Weight Loss Calculator
How This Calculator Works
Simple energy-balance math: each kilogram of body fat stores about 7,700 kcal. So sustained daily deficit times days, divided by 7,700, gives kilograms lost.
Δweight (kg) ≈ (deficit_kcal_per_day × days) / 7,700
Δweight (lb) ≈ (deficit_kcal_per_day × days) / 3,500
Projected weight = current weight − ΔweightCitations: The 3,500 kcal/lb (= 7,700 kcal/kg) rule traces to Wishnofsky M., "Caloric equivalents of gained or lost weight," American Journal of Clinical Nutrition, 1958;6(5):542–546. For a more accurate dynamic model that accounts for metabolic adaptation, see Hall KD, Sacks G, Chandramohan D, et al. "Quantification of the effect of energy imbalance on bodyweight." The Lancet, 2011;378(9793):826–837.
Why Real Results Differ
The 3,500 kcal-per-pound rule overestimates long-term loss. As you lose weight your TDEE drops (a smaller body burns less), and spontaneous activity tends to drop too. After a few months, the same diet that produced a 500 kcal/day deficit at the start might be only a 200 kcal/day deficit. Recalculate your TDEE every 5–10 lb of progress and adjust your calorie target downward to keep moving.
Limitations
The first 5–10 lb of any deficit-driven loss is largely water and glycogen, not fat — so early loss tends to outpace the math. After that, fat loss is slower than the projection because of adaptive thermogenesis (Hall et al., 2011, models this in detail). Use the projection for planning expectations, not as a precise schedule.
Lower bounds: avoid sustained calorie targets below ~1,200 kcal (women) or ~1,500 kcal (men) without clinical guidance. If your math pushes you that low, consider extending the timeline or adding activity instead of cutting more.
Frequently Asked Questions
Where does the 3,500 kcal per pound rule come from?
It's a 1958 estimate by Max Wishnofsky, based on the energy density of pure body fat. It's a useful shorthand for short-term planning — at a 500 kcal/day deficit, you can expect to lose about 1 lb of fat per week, at first. The rule overestimates loss long-term because metabolism adapts as you get smaller. Hall et al. (2011) published a more accurate dynamic model that accounts for that adaptation.
Why does the calculator show a slowdown?
Real bodies adapt to deficits. As you lose weight, your TDEE drops because a smaller body needs fewer calories. Your activity tends to drop slightly too (less spontaneous movement). After a few months, you might be in only a 200 kcal deficit even though you're still eating the same number that produced a 500 kcal deficit at the start. The simple model ignores this; we project the simple version, but you should expect to plateau and need to adjust.
Is 1 lb per week realistic long-term?
Realistic for the first few weeks. After 8–12 weeks, most people see the rate slow to 0.5 lb/week or less, even at the same calorie target. This is normal and expected — recalculate your TDEE every 5–10 lb of weight change and you'll keep results moving. Going below ~1,200 kcal (women) or ~1,500 kcal (men) to maintain a high deficit is generally not recommended.
What deficit should I pick?
For most healthy adults, 500 kcal/day (~1 lb/week) is sustainable for the first 2–3 months. 250 kcal/day (~0.5 lb/week) is gentler and easier to maintain long-term. Above 750 kcal/day in deficit tends to come with more muscle loss, faster metabolic adaptation, and higher dropout rates — even though the scale moves faster initially.
Will I lose only fat?
No, but you can lose mostly fat. With a moderate deficit (under 1% of bodyweight per week), enough protein (1.6–2.2 g/kg), and resistance training, the majority of weight loss is fat. Without those things — especially without strength training — you can lose 25% or more as muscle. The protein and lifting matter more than the deficit size.
What does the projection ignore?
Glycogen and water — the first 5–10 lb of any deficit-driven loss is largely water and glycogen depletion, which is fast and doesn't follow the 3,500 kcal rule. After that, fat loss kicks in. The projection treats the entire change as fat. For 8+ week projections this is fine; for shorter periods, expect the early loss to be faster than the math suggests.
Can I trust this projection?
For directional planning, yes. For exact numbers, no. The actual rate depends on adherence, metabolic adaptation, water and glycogen swings, and your individual physiology. Use the projection to set expectations and pick a deficit, not to predict your weight on a specific date. If you're planning around a date, give yourself a buffer.
What if I want to gain weight?
Same math, opposite direction — enter a negative deficit (i.e., a surplus). For lean muscle gain, 250 kcal/day surplus is realistic for most. 500 kcal/day surplus tends to come with more fat gain. You can't add muscle infinitely fast: roughly 2 lb/month for beginners, less for intermediate lifters, well under that for advanced.