For educational and tracking purposes only. Results are estimates and are not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare professional.
Calculator tool
How this calculator works
Use the explanation to understand the formula, assumptions, and practical limits behind the calculator result.
The Calorie Deficit Formula
Weight change is driven by the cumulative energy imbalance between intake and expenditure:
- TDEE: Total Daily Energy Expenditure — your maintenance calories (estimated from TDEE calculator or tracked over 2–3 weeks)
- 7,700 kcal/kg: the approximate energy content of 1 kg of body fat (1 lb ≈ 3,500 kcal)
- Safe limit: 500–1,000 kcal/day deficit for most adults (0.5–1.0 kg/week loss)
Worked Example
Goal: lose 8 kg in 16 weeks | TDEE: 2,400 kcal/day
Total deficit: 8 kg × 7,700 = 61,600 kcal
Daily deficit: 61,600 ÷ 112 days = 550 kcal/day
Daily target: 2,400 − 550 = 1,850 kcal/day
This is within the safe range and projects 0.5 kg/week — achievable with a sustainable diet approach.
Why the 3,500 kcal/lb Rule Is an Approximation
The classic rule overestimates actual weight loss for two reasons:
- Mixed tissue loss: weight lost is not pure fat — water, glycogen, and some lean mass are also shed, especially early in a deficit
- Metabolic adaptation: as you lose weight, TDEE decreases — both from carrying less mass and from adaptive thermogenesis (the body reducing BMR by 10–15% more than weight loss alone would predict)
In practice, the first 1–2 kg lost is often partially water; long-term fat loss runs 10–20% slower than the simple formula suggests. Revalidate your TDEE every 4–6 weeks.
Safe Deficit Ranges
| Weekly Goal | Daily Deficit | Context |
|---|---|---|
| 0.25 kg/week | ~270 kcal/day | Slow, maximum lean mass preservation |
| 0.5 kg/week | ~550 kcal/day | Standard recommendation |
| 0.75 kg/week | ~820 kcal/day | Aggressive; acceptable short-term |
| 1.0 kg/week | ~1,100 kcal/day | Maximum safe; risk of lean mass loss |
| >1.0 kg/week | >1,100 kcal/day | Not recommended without medical supervision |
Frequently asked questions
Is the 3,500 kcal = 1 lb of fat rule accurate?
It is a useful first approximation, not a precise rule. The 3,500 kcal/lb estimate from Wishnofsky (1958) applies to pure adipose tissue. In practice, weight loss includes water, glycogen, and some lean mass — so the actual calorie deficit required per pound of weight lost varies and tends to increase over time as adaptation occurs. The NIH Body Weight Planner uses a dynamic model accounting for metabolic adaptation and produces more accurate long-term predictions. For planning purposes, the 3,500 kcal/lb rule is sufficient; just expect some deviation and recalibrate based on actual results.
How should I adjust my target as I lose weight?
As your body weight decreases, your TDEE falls — you have less mass to sustain and some adaptive metabolic reduction occurs. A common approach is to recalculate TDEE every 4–6 weeks or after 4–5 kg of weight loss and update your daily target accordingly. Tracking your actual weekly weight trend (average 7-day weight, not daily fluctuations) and adjusting calories by 100–200 kcal based on results is more accurate than relying on formula recalculation alone. If weight loss stalls for 3+ weeks, reduce calories by 100–150 kcal or increase daily steps.
What happens physiologically with an aggressive deficit above 1,000 kcal/day?
Deficits beyond 1,000 kcal/day trigger disproportionate lean mass loss — the body catabolises muscle tissue for gluconeogenesis when fat mobilisation cannot keep pace with energy demand. Cortisol rises, suppressing anabolic signalling. Adaptive thermogenesis deepens: BMR may fall 15–25% more than weight loss alone would predict. Appetite hormones become dysregulated — ghrelin surges and leptin falls sharply, making hunger substantially harder to manage. Research consistently shows that moderate deficits produce a higher proportion of fat loss to total weight loss than crash diets. Slower is biologically more effective for body composition.
What are diet breaks and why do they help?
A diet break is a planned 1–2 week period at maintenance calories after 8–12 weeks of deficit eating. Returning to TDEE temporarily reverses some adaptive thermogenesis, normalises leptin and other hunger hormones, restores performance and glycogen stores, and provides psychological relief that improves long-term adherence. Research on intermittent energy restriction — alternating 2 weeks dieting with 2 weeks at maintenance — shows similar total fat loss to continuous restriction but better lean mass preservation. Diet breaks do not 'undo' a deficit; they are a strategic pause, not a failure.
