To gain weight, you need to eat more calories than your body burns each day. The exact number depends on your age, sex, height, weight, and activity level, but most adults need to add 300 to 500 extra calories on top of their maintenance level. You can estimate your personal target in a few steps using a well-validated formula and some simple math.
Step 1: Calculate Your Resting Metabolic Rate
Your resting metabolic rate (RMR) is the number of calories your body burns just to stay alive: breathing, circulating blood, maintaining body temperature. It accounts for the largest chunk of your daily calorie burn, so getting this number right matters most.
The most accurate formula for most people is the Mifflin-St Jeor equation. A systematic review in the Journal of the American Dietetic Association found it predicted resting metabolic rate within 10% of lab-measured values more often than any competing formula, with the narrowest error range. Here’s how it works:
- Men: (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + 5
- Women: (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) − 161
If you’re more comfortable with pounds and inches, convert first: divide your weight in pounds by 2.2 to get kilograms, and multiply your height in inches by 2.54 to get centimeters. For example, a 28-year-old man who weighs 150 pounds (68 kg) and stands 5’10” (178 cm) would calculate: (10 × 68) + (6.25 × 178) − (5 × 28) + 5 = 1,658 calories per day at rest.
Step 2: Factor In Your Activity Level
You don’t lie in bed all day, so your actual calorie needs are higher than your RMR. Multiply your result by an activity factor to get your total daily energy expenditure (TDEE):
- Sedentary (desk job, little exercise): RMR × 1.2
- Lightly active (light exercise 1–3 days/week): RMR × 1.375
- Moderately active (moderate exercise 3–5 days/week): RMR × 1.55
- Very active (hard exercise 6–7 days/week): RMR × 1.725
- Extremely active (physical job + intense training): RMR × 1.9
Using the example above, the 28-year-old man with an RMR of 1,658 who exercises moderately would multiply by 1.55, giving a TDEE of roughly 2,570 calories. That’s his estimated maintenance level, the amount he’d need to eat just to stay at his current weight.
Step 3: Add Your Calorie Surplus
Once you know your TDEE, add a surplus on top of it. The current consensus, supported by sports nutrition research, is that 300 to 500 extra calories per day is the ideal range for gaining weight. This surplus is large enough to support muscle growth but controlled enough to limit unnecessary fat gain. The NHS recommends the same 300 to 500 calorie range for adults looking to gain weight gradually.
So our example person would aim for roughly 2,870 to 3,070 calories per day. If you’re starting from a place of being significantly underweight, your doctor or dietitian may recommend a larger surplus, but for most people, this range works well. Expect to gain about 0.5 to 1 pound per week at this rate, which is a healthy, sustainable pace.
Why Your Calculator Might Be Off
Online calculators give you a starting estimate, not a precise number. One major reason: they can’t account for how much you move outside of formal exercise. This non-exercise activity, everything from fidgeting to walking around your kitchen, varies enormously between people and can significantly shift your actual calorie burn.
Research published in The American Journal of Clinical Nutrition measured the energy cost of these small movements and found striking differences. Fidgeting while seated increased calorie burn by about 54% compared to sitting still. Standing and fidgeting nearly doubled it (94% increase). Even walking at a slow pace of 1.6 km/h (about 1 mph) raised energy expenditure by 154%. The variation between individuals was large, meaning two people with the same height, weight, and exercise routine can have very different daily calorie needs simply because one of them is naturally more restless.
This is why the calculator gives you a starting point, not a final answer. If you’re eating at your calculated surplus and not gaining weight after two to three weeks, you likely need to add another 200 to 300 calories. If you’re gaining faster than a pound per week and noticing more fat than you’d like, scale back slightly.
How to Split Your Calories
Where your calories come from matters, especially if you want more of your weight gain to be muscle rather than fat. Protein is the most important piece: aim for 1 to 1.5 grams of protein per kilogram of your body weight each day. For a 68 kg person, that’s 68 to 102 grams. Going above 1.5 grams per kilogram doesn’t provide additional muscle-building benefits.
Fill the remaining calories with a mix of carbohydrates and healthy fats. Carbs fuel your workouts and replenish energy stores in your muscles. Fats are calorie-dense (9 calories per gram versus 4 for protein and carbs), which makes them useful when you’re struggling to eat enough volume. A reasonable split for most people gaining weight is roughly 25 to 30% of calories from protein, 45 to 55% from carbohydrates, and 20 to 30% from fat.
High-Calorie Foods That Make It Easier
The hardest part of gaining weight for many people isn’t doing the math. It’s actually eating enough. If your appetite is small, calorie-dense foods and combination meals help you hit your target without feeling stuffed. Some practical options:
- Protein smoothie (Greek yogurt, banana, milk, whey protein, peanut butter): about 538 calories in one glass
- Bagel with cream cheese and jelly: 584 calories
- Turkey sandwich with avocado and mayo: 555 calories
- Oatmeal made with milk, honey, bananas, and raisins: 458 calories per cup
- Cottage cheese with canned fruit and chia seeds: 459 calories
- Trail mix (almonds, walnuts, raisins, wheat cereal): 370 calories per serving
- Peanut butter and jelly on whole wheat bread: 400 calories
- Turkey chili with beans over a baked potato: 420 calories
Liquids are especially helpful if you struggle with volume. A single smoothie can deliver over 500 calories without the heavy, full feeling of a solid meal. Eating more frequently, four to six smaller meals instead of three large ones, also makes it easier to hit a higher calorie target consistently.
Tracking and Adjusting Over Time
Weigh yourself at the same time each day (first thing in the morning works best) and look at the weekly average rather than any single reading. Daily weight fluctuates by 2 to 4 pounds from water, food in your digestive system, and sodium intake, so individual readings are misleading.
If your weekly average is trending up by about half a pound to one pound, your surplus is in the right zone. If the scale isn’t moving after two to three consistent weeks, increase your daily intake by 200 to 300 calories and reassess. If you’re gaining faster than one pound per week and you’re not a complete beginner to resistance training, a larger portion of that gain is likely fat, and trimming your surplus slightly will produce better results over time.
Your calorie needs will also shift as you gain weight. Every 10 to 15 pounds you add, recalculate your RMR and TDEE. A heavier body burns more calories at rest, so your maintenance level rises and your old surplus may no longer be enough.