Find out how many calories you need to eat to gain weight and reach your goal. The calculator estimates your maintenance calories with the Mifflin-St Jeor equation, then adds a surplus based on the pace you choose — and shows how long it will take to hit your target weight.
The weight gain calculator tells you how many calories you need to eat each day to gain weight and reach a target weight. Gaining weight in a healthy, controlled way is just as much a numbers game as losing it: you need to eat more calories than your body burns, creating a calorie surplus. This tool estimates your daily energy needs with the trusted Mifflin-St Jeor equation, adds the surplus that matches your chosen pace, and shows both your target daily intake and how long it will take to reach your goal. It is built for anyone who is underweight, recovering, building muscle, or simply trying to put on healthy weight.
This page explains what a calorie surplus is, the formulas the calculator uses, worked examples you can reproduce, how to gain weight healthily, and answers to the questions people ask most. By the end you will know exactly how many calories to aim for — and how to turn that number into real, lasting weight gain.
A calorie surplus is the state of eating more calories than your body burns in a day. Your body spends energy every day just staying alive and moving around — a total called your total daily energy expenditure, or TDEE. When the calories you eat exceed that total, the extra energy is stored, mostly as body tissue, and you gain weight. Put simply: to gain weight you must consistently eat above your maintenance level. The size of the surplus controls how fast you gain: a modest surplus produces slow, steady gain that favors muscle over fat, while a large surplus produces faster gain that includes more fat. This calculator works out your maintenance level first, then adds the right surplus for your goal.
The calculator combines three well-established relationships. First, it estimates your basal metabolic rate (BMR) — the calories you burn at complete rest — with the Mifflin-St Jeor equation:
Men: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age + 5
Women: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age − 161
Second, it multiplies your BMR by a physical activity factor to get your maintenance calories (TDEE):
maintenance calories = BMR × activity factor
Third, it adds a daily surplus based on how fast you want to gain, using the estimate that about 7,000 calories are needed to gain one kilogram of body weight:
daily surplus = weekly gain (kg) × 7,000 ÷ 7
So gaining 0.5 kg per week needs a surplus of about 500 calories a day, and gaining 1 kg per week needs about 1,000 calories a day. Your target intake is your maintenance calories plus this surplus.
Your activity factor scales your resting metabolism up to your real daily burn. The six levels are:
| Activity level | Factor |
|---|---|
| Little/no exercise (sedentary) | 1.2 |
| Light exercise 1–2 times/week | 1.4 |
| Moderate exercise 2–3 times/week | 1.6 |
| Hard exercise 4–5 times/week | 1.75 |
| Physical job or hard exercise 6–7 times/week | 2.0 |
| Professional athlete | 2.4 |
Consider a 28-year-old woman who is 170 cm tall, weighs 60 kg, does light exercise (factor 1.4), and wants to reach 65 kg at a normal pace of 0.5 kg per week. First, her BMR:
BMR = 10 × 60 + 6.25 × 170 − 5 × 28 − 161 = 1,361.5 kcal
Her maintenance calories are 1,361.5 × 1.4 ≈ 1,906 kcal/day. To gain 0.5 kg per week she needs a surplus of about 500 calories, so her target intake is roughly 2,406–2,476 kcal/day — the lower figure at her current weight and the higher figure as she approaches her goal, because a heavier body burns more. Reaching 65 kg from 60 kg at 0.5 kg per week takes 10 weeks. These are the calculator's default values, so you can press Calculate to see the full result and then enter your own numbers.
For most people, a modest surplus of about 300 to 500 calories a day is the sweet spot for healthy weight gain, producing roughly 0.25 to 0.5 kg (0.5 to 1 lb) of gain per week. A gentler surplus favors lean muscle over fat, especially when paired with strength training, and is easier on digestion. A larger surplus — around 1,000 calories a day for 1 kg per week — achieves faster gain but tends to add more fat, so it is usually reserved for people who are significantly underweight or "bulking" under guidance. As with weight loss, consistency beats extremes: a moderate surplus you can maintain for months will build far more lasting weight than an aggressive one you abandon after a week. If you are underweight or have a medical condition, work with a doctor or dietitian to set a safe target.
Hitting a calorie target is easier and healthier when the calories come from nourishing, energy-dense foods rather than junk. Prioritise protein — from meat, fish, eggs, dairy, legumes, and protein supplements — because it provides the building blocks for muscle and ensures your gains are not just fat. Add healthy calorie-dense foods like nuts, nut butters, olive oil, avocado, whole-milk dairy, oats, rice, and dried fruit, which pack a lot of energy into a small volume, making the surplus easier to eat. Eating more frequently — larger meals plus snacks — helps when a big appetite is a challenge, and drinking some of your calories in smoothies or milk avoids filling up too fast. Crucially, combine the surplus with resistance training: lifting weights signals your body to turn the extra calories into muscle rather than fat, transforming weight gain into strength and a healthier body composition.
An important detail the calculator captures is that your maintenance level is a moving target. As you gain weight, your body becomes larger and burns more calories at rest and in motion, so your TDEE rises. This means the surplus that produced steady gain at the start will gradually become just maintenance as you get heavier, and your gain will slow unless you keep eating more. That is why the target-intake figure is shown as a range — from what you need at your current weight to what you will need near your goal — and why it is worth recalculating every few kilograms. Muscle gained through training also raises your metabolism a little, since muscle is more metabolically active than fat, which is another reason a training-supported bulk pays off over time.
Intentional weight gain is beneficial for several groups. People who are underweight — generally a body mass index below 18.5 — may face weakened immunity, fatigue, nutrient deficiencies, and bone or fertility problems, and gaining to a healthy range improves all of these. Athletes and lifters deliberately eat in a surplus to build muscle and strength. People recovering from illness, surgery, an eating disorder, or a period of unintended loss often need to rebuild weight safely. Naturally slim "hardgainers" with fast metabolisms may struggle to gain and benefit from a structured calorie target. In every case, the goal is healthy, mostly-lean weight gain, and knowing your calorie number is the essential first step. If weight loss is unexplained or rapid, see a doctor before trying to gain it back, as it can signal an underlying condition.
Gaining and losing weight are governed by the same principle — energy balance — just pointed in opposite directions. To lose weight you eat below your maintenance level, creating a deficit; to gain weight you eat above it, creating a surplus. The arithmetic is nearly identical: find your maintenance calories, then add or subtract a daily amount based on how fast you want to change. There is one interesting asymmetry, though. Weight loss is usually estimated at about 7,700 calories per kilogram (the energy in a kilogram of body fat), while weight gain is estimated at about 7,000 calories per kilogram, because the tissue you add is a mix of muscle, fat, glycogen, and water that is, on average, slightly less energy-dense than pure fat. That is why the same weekly pace corresponds to a slightly different daily calorie figure depending on whether you are gaining or losing. Otherwise, the strategy is a mirror image: consistency, a moderate rate of change, and attention to protein and training.
Several missteps slow people down. The most common is simply not eating enough — underestimating intake and hovering near maintenance instead of a true surplus, so the scale never moves. Another is relying on junk food to hit the calorie target, which adds fat and harms health rather than building quality weight. Some people do too much cardio, burning off the surplus they worked to eat, when strength training would serve them better. Impatience is another trap: expecting rapid gains, then quitting when weight rises slowly, even though slow gain is the healthiest kind. Finally, many forget that maintenance rises as they grow, so they stop gaining and assume the plan failed when they simply need to eat a little more. Avoiding these mistakes — eat a genuine surplus of quality food, train with weights, be patient, and recalculate as you go — turns the calorie target from this calculator into steady, healthy progress.
Eat above your maintenance calories. A surplus of about 500 calories a day adds roughly 0.5 kg (1 lb) per week. The calculator works out your exact maintenance level and target intake.
Roughly 7,000 calories are needed to gain one kilogram of body weight, which is why 0.5 kg per week corresponds to about a 500-calorie daily surplus.
A normal, healthy pace is about 0.5 kg (1 lb) per week, which favors muscle over fat. A stricter 1 kg per week is faster but adds more fat and is best done under guidance.
Both, but the balance depends on your surplus size and training. A moderate surplus combined with resistance training maximizes muscle gain and limits fat gain.
As you gain weight, your body burns more calories, so your maintenance level rises. To keep gaining at the same pace, you gradually need to eat more — which is why the target is shown as a range.
Protein-rich foods plus calorie-dense choices like nuts, nut butters, olive oil, avocado, whole-milk dairy, oats, and rice make it easier to reach a surplus without relying on junk food.
This Weight Gain Calculator is provided for general informational and educational purposes and gives estimates based on the Mifflin-St Jeor equation and the standard ~7,000 kcal-per-kilogram figure for weight gain. It is not medical or nutritional advice. Individual needs vary; consult a qualified health professional before starting a weight-gain program, especially if you are underweight or have a health condition.