TDEE calculator.Mifflin or Katch.

§ 01 · How TDEE works

BMR is the energy your body burns at rest — breathing, circulation, organ function. TDEE is BMR multiplied by an activity factor that accounts for everything else: walking, training, fidgeting, thinking.

Your daily intake against TDEE is your deficit (loss) or surplus (gain). One pound of fat is roughly 3,500 kcal — but that's a rule of thumb, not a contract. Real-world results lag the math by a week or two, and water shifts can mask actual change for several days at a time.

§ 02 · Mifflin–St Jeor vs. Katch–McArdle

Mifflin–St Jeor uses sex, age, height, and weight. Reasonable for most people. Tends to over-estimate at high body fat and under-estimate for very lean athletes.

Katch–McArdleuses lean body mass instead of total weight. More accurate at the tails of body composition. The catch: you need a body-fat number you trust. If you don't, the US Navy body-fat calculator gets you a usable estimate in two minutes.

§ 03 · Activity factor, demystified

The activity factor is the softest part of the calculation. The five-level breakdown is conventional but coarse:

• Sedentary (1.2) — desk job, almost no exercise.
• Lightly active (1.375) — 1–3 sessions/week.
• Moderately active (1.55) — 3–5 sessions/week.
• Very active (1.725) — 6–7 sessions/week.
• Extra active (1.9) — physical job + training.

If you're between two levels, pick the lower one for two weeks and adjust based on actual weight change. The number is an estimate either way.

§ 04 · How BiteDeck uses this

In the app, this calculation runs daily — against your logged weight, your saved body fat %, and your activity (HealthKit imports help here). The forecast watches the rolling average against the projection and tells you when the trend reverses. See On-device weight forecasting for the rest of the math.