What this calculator does
A weight gain calculator works out the calorie surplus needed above maintenance to gain weight at a chosen, gradual rate. It starts from your total daily energy expenditure (TDEE), estimated from weight, height, age, sex and activity level, then adds a surplus sized to a weekly gain target you choose.
The arithmetic mirrors a calorie deficit calculator in reverse: instead of subtracting calories to lose weight, this adds them to gain it, using the same widely used approximation that roughly 7,700 kilocalories corresponds to one kilogram of body weight change. Weight gained this way is not purely muscle or purely fat; how much of each depends heavily on training, protein intake and the size of the surplus, none of which this calculator can see.
The formula
TDEE is estimated with the Mifflin-St Jeor equation for basal metabolic rate, multiplied by an activity factor. The daily surplus needed is the target weekly weight gain, in kilograms, multiplied by 7,700 kilocalories and divided by 7 days. The daily calorie target is TDEE plus that surplus.
| Term | Meaning |
|---|---|
| TDEE | Total daily energy expenditure: the calories burned in an average day at your stated activity level. |
| Daily surplus | Extra calories eaten above TDEE each day, sized to the chosen weekly weight-gain rate. |
| 7,700 kcal/kg | The standard approximation for the calorie surplus that corresponds to one kilogram of weight gained. |
The inputs explained
| Field | What to enter |
|---|---|
| Weight (kg) | Current body weight in kilograms. |
| Height (cm) | Height in centimetres. |
| Age (years) | Age in years. |
| Sex | Biological sex, used in the Mifflin-St Jeor equation. |
| Activity level | How active a typical week is, outside of any specific training goal. |
| Target weekly weight gain (kg/week) | How much weight you are aiming to gain per week, in kilograms. |
When to use it
Planning a gradual, controlled surplus
Choosing a slower weekly rate keeps the daily surplus modest, which tends to favour lean tissue gain over a rapid weight increase that is more likely to be mostly fat.
Working from a training goal rather than a diet
Someone training for strength or size gains typically wants a known, deliberate surplus rather than eating however much feels right, since an unmeasured surplus can run much higher or lower than intended.
Checking whether a current intake is already enough
Comparing the calculated daily target against actual recorded intake shows whether a current diet is already close to, above, or well short of what a given weight-gain rate needs.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How the daily calorie target changes with the weekly gain rate
The same body at a range of weekly weight-gain targets.
| Target weekly weight gain | Daily calorie target | Daily surplus | Surplus as % of TDEE |
|---|---|---|---|
| 0.1 kg/week | 2,555 kcal/day | 110 kcal/day | 4.50% |
| 0.25 kg/week | 2,720 kcal/day | 275 kcal/day | 11.2% |
| 0.5 kg/week | 2,995 kcal/day | 550 kcal/day | 22.5% |
| 0.75 kg/week | 3,270 kcal/day | 825 kcal/day | 33.7% |
| 1 kg/week | 3,545 kcal/day | 1,100 kcal/day | 45.0% |
Questions
How much should I aim to gain per week?
There is no single correct rate, and this calculator does not set one for you: it is common for gradual gains aimed mostly at muscle to sit well under 0.5 kg a week, while faster rates bring a larger share of fat gain. A doctor or dietitian can help set a rate that fits your specific goals and health.
Why does a bigger surplus not just mean faster muscle gain?
Muscle can only be built at a limited rate regardless of how much surplus is available, so calories beyond what the body can use for that are stored mainly as fat. A very large surplus tends to shift the balance of weight gained away from muscle.
Does this calculator account for exercise?
Only indirectly, through the activity level you select, which is a broad estimate rather than a precise log of training. If you track workouts separately with a fitness device, its calorie-burn estimate may differ from the activity multiplier used here.
How is this different from a calorie deficit calculator?
The maths is the same in reverse. A calorie deficit calculator subtracts a deficit from TDEE to lose weight; this one adds a surplus to TDEE to gain it, using the same 7,700 kcal per kilogram approximation either way.
For the equivalent calculation in the loss direction, see the calorie deficit calculator. To check your energy needs at maintenance first, use the TDEE calculator.