Weight gain calculator

What is a weight gain calculator?

A weight gain calculator is a free online tool that works out how many calories you need to eat each day to put on weight at a controlled, sustainable pace. It starts from the energy your body burns in a day (your total daily energy expenditure, or TDEE) and adds the extra calories — the surplus — needed to build up the weight you want over the time frame you choose.

Gaining weight is the mirror image of losing it: when you consistently eat more calories than your body uses, the surplus is stored as new tissue and your body mass goes up. This calculator turns your age, sex, height, weight, activity level, target gain, and timeline into three practical numbers — your maintenance calories, the daily surplus you need, and the total daily calories to eat to hit your goal.

How does the calculator work?

The calculator runs in three stages:

1. Estimate your basal metabolic rate (BMR) — the calories your body needs at complete rest — using the Mifflin-St Jeor equation.
2. Estimate your maintenance calories (TDEE) by multiplying BMR by an activity factor.
3. Compute the surplus needed to gain your target weight over the chosen number of weeks, then add it to your maintenance calories to give the daily intake required.

Activity levels

Your activity level scales BMR up to your real-world energy expenditure:

• Sedentary (1.2): little to no exercise.
• Light exercise (1.375): light exercise or sports 1–3 days/week.
• Moderate exercise (1.55): moderate exercise or sports 3–5 days/week.
• Heavy exercise (1.725): hard exercise or sports 6–7 days/week.
• Extra active (1.9): very intense exercise or a physical job.

Formula

The Mifflin-St Jeor equation estimates BMR from weight in kilograms, height in centimetres, and age in years:

For men:

$$\text{BMR} = (10 \times \text{weight}) + (6.25 \times \text{height}) - (5 \times \text{age}) + 5$$

For women:

$$\text{BMR} = (10 \times \text{weight}) + (6.25 \times \text{height}) - (5 \times \text{age}) - 161$$

Maintenance calories (TDEE) follow from BMR:

$$\text{TDEE} = \text{BMR} \times \text{Activity Factor}$$

Because roughly 7700 kcal is stored in 1 kg of body mass (about 3500 kcal per pound), the daily surplus needed to gain a target weight over a number of weeks is:

$$\text{Surplus} = \frac{\text{Target gain (kg)} \times 7700}{\text{Weeks} \times 7}$$

The recommended daily intake to gain weight is then:

$$\text{Daily calories} = \text{TDEE} + \text{Surplus}$$

Examples

Example 1: 30-year-old woman, sedentary, gaining 5 kg in 10 weeks

Individual: 30 years old, 60 kg, 165 cm, sedentary, aiming to gain 5 kg over 10 weeks.

• Calculate BMR:

$$\text{BMR} = (10 \times 60) + (6.25 \times 165) - (5 \times 30) - 161 = 1320.25$$

• Calculate TDEE (Sedentary factor = 1.2):

$$\text{TDEE} = 1320.25 \times 1.2 = 1584$$

• Calculate the daily surplus:

$$\text{Surplus} = \frac{5 \times 7700}{10 \times 7} = 550 \text{ kcal/day}$$

• Calculate the daily calories to gain weight:

$$\text{Daily calories} = 1584 + 550 = 2134 \text{ kcal/day}$$

Example 2: 30-year-old man, moderate exercise, gaining 4 kg in 8 weeks

Individual: 30 years old, 80 kg, 180 cm, moderate exercise, aiming to gain 4 kg over 8 weeks.

• Calculate BMR:

$$\text{BMR} = (10 \times 80) + (6.25 \times 180) - (5 \times 30) + 5 = 1780$$

• Calculate TDEE (Moderate factor = 1.55):

$$\text{TDEE} = 1780 \times 1.55 = 2759$$

• Calculate the daily surplus:

$$\text{Surplus} = \frac{4 \times 7700}{8 \times 7} = 550 \text{ kcal/day}$$

• Calculate the daily calories to gain weight:

$$\text{Daily calories} = 2759 + 550 = 3309 \text{ kcal/day}$$

Notes

• A moderate, sustainable surplus of about 300–500 kcal per day is widely recommended for steady weight gain that favours muscle over fat, especially when paired with resistance training.
• A larger surplus and a faster timeline put on weight more quickly, but a greater share of that gain tends to be body fat.
• The 7700 kcal-per-kg rule is an approximation. Real weight change also depends on water balance, glycogen stores, training, and metabolic adaptation, so actual results vary.
• For specific health goals or medical conditions, consult a registered dietitian or healthcare provider before changing your diet substantially.

FAQs

How many calories do I need to gain weight?

You need to eat more than your maintenance calories (TDEE). The size of the surplus depends on how fast you want to gain: about 7700 kcal of extra energy is stored for each kilogram of body mass, so spreading that target over more weeks means a smaller daily surplus.

How fast should I gain weight?

For most people, aiming for roughly 0.25–0.5 kg (0.5–1 lb) per week is a sensible pace. Slower gains, combined with strength training and enough protein, tend to add more muscle and less fat than aggressive bulking.

Why is the result only an estimate?

The calculator assumes about 7700 kcal per kilogram of body mass and a constant metabolism. In reality your TDEE rises as you gain weight, and short-term scale movements include water and digestive contents, so your real rate of gain will fluctuate.

Can I gain weight without gaining fat?

Some fat gain is normal when you eat in a surplus, but you can shift the balance toward muscle by keeping the surplus moderate, training with resistance, and eating enough protein. A very large surplus speeds up the scale but usually adds more fat.

What if I am not gaining weight at all?

If the scale is not moving after a couple of weeks, you may be eating less than you think or burning more through activity. Increase your daily intake gradually — for example by 100–200 kcal — and reassess after another week or two.