Gaining weight is as much a numbers problem as losing it. Your body only builds new tissue when you eat more energy than you use, and the size of that surplus decides how fast the scale moves. This calculator estimates your maintenance calories, adds the surplus you pick, and then recalculates every week as you get heavier, because a bigger body uses more energy and shrinks the surplus. The result is a timeline that is longer, and more realistic, than the straight-line sums often quoted.
It also checks the pace against sports-nutrition guidance and shows the weight at which your chosen intake would stop producing gain.
How to use the weight gain calculator
- Enter your sex, age, height and current weight.
- Enter your goal weight, which must be higher than your current weight.
- Choose your activity level and a daily surplus. Smaller surpluses give a slower, leaner gain.
- Read the calories to eat, weeks to goal, weights at 4, 12 and 26 weeks and the pace check.
Weight gain formula
Intake = maintenance at starting weight + daily surplus
Weekly gain (kg) = (intake − maintenance(w)) × 7 ÷ 7,700
Pace = surplus × 7 ÷ 7,700 ÷ weight × 100 (% of body weight per week)
Maintenance uses the Mifflin-St Jeor equation (Am J Clin Nutr 1990;51:241–247) with the activity levels from the Institute of Medicine’s Dietary Reference Intakes for Energy (2005). The 7,700 kcal per kilogram figure is the traditional approximation of the energy stored in gained tissue; real values depend on whether the gain is fat or lean. The 0.25 to 0.5 percent of body weight per week guide comes from Iraki J, Fitschen P, Espinar S, Helms E, Sports 2019;7:154 (nutrition recommendations for bodybuilders in the off-season).
Worked example
A 25-year-old man, 180 cm, 65 kg, low active, wants to reach 72 kg with a 500 kcal surplus.
- Maintenance = (650 + 1,125 − 125 + 5) × 1.5 = 2,482 kcal
- Intake = 2,982 kcal a day
- Week 1 gain = 500 × 7 ÷ 7,700 = 0.45 kg. Each kilogram gained raises maintenance by 15 kcal.
- Time to goal: 17 weeks, against 15 weeks by the fixed-surplus rule.
- Weight after 4, 12 and 26 weeks: 66.8, 70.1 and 72.0 kg.
Week-by-week projection
| Week | Weight | Gained so far | Maintenance at that weight |
|---|---|---|---|
| 0 | 65.0 kg (143 lb) | 0.0 kg | 2,482 kcal |
| 4 | 66.8 kg (147 lb) | 1.8 kg | 2,509 kcal |
| 8 | 68.5 kg (151 lb) | 3.5 kg | 2,535 kcal |
| 12 | 70.1 kg (154 lb) | 5.1 kg | 2,558 kcal |
| 20 | 72.0 kg (159 lb) | 7.0 kg | 2,588 kcal |
| 26 | 72.0 kg (159 lb) | 7.0 kg | 2,588 kcal |
Gaining the right kind of weight
- Lift weights. Resistance training directs more of the surplus to muscle.
- Eat enough protein. A meta-analysis of 49 studies (Morton et al., Br J Sports Med 2018;52:376–384) found benefits for muscle gain up to about 1.6 g of protein per kg per day; use the Protein Intake Calculator.
- Keep the surplus modest. Larger surpluses mainly add fat, and a 250 to 500 kcal gap is usually enough.
- Be patient. Muscle gain is slow, often under 1 kg a month for experienced lifters.
- Track more than weight. Waist, strength and photos tell you whether the gain is useful; the Body Fat Calculator and FFMI Calculator help.
When gaining is hard
Some people struggle to eat enough because of a small appetite, a high activity level or an underlying medical condition. Calorie-dense foods such as nuts, whole milk, olive oil and smoothies can help. If your BMI is below 18.5, check your goal range with the Healthy Weight Range Calculator and speak to a doctor about unintended weight loss. For daily needs see the Calorie Calculator, TDEE Calculator and Macro Calculator.
This page is for information only and is not medical or dietary advice. Speak to a doctor or registered dietitian about your own situation.
Sources
- Mifflin MD, St Jeor ST, et al. Am J Clin Nutr 1990;51:241–247.
- Institute of Medicine. Dietary Reference Intakes for Energy, 2005 (Physical Activity Level categories).
- Iraki J, Fitschen P, Espinar S, Helms E. Nutrition recommendations for bodybuilders in the off-season: a narrative review. Sports 2019;7:154.
- Morton RW, et al. A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength in healthy adults. Br J Sports Med 2018;52:376–384.
Frequently asked questions
How many calories do I need to gain weight?
You need to eat above your maintenance calories. A surplus of 250 to 500 kcal a day is a common starting point. The calculator estimates your maintenance with the Mifflin-St Jeor equation and adds the surplus you choose.
How fast should I gain weight?
For gaining mainly muscle, sports nutrition reviews (Iraki et al., Sports 2019;7:154) suggest a rate of roughly 0.25 to 0.5 percent of body weight per week for experienced lifters, with beginners able to gain somewhat faster. Faster gain tends to add more fat. The calculator warns if your pace is above 0.5 percent a week.
Will the extra weight be muscle?
Only partly. Without resistance training, most of a calorie surplus is stored as fat. Combining a modest surplus with strength training and enough protein (about 1.6 g per kg of body weight, according to the meta-analysis by Morton et al., Br J Sports Med 2018;52:376–384) favours muscle gain.
Why does the timeline differ from a simple calculation?
As you gain weight your maintenance calories rise, so the same intake leaves a smaller surplus each week. The calculator recalculates this weekly. If your intake is eventually equal to your new maintenance, weight gain stops, which is the ceiling shown.
When should I see a doctor about being underweight?
If your BMI is under 18.5, you have lost weight without trying, or you find it hard to eat enough, speak to a doctor. Unintended weight loss can have medical causes and may need investigation.