How this works
The classic energy-balance equation says fat loss equals calorie deficit divided by the energy density of body tissue: about 7700 kcal per kg of fat (or 3500 kcal per pound — the famous "3500 rule"). To lose 1 kg of fat in a week, you'd need a daily deficit of 7700 / 7 ≈ 1100 kcal. Combined with your TDEE (maintenance calories), that gives a target intake. Smaller deficits are slower but safer; larger deficits work but get harder to sustain and cause more lean-mass loss along with fat.
The "1% of body weight per week" rule is the safe-pace heuristic most evidence-based dietitians use. Faster than that and you're likely losing more lean tissue, your hunger and fatigue increase disproportionately, and adherence drops. This calculator flags the pace based on the implied weekly loss as a percentage of current body weight: under 0.6% is safe and sustainable, 0.6–1.1% is aggressive but workable for short cuts (4–8 weeks), over 1.1% is not recommended without medical supervision.
Reality is messier than the math suggests. The 7700 kcal/kg figure assumes pure fat loss, but real-world weight loss includes water, glycogen, and some lean tissue. Initial weeks of a diet show faster scale loss because of glycogen and water depletion (a 5–10 kg overestimate is common in the first 2 weeks). The body also adapts: TDEE drops as you lose weight, so a fixed deficit produces diminishing returns. Plan to recalculate every 5–10 kg of progress.
The formula
TDEE is your maintenance calories — see the TDEE Calculator. 7700 kcal/kg is the fat-tissue energy density (approximate; uses 3500 kcal/lb in imperial). The pace bands come from the 1%-per-week heuristic widely used in evidence-based nutrition.
Example calculation
- Current 85 kg, target 75 kg, timeline 12 weeks, TDEE 2400 kcal.
- Total deficit needed = 10 × 7700 = 77000 kcal. Daily = 77000 / 84 ≈ 917 kcal. Target intake ≈ 2400 − 917 ≈ 1483 kcal.
- Weekly loss = 0.83 kg (0.98% of 85 kg) → aggressive but workable. Recompute after 5 kg of loss as TDEE will have dropped.
Frequently asked questions
Is the 7700 kcal/kg rule actually accurate?
It's a useful planning approximation. Pure adipose tissue is about 7700 kcal/kg (3500 kcal/lb), and over moderate cuts (5–15% of body weight) the math holds within ±10%. Where it breaks: very long, large cuts where TDEE adapts downwards (a phenomenon called "metabolic adaptation"); very low-fat individuals where lean-mass loss becomes a larger share; and the first 1–2 weeks of any cut where glycogen and water shifts dominate. Use it for planning, recalculate periodically.
Why does my weight loss stall after a few weeks?
Two reasons combined. First, TDEE drops as you lose weight — a smaller body burns fewer calories at rest and during activity, so your fixed-deficit shrinks in absolute terms. Second, behavioural drift: tracking gets less precise, "small" cheats accumulate, and exercise frequency tends to slip when restriction is high. Solutions: recalculate TDEE every 5–10 kg, periodically log meals strictly for a week to recalibrate, and consider planned diet breaks (1–2 weeks at maintenance every 6–10 weeks of cutting) to maintain compliance.