# How long will my weight gain take?

Estimates how long it takes to reach a goal weight eating a fixed number of calories a day above your needs, with daily energy needs (Mifflin-St Jeor or Harris-Benedict × activity) rising as weight rises.

- Page: https://www.acalculator.org/health/weight-gain-calculator
- JSON spec: https://www.acalculator.org/health/weight-gain-calculator.json
- Version: 424fe7c1be9e

## Default answer

Example with the default inputs (Sex Female, Age 30, Height 5 ft 5 in, Weight 120 lb, Activity Light: exercise 1 to 3 days, Goal weight 130 lb, Calories you eat a day 2,200, Start date September 30, 2026, BMR formula Mifflin-St Jeor) on the example date Wednesday, September 30, 2026: Eating 2,200 calories a day, you reach your goal weight in about 11.7 weeks, on December 21, 2026.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| sex | Sex | Sex at birth, which the body formulas use. |
| age | Age | Your age in whole years. |
| height | Height | Your height without shoes. |
| weight | Weight | Your body weight. |
| activity | Activity | How active you are in a usual week, which sets the multiplier on your BMR. |
| goal | Goal weight | The weight you want to reach. |
| intake | Calories you eat a day | The calories you plan to eat each day until you reach your goal. |
| start | Start date | The day you start. Today, unless you change it. |
| formula | BMR formula | The equation for basal metabolic rate: Mifflin-St Jeor (1990) or the revised Harris-Benedict (1984). |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| date | You reach your goal on | The date you reach the goal weight: the start date plus the days to the goal, rounded up. |
| weeks | Weeks to your goal | Time to the goal weight in weeks, with daily needs rising as weight rises. |
| days | Days to your goal | Time to the goal weight in whole days, rounded up. |
| tdee | Calories you burn now (TDEE) | Total daily energy expenditure at your current weight: BMR × activity factor. |
| surplus | Daily surplus now | The calories you eat minus your TDEE at your current weight. |
| tdeeGoal | Calories you burn at your goal weight | TDEE at the goal weight, which is higher, so the surplus shrinks as you gain. |
| poundWeek | Calories for about 1 lb a week now | TDEE now + 500: a 3,500-calorie surplus a week, a pound a week at the start by the 3,500-calorie rule. |
| firstWeek | Gain in the first week | Weight gained in the first 7 days. |
| simpleWeeks | Weeks by the 3,500-calorie rule | Weeks to the goal if the surplus stayed at its starting size. |

## Method

Days to goal = (7,716.2 ÷ s) × ln(S₀ ÷ S_g), where S₀ and S_g are the daily surpluses (intake − TDEE) at the current and goal weights and s is the rise in TDEE per kilogram (activity factor × BMR weight coefficient).

## Assumptions

- A pound of body weight is 3,500 calories (7,716.2 calories a kilogram).
- Daily needs are the BMR (Mifflin-St Jeor or revised Harris-Benedict) × the activity factor, recomputed continuously as weight rises; age, height and activity stay as entered.
- The daily intake stays the same until the goal. The goal date rounds the days up to a whole day.
- This is an estimate for planning, not medical advice. Talk to a doctor or a registered dietitian if you need to gain weight for your health.

## Worked examples

1. sex = female, age = 30, height = 1.651, weight = 54.431084, activity = light, formula = mifflin, goal = 58.967008, intake = 2,200, start = 2026-10-01 gives tdee = 1,739.630536, surplus = 460.369464, tdeeGoal = 1,801.999486, poundWeek = 2,239.630536, weeks = 11.670555, days = 82, date = 2026-12-22, firstWeek = 0.415046, simpleWeeks = 10.860842. Source: Mifflin et al. 1990; 3,500 kcal per lb (Wishnofsky 1958).
2. sex = male, age = 25, height = 1.78, weight = 65, activity = moderate, formula = mifflin, goal = 70, intake = 3,000, start = 2027-01-01 gives tdee = 2,545.875, surplus = 454.125, weeks = 13.307564, date = 2027-04-05, simpleWeeks = 12.136651. Source: Mifflin et al. 1990.
3. sex = female, age = 40, height = 1.6, weight = 50, activity = sedentary, formula = harris, goal = 55, intake = 1,900, start = 2026-11-15 gives tdee = 1,478.9076, weeks = 14.035088, days = 99, date = 2027-02-22. Source: Roza and Shizgal 1984 (revised Harris-Benedict).

## FAQ

### How many calories do I need to gain weight?

More than you burn. Your total daily energy expenditure (TDEE) is your basal metabolic rate times an activity factor; eating above it gives a surplus. By the 3,500-calorie rule, 500 calories a day above your TDEE is about a pound a week at the start. The calculator shows that number as "Calories for about 1 lb a week now".

### How long will it take to reach my goal weight?

It depends on the surplus. For example, a 30-year-old woman who is 5 ft 5 in, 120 lb and lightly active burns about 1,740 calories a day. Eating 2,200 a day, she reaches 130 lb in about 11.7 weeks.

### Why is this slower than the 3,500-calorie rule?

A heavier body burns more calories, so as you gain, the same intake gives a smaller surplus each week. This calculator lets your needs rise as your weight rises, so the time to your goal is longer than the simple rule says.

### Why does it say my weight levels off before my goal?

At a fixed intake, your weight moves toward the weight at which you burn exactly what you eat. If that weight is below your goal, you would never reach the goal at that intake. Eat more, or check your activity level.

### Is this for muscle gain or fat gain?

The maths counts body weight only. How much of the gain is muscle depends on training, protein and many other things this model does not include.

### Who should get advice before trying to gain weight?

Anyone who is underweight without knowing why, has lost weight without trying, or has a medical condition. A doctor or registered dietitian can check the cause and suggest a safe plan.

## Sources

- Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr. 1990;51(2):241-247. https://doi.org/10.1093/ajcn/51.2.241
- Roza AM, Shizgal HM. The Harris Benedict equation reevaluated: resting energy requirements and the body cell mass. Am J Clin Nutr. 1984;40(1):168-182. https://doi.org/10.1093/ajcn/40.1.168
- FAO/WHO/UNU. Human energy requirements (2004), section 5.3.1, Classification of physical activity levels (PAL = TEE ÷ BMR). https://www.fao.org/4/y5686e/y5686e07.htm
- Wishnofsky M. Caloric equivalents of gained or lost weight. Am J Clin Nutr. 1958;6(5):542-546. https://doi.org/10.1093/ajcn/6.5.542
- Hall KD, Sacks G, Chandramohan D, et al. Quantification of the effect of energy imbalance on bodyweight. Lancet. 2011;378(9793):826-837. https://doi.org/10.1016/S0140-6736(11)60812-X