Weight Gain Calculator
The daily calories needed to gain weight at a chosen rate — with the surplus that adds muscle rather than mostly fat.
A kilogram of body mass gained is roughly 5,000 to 7,000 kcal of surplus, depending on how much is lean tissue.
How the weight gain calculator works
A kilogram of body mass gained is roughly 5,000 to 7,000 kcal of surplus, depending on how much is lean tissue. This uses 6,000 kcal/kg, so a 0.25 kg per week gain needs about 215 kcal a day above maintenance.
The rate matters more than the total. Gaining faster than about 0.5% of body weight per week adds disproportionately more fat, because muscle synthesis has a ceiling that eating more does not raise.
Formula: surplus = rate × 6000 / 7 kcal per day
Worked examples
| Inputs | Daily calories | Note |
|---|---|---|
| 65 kg, 2,400 maintenance, 0.25 kg/week | 2,614 kcal | 2,614 kcal |
| Faster gain | 3,043 kcal | flagged as very fast |
| A larger person | 3,200 kcal | the same percentage rate |
FAQFrequently asked questions
How many calories to gain weight?
About 6,000 kcal of surplus per kilogram gained. For 0.25 kg a week that is roughly 215 kcal a day above maintenance.
How fast should I gain?
Around 0.25 to 0.5% of body weight per week. Faster adds disproportionately more fat, because muscle synthesis has a ceiling.
Why can I not just eat more to build muscle faster?
Because the rate of muscle protein synthesis is limited by training and recovery, not by calories. Surplus beyond what it can use becomes fat.
How much protein do I need?
Around 1.6 to 2.2 g per kilogram of body weight for building muscle. More than that shows no additional benefit in the research.
Why am I not gaining?
Almost always because intake is lower than estimated. Maintenance figures are approximations, and people under-report food intake substantially.
Where these figures come from
- Mifflin MD et al., "A new predictive equation for resting energy expenditure in healthy individuals", Am J Clin Nutr 1990 — the BMR equation used here
- World Health Organization — Waist circumference and waist–hip ratio (2008 expert consultation) — waist-to-hip cut-offs
- Tanaka H, Monahan KD, Seals DR, "Age-predicted maximal heart rate revisited", J Am Coll Cardiol 2001 — 208 − 0.7 × age
- Nathan DM et al., "Translating the A1C assay into estimated average glucose values" (ADAG), Diabetes Care 2008 — eAG = 28.7 × A1c − 46.7
- Mosteller RD, "Simplified calculation of body-surface area", N Engl J Med 1987 — BSA = √(height × weight / 3600)
- NHS — national health guidance
Last checked: September 2026. The equations are fixed by their published papers and do not change with tax years or regulations; the thresholds quoted in verdicts name the body that set them.