in body stores = intake − expenditure. So if you know your average intake and how your weight actually trended, you can solve for your real expenditure: TDEE ≈ average intake minus the daily energy implied by the weight change. Lose weight faster than expected and your true burn is higher than any calculator guessed. This is the same identity MacroFactor-style adaptive engines run.

Converting a weight trend into an energy gap uses the classic figure of about 7,700 kcal per kg (≈3,500 kcal per lb) for the fat-mass component. It's an approximation — early-diet water shifts and lean-mass changes muddy it — which is why we recommend a window of at least two weeks and a smoothed (7-day average) weight at each end.

Morton et al. 2018 pooled 49 randomized trials (n≈1,863) and found the gains in fat-free mass flatten beyond a breakpoint of ~1.62 g/kg/day, with the confidence interval reaching ~2.2 g/kg. We set the floor at 1.6 (higher, 1.8, when you're cutting, since protein needs rise in a deficit) and the ceiling at 2.2. Above that, the meta-analytic evidence shows no further gain.