To gain muscle, you need to eat roughly 5 to 20% more calories than your body burns each day. For someone maintaining their weight on 2,000 calories, that means eating between 2,100 and 2,400 calories. The exact number depends on your body size, activity level, training experience, and how much fat you’re willing to gain alongside new muscle.
Why You Need a Caloric Surplus
Your body can’t build new tissue out of nothing. Synthesizing muscle protein is an energy-intensive process. Estimates from metabolic research put the cost at roughly 5 calories per gram of new tissue produced, which means even building half a pound of muscle in a week requires hundreds of extra calories once you factor in the energy demands of harder training, recovery, and the metabolic overhead that comes with gaining weight.
A surplus also sends a hormonal signal that it’s safe to invest resources in growth. When energy is abundant, your body partitions more of the incoming calories toward building lean tissue rather than just storing fat. When energy is scarce, it shifts into conservation mode, which is why trying to build significant muscle in a calorie deficit is an uphill battle for most people beyond the beginner stage.
Finding Your Maintenance Calories
Before you can add a surplus, you need a starting point: your maintenance calories, or the total amount you burn in a day. This number has several components. Your resting metabolic rate (the calories your organs and tissues burn just to keep you alive) accounts for the largest share. On top of that, you burn calories through exercise, digestion, and all the movement you do outside the gym, like walking, fidgeting, and standing at work. That last category alone can vary by as much as 2,000 calories per day between two people of similar size, depending on occupation and lifestyle.
The most widely recommended formula for estimating resting metabolic rate is the Mifflin-St. Jeor equation, which the Academy of Nutrition and Dietetics considers the most accurate option when lab testing isn’t available. For men, it works out to: (9.99 × weight in kg) + (6.25 × height in cm) – (4.92 × age) + 5. For women, the same formula applies but you subtract 161 instead of adding 5. Multiply the result by an activity factor (typically 1.4 to 1.7 for people who lift weights several times a week) and you get a rough estimate of total daily expenditure.
That said, formulas are just a starting estimate. The most reliable method is to track your body weight and calorie intake for two to three weeks. If your weight holds steady, you’ve found your maintenance number. From there, you build your surplus.
How Big Your Surplus Should Be
A surplus of 5 to 20% above maintenance is the range that builds muscle while keeping fat gain relatively modest. For most people, that works out to somewhere between 100 and 400 extra calories per day. Where you land in that range depends on a few factors.
If you’re carrying more body fat than you’d like, start at the lower end. Higher body fat levels tend to shift the balance of weight gain toward more fat and less muscle, partly because of changes in insulin sensitivity. A smaller surplus gives you room to build muscle without digging a deeper hole on the fat loss side. On the other hand, if you’re already lean and highly active, especially if you’re a competitive athlete, a surplus closer to 20% helps ensure your body has enough energy to actually fuel new growth on top of intense training demands.
Training experience matters too. Beginners can build muscle faster, so they can get away with a slightly larger surplus without excessive fat gain. Someone who’s been lifting seriously for five or more years will add muscle slowly no matter what, and a large surplus just means more fat to cut later.
Rate of Weight Gain to Target
A practical way to check whether your surplus is dialed in is to monitor the scale. Aim for a weight gain of about 0.25 to 0.5% of your body weight per week. For a 180-pound person, that’s roughly 0.5 to 0.9 pounds per week. Gaining faster than that typically means you’re overshooting and accumulating more fat than necessary. Gaining slower might mean your surplus is too small to maximize growth, though a slower rate is appropriate for experienced lifters.
Weigh yourself under consistent conditions, ideally first thing in the morning, and use a weekly average rather than any single day’s reading. Water retention, meal timing, and digestive contents can swing the number by several pounds day to day.
How to Split Those Calories
Total calories matter most, but how you distribute them across protein, carbohydrates, and fat has a meaningful impact on whether the weight you gain is mostly muscle or mostly fat.
Protein
Protein provides the raw material for muscle repair and growth. People who lift weights regularly need about 1.2 to 1.7 grams of protein per kilogram of body weight each day. For a 180-pound (82 kg) person, that’s roughly 98 to 139 grams. Spreading your protein intake across three to five meals helps keep muscle protein synthesis elevated throughout the day rather than spiking it once and letting it drop.
Carbohydrates
Carbohydrates fuel your training sessions and replenish the glycogen your muscles burn during resistance exercise. Recommendations for people who lift regularly fall in the range of 3 to 7 grams per kilogram of body weight per day, with strength athletes typically needing 3 to 5 g/kg and bodybuilders aiming for 4 to 7 g/kg. For that same 180-pound person, this means roughly 245 to 575 grams of carbohydrates daily, depending on training volume and intensity. If you’re doing four to five hard lifting sessions a week, you’ll need more carbs than someone training three times.
Fat
Dietary fat plays a critical role in hormone production, including testosterone, which directly supports muscle growth. Dropping fat intake too low can impair hormonal health. A minimum of about 0.8 to 1 gram of fat per kilogram of body weight keeps hormone levels in a healthy range. For an 82 kg person, that’s roughly 66 to 82 grams of fat, or about 590 to 740 calories from fat. After you’ve set protein and fat minimums, fill the remaining calories with additional carbohydrates or fat based on your preference and how you perform in the gym.
Why Some People Gain More Fat Than Muscle
When you eat in a surplus, not every calorie goes toward building muscle. Your body gains both lean tissue and fat simultaneously, and the ratio between the two isn’t fixed. Researchers describe this ratio as a “p-ratio,” the proportion of weight gained as lean mass versus total weight gained. But this ratio isn’t a single biological dial your body controls. It’s the combined result of separate processes: factors that promote or inhibit fat storage, and factors that promote or inhibit muscle growth, all happening at the same time.
The biggest levers you can pull to shift that ratio in favor of muscle are consistent progressive resistance training, adequate protein, and keeping your surplus moderate rather than extreme. Starting a bulk at a lower body fat percentage also helps, since higher body fat tends to push the ratio toward more fat gain. Sleep and stress management play supporting roles by influencing the hormonal environment that governs recovery and growth.
A Practical Example
Here’s what this looks like in practice for a 30-year-old man who weighs 180 pounds (82 kg), stands 5’10” (178 cm), and lifts four days a week. Using the Mifflin-St. Jeor equation, his resting metabolic rate comes out to about 1,775 calories. Multiplied by an activity factor of 1.55 for moderate exercise, his estimated maintenance is roughly 2,750 calories per day.
A 10 to 15% surplus puts him at about 3,025 to 3,160 calories daily. Within that, he’d aim for around 115 to 130 grams of protein (460 to 520 calories), 70 to 80 grams of fat (630 to 720 calories), and fill the rest with carbohydrates, which works out to roughly 400 to 475 grams of carbs. He’d track his weight weekly, looking for a gain of about 0.5 to 0.9 pounds per week, and adjust his intake up or down by 100 to 200 calories if the scale moves too slowly or too fast.
This kind of structured, moderate approach builds muscle steadily while keeping fat gain manageable enough that you won’t need months of aggressive dieting afterward to see the muscle you built.