💪 Nutrition & Fitness Tool

Weight Gain
Calculator

Calculate exactly how many calories you need to gain weight and build muscle, using the Mifflin-St Jeor formula with activity-adjusted maintenance, calorie surplus targets, and macro breakdown.

🔬 Science-Based Nutrition

💪 Fitness Friendly

📊 Healthy Weight Gain Planning

Gender

Weight (kg)

Height (cm)

Age (years)

Activity level

Weight gain goal

Daily calorie target

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to gain weight at your selected rate

BMR

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Maintenance

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Surplus

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Gain rate

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Goal

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⚡ Maintenance vs target calories

Maintain

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Target

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🥗 Macro breakdown

Protein

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Carbs

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Fat

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📅 Weight gain projection

📐 Step-by-step calculation

💡 Nutrition insights:
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ℹ️ This calculator uses the Mifflin-St Jeor equation for BMR estimation. Results are estimates, individual metabolism, body composition, and response to calorie surplus vary. Consult a registered dietitian or physician before making significant dietary changes, especially if you have any medical conditions.

Weight Gain Calculator: Estimate Calories for Healthy Weight Gain

Whether you’re trying to build muscle, recover from illness-related weight loss, support athletic performance, or simply reach a healthier bodyweight, knowing your exact calorie target is the foundation of any successful weight gain plan. This free weight gain calculator uses the clinically validated Mifflin-St Jeor equation to calculate your BMR (basal metabolic rate), applies your activity level multiplier to find your maintenance calories, then adds your chosen calorie surplus for your weight gain goal, complete with macro recommendations and a week-by-week projection timeline.

💪 Mifflin-St Jeor BMR formulas:
Male: BMR = (10 × weight kg) + (6.25 × height cm) − (5 × age) + 5
Female: BMR = (10 × weight kg) + (6.25 × height cm) − (5 × age) − 161
Maintenance = BMR × Activity multiplier (1.2 – 1.9)
Weight gain target = Maintenance + Calorie surplus (250–1,000 kcal)

The Mifflin-St Jeor equation was published in 1990 and has since been validated as the most accurate widely-used resting metabolic rate formula in systematic comparisons against measured metabolic rate, outperforming the older Harris-Benedict equation from 1919.

How the Weight Gain Calculator Formula Works

This calculator measures the daily calorie target needed to gain weight at a chosen rate. It builds that number in three stages: BMR (calories burned at complete rest), maintenance calories (BMR adjusted for activity), and a target calorie intake (maintenance plus a surplus).

Step	Formula	Units
BMR (men)	(10 × weight kg) + (6.25 × height cm) − (5 × age) + 5	kcal/day
BMR (women)	(10 × weight kg) + (6.25 × height cm) − (5 × age) − 161	kcal/day
Maintenance	BMR × activity multiplier (1.2 to 1.9)	kcal/day
Target	Maintenance + goal surplus (250, 500, 750, or 1,000 kcal)	kcal/day

Step-by-step calculation walkthrough

Step 1: Identify the inputs. Gender: male. Age: 24. Weight: 68 kg. Height: 175 cm. Activity: Moderate (×1.55). Goal: Moderate gain (+500 kcal).

Step 2: Apply the formula. BMR = (10 × 68) + (6.25 × 175) − (5 × 24) + 5. Maintenance = BMR × 1.55. Target = Maintenance + 500.

Step 3: Perform the calculation. 10 × 68 = 680. 6.25 × 175 = 1,093.75. 5 × 24 = 120. BMR = 680 + 1,093.75 − 120 + 5 = 1,658.75 kcal, shown rounded as 1,659. Maintenance = 1,658.75 × 1.55 = 2,571 kcal (rounded). Target = 2,571 + 500 = 3,071 kcal/day (rounded).

Step 4: Interpret the result. This 24-year-old burns roughly 1,659 kcal at complete rest, and about 2,571 kcal on an average moderately active day. Eating around 3,071 kcal daily, a 500 kcal surplus, targets a gain of about 0.5 lb (0.23 kg) per week. The macro split for this target works out to roughly 120g protein, 441g carbs, and 92g fat.

📐 The macro breakdown always follows the same order: protein is set first as a fixed amount per pound of bodyweight (0.8 g/lb for lean gains, 1.0 g/lb for aggressive or dirty bulks), fat is set as 27% of total target calories, and carbohydrates absorb whatever calories remain. This is why carbs make up the largest share of the macro split for most profiles.

Assumptions and limitations: the Mifflin-St Jeor equation predicts BMR within roughly ±10% for most healthy adults, but it doesn’t measure your actual metabolism, body composition, or hormonal status. The activity multiplier is self-selected, which is the single largest source of error in the maintenance estimate, since most people round their activity level up. The fixed calorie surplus for each goal (250 to 1,000 kcal) doesn’t automatically adjust as your weight changes, so recalculating every few weeks as your weight shifts keeps the target accurate.

How Weight Gain Works: Calorie Surplus Explained

Weight gain (whether muscle, fat, or a combination) requires consuming more calories than your body expends. This is called a calorie surplus. When you consistently eat above your maintenance calories, your body has extra energy available, which it uses to synthesise new tissue (muscle, with appropriate training) or store as body fat when surplus is large or training is absent.

The energy content of body tissue is approximately: 1 lb (0.45 kg) of fat ≈ 3,500 calories. 1 lb of muscle tissue ≈ 700–1,400 calories (muscle is mostly water). The often-cited “3,500 kcal = 1 lb” rule assumes all of it converts to stored fat; in practice, gained weight is a mix of muscle, fat, and water, and rising maintenance calories as you gain weight eat into the surplus. That’s why this calculator estimates a 500 kcal/day surplus at closer to 0.5 lb of tissue gained per week rather than the full 1 lb the naive rule would suggest. In practice, the relationship isn’t perfectly linear, metabolic adaptation, individual variation, and training status all affect outcomes, but the calorie surplus principle remains the most reliable predictor of weight gain.

Choosing the Right Calorie Surplus

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Slow lean gain (+250 kcal)

The most muscle-to-fat-efficient approach. At a 250 kcal surplus, weight gain is slow (~0.25 lb/week) but the majority is lean mass with consistent resistance training. Best for intermediate-advanced lifters who want to minimise fat gain, or anyone who wants gradual body composition improvement.

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Moderate gain (+500 kcal)

The most widely recommended bulking surplus, enough extra energy for solid muscle growth without excessive fat accumulation. At ~0.5 lb/week gain, expect roughly a 50/50 muscle-to-fat ratio in gained weight for typical lifters. The standard recommendation for beginners and intermediates starting a bulk.

🚀

Aggressive bulk (+750 kcal)

Faster weight gain (~0.75 lb/week) with more significant fat gain alongside muscle. Best suited for those significantly underweight, athletes in high-volume training phases, or hard gainers who struggle to eat enough. Expect to carry more body fat and require a cut phase later.

🍔

Dirty bulk (+1,000 kcal)

Maximum calorie surplus, fast weight gain (~1 lb/week) but a larger fraction is fat. Rarely recommended for general population; sometimes used by competitive bodybuilders in an off-season or athletes recovering from significant weight loss. Requires a subsequent fat loss phase.

Calorie Surplus Reference Table

Surplus	Gain rate (lbs/wk)	4 weeks	12 weeks	Muscle:fat ratio
+250 kcal/day	~0.25 lb	~1 lb	~3 lbs	High muscle fraction
+500 kcal/day	~0.5 lb	~2 lbs	~6 lbs	Balanced (recommended)
+750 kcal/day	~0.75 lb	~3 lbs	~9 lbs	More fat accumulation
+1,000 kcal/day	~1.0 lb	~4 lbs	~12 lbs	Significant fat gain

Macronutrients for Weight Gain

Total calories determine whether you gain or lose weight. Macronutrient distribution, the ratio of protein, carbohydrates, and fat, determines the quality of that weight gain. For muscle building, protein is the critical macro. For energy and gym performance, carbohydrates are primary. Fat supports hormonal function and recovery.

Protein (4 kcal/g): Target 0.7–1.0 g per lb of bodyweight (1.6–2.2 g/kg). Protein provides amino acids for muscle protein synthesis. At 0.8 g/lb, a 175 lb person needs approximately 140 g protein/day. Distribute across 4–5 meals for optimal absorption (25–40 g per serving).
Carbohydrates (4 kcal/g): The primary training fuel. A standard bulking macro split allocates 40–50% of calories to carbohydrates. Carbs replenish muscle glycogen, support training performance, and trigger insulin release which aids amino acid uptake by muscle cells.
Dietary fat (9 kcal/g): Target 20–30% of total calories. Fat is essential for testosterone and other anabolic hormone production. Prioritise unsaturated fats (olive oil, avocado, nuts, fatty fish) while including some saturated fat from whole food sources.

3 Real-Life Examples

Three different profiles and goals, calculated the way the tool above does it.

Situation	Inputs	Result	What it means
Underweight student aiming for a slow, lean gain	Female, 21, 50 kg, 162 cm, light activity, slow gain (+250 kcal).	BMR: 1,247 kcal. Maintenance: 1,714 kcal. Target: 1,964 kcal/day. Macros: 88g protein, 270g carbs, 59g fat.	The light surplus and light activity level keep the daily target modest, appropriate for someone whose main goal is minimising fat gain while slowly building weight.
Self-described hard gainer pushing an aggressive bulk	Male, 28, 62 kg, 180 cm, very active, aggressive bulk (+750 kcal).	BMR: 1,610 kcal. Maintenance: 2,777 kcal. Target: 3,527 kcal/day. Macros: 137g protein, 507g carbs, 106g fat.	The high activity multiplier and large surplus combine to push this target well above 3,500 kcal, reflecting both a demanding training schedule and an intentionally aggressive gain rate.
Comparing two surplus sizes for the same person	Female, 30, 58 kg, 165 cm, moderate activity, comparing slow (+250) vs. moderate (+500) surplus.	Maintenance: 2,015 kcal. Slow gain target: 2,265 kcal/day. Moderate gain target: 2,515 kcal/day.	The 250 kcal difference between the two goals is exactly the gap between the two surplus tiers, giving her a concrete before-and-after to plan around if she decides to switch pace partway through a bulk.

These are illustrative calculations using the same Mifflin-St Jeor formula the calculator above applies. They’re a planning starting point, not personalised medical or nutrition advice.

Important Notes

These are population-average estimates, not a metabolic test. The Mifflin-St Jeor equation predicts BMR to within roughly ±10% for most healthy adults, but only a lab-based indirect calorimetry test measures your actual resting metabolism.
Rounding. Displayed calorie and macro figures round to the nearest whole number.
Activity level is self-reported and the biggest source of error. Choosing a multiplier one level too high is the most common reason a calculated “maintenance” target ends up producing faster gain than expected.
The surplus is fixed, not personalised to your rate of change. The 250 to 1,000 kcal surplus options are common starting points, not individually calculated for your specific metabolism or genetics.
Recalculate as your weight changes. As you gain weight, your BMR and maintenance calories rise too, so the same fixed surplus produces a smaller relative gain over time unless you periodically update your inputs.
The macro split assumes standard bulking ratios. Protein is set by bodyweight, fat by percentage of total calories, and carbs fill the remainder. Individual needs may differ based on training style, food preferences, or medical guidance.
Not validated for every population. This formula isn’t intended for children, pregnant or breastfeeding women, or people recovering from significant illness, where a healthcare provider’s guidance should take priority.
Data privacy. All calculations run in your browser. Your inputs aren’t sent to a server, and the PDF is generated locally on your device.

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Frequently Asked Questions

How many calories do I need to gain weight?

You need to eat more than your maintenance calories: the amount you burn daily. This calculator determines your maintenance level using the Mifflin-St Jeor formula (BMR × activity multiplier), then adds your chosen surplus: +250 kcal for slow lean gain, +500 kcal for moderate gain, +750 kcal for aggressive bulking, or +1,000 kcal for a dirty bulk. The most commonly recommended starting point is a +500 kcal surplus, which produces approximately 0.5 lb of weight gain per week: enough to support solid muscle growth without excessive fat accumulation.

What is a calorie surplus?

A calorie surplus means consuming more calories than your body expends in a day. When you’re in a surplus, your body has extra energy it uses to build new tissue: muscle (with resistance training) or fat (without training, or with a very large surplus). Your maintenance calories are the amount you need to maintain your current weight. Any amount above maintenance is your surplus. A 500 kcal/day surplus over 7 days totals 3,500 excess calories: the textbook energy equivalent of 1 lb of fat, though real-world gains run closer to 0.5 lb of body tissue per week once you factor in that gained weight is a mix of muscle, fat, and water.

How fast can I gain weight safely?

For muscle-focused weight gain, 0.25–0.5 lb per week (0.1–0.25 kg/week) is the generally recommended rate. This range maximises the proportion of gained weight that is muscle rather than fat, particularly for natural lifters. Faster gains (1+ lb/week) tend to produce more fat accumulation relative to muscle. Very rapid weight gain (2+ lb/week) in adults is almost entirely fat and water. Beginners and those returning after a break can often gain faster initially due to “newbie gains”: enhanced muscle protein synthesis responsiveness. After 6–12 months of consistent training, gains slow to the natural rate.

What is lean bulking?

Lean bulking (also called a clean bulk) means using a modest calorie surplus (+250–500 kcal/day) combined with progressive resistance training to build muscle while minimising fat gain. It prioritises food quality, adequate protein, complex carbohydrates, and healthy fats, over eating in excess. The benefit is that less fat gain means shorter and less severe cut phases are needed. The trade-off is slower overall weight gain compared to a dirty bulk. Most fitness professionals recommend lean bulking for natural athletes outside of specific therapeutic weight restoration contexts.

How much protein do I need to gain muscle?

Research consistently supports 1.6–2.2 g of protein per kg of bodyweight (0.73–1.0 g/lb) as optimal for muscle protein synthesis. For a 175 lb (80 kg) person, that’s 130–175 g protein/day. Higher intakes (up to 3 g/kg) show no harm but don’t provide additional muscle-building benefit. Distribute protein across 4–6 meals of 25–40 g per serving to maximise leucine-triggered muscle protein synthesis. Complete protein sources (meat, fish, eggs, dairy, or complementary plant proteins) should form the foundation. This calculator targets approximately 0.8 g/lb as the protein recommendation.

Can I gain muscle without gaining fat?

Pure muscle gain with zero fat gain (“body recomposition”) is possible but limited. It occurs primarily in: beginners with untrained muscles (newbie gains), people returning after time off, individuals with significant body fat who can mobilise stored fat as energy while building muscle, and people on certain performance-enhancing substances (not recommended). Natural, trained individuals in a calorie surplus will inevitably gain some body fat alongside muscle. The goal is to minimise this ratio through appropriate surplus size, consistent progressive training, and adequate protein. A +250 kcal lean bulk is the closest practical approach to muscle-focused gains with minimal fat.

How many calories equal 1 pound of body weight?

The widely cited figure is 3,500 calories per pound of body fat, so a 500 kcal/day surplus × 7 days = 3,500 kcal in theory. In practice this calculator estimates closer to 0.5 lb gained per week at that surplus, not the full 1 lb the raw math implies. That’s because muscle tissue has a different energy density (approximately 700–1,400 kcal per pound, as muscle is mostly water), and metabolic adaptation means that as you gain weight, your maintenance calories increase, making the same surplus produce less additional gain over time. The 3,500 kcal/lb figure is a useful starting reference, but treat the calculator’s per-goal gain rates as the more realistic estimate.

Is this calculator accurate?

This calculator uses the Mifflin-St Jeor equation, validated as the most accurate BMR prediction formula for most adults across multiple clinical comparisons. It predicts BMR within ±10% for approximately 80% of the population. The activity multipliers are standard values used by sports nutritionists and dietitians worldwide. Individual variation is real, metabolism varies due to genetics, lean body mass, hormonal status, and other factors. Use this as a starting point: track your weight for 2–3 weeks at the suggested intake. If you’re not gaining at the expected rate, adjust calories by 100–200 kcal in the appropriate direction.

What foods are best for healthy weight gain?

Focus on calorie-dense, nutrient-rich foods rather than junk food: lean meats (chicken, beef, turkey), fatty fish (salmon, tuna), whole eggs, full-fat dairy (Greek yoghurt, cottage cheese, milk), legumes (lentils, chickpeas), whole grains (oats, brown rice, quinoa), nuts and nut butters (extremely calorie-dense), avocado, dried fruits, and olive oil. For people struggling to eat enough, liquid calories (whole milk smoothies, protein shakes) are useful as they don’t create the same satiety as solid food. Prioritise whole foods for micronutrients, but caloric density matters when hitting a surplus is difficult.

Should I use supplements for weight gain?

Whole food should always be the foundation. That said, some supplements have strong evidence: Creatine monohydrate (3–5 g/day) consistently improves strength, training volume, and lean mass gain: the most evidence-supported muscle-building supplement. Whey or plant-based protein powder can help meet protein targets conveniently when whole food isn’t practical. Mass gainers (high-calorie shakes) can help people who genuinely struggle to eat enough, but check the label: many contain mostly sugar. Vitamin D and zinc may support testosterone and overall health. Expensive proprietary “weight gainer” blends offer little beyond their basic macros. Save money and prioritise food quality first.

Can I download my results as a PDF?

Yes. Use the “Download results as PDF” button below your results to save a summary of your inputs, BMR, maintenance calories, calorie target, macros, and the weight gain projection timeline, generated entirely in your browser.

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