How this calculator works

No mystery maths. It calculates your basal metabolic rate with the Mifflin-St Jeor equation, published in 1990 and still the standard used by dietitians because it outperforms older formulas for most adults. That figure is multiplied by an activity factor between 1.2 and 1.9 to estimate your total daily energy expenditure, and your gain target adds 300 or 500 calories on top, because roughly 7,700 surplus calories become a kilogram of body weight. The full explanation of these numbers, and why they vary person to person, lives in our guide to how many calories you need to gain weight.

How to use your number

Treat it as a starting estimate, not a verdict; equations like this are typically within about 10 percent, which is why the result is a range. Eat within it for two to three weeks, weigh yourself once a week under the same conditions, and let the scale referee: not moving, add around 200; gaining faster than about half a kilo a week, ease back by the same. The consistency matters far more than the precision, and how to eat in a calorie surplus without obsessing is the guide to holding the habit without letting the numbers run your life. From there it's execution: 1,000 calorie meals for the individual plates, 2,000 calorie meals when a single sitting has to carry more, and the daily plans your result links to for the full structure. If you're training for muscle, the surplus works alongside the lifting, not instead of it, and our bulkingguide covers that pairing.

Why the modest surplus, when other calculators offer more

Because the faster paces mostly add fat, not progress. At 300 to 500 over maintenance, a meaningful share of what you gain can be lean tissue, especially with training; push the surplus far past that and the extra almost all banks as fat, while the eating itself gets much harder. Slow is not the cautious option here, it's the effective one.

When this calculator is the wrong tool

Three honest exclusions. Under 18, your energy needs include growth, and no adult formula handles that; your GP or a dietitian is the right guide. During pregnancy or breastfeeding, weight change guidance belongs with your midwife or GP, not a formula on a food brand's website. And if numbers around food feel harmful to you right now, or eating is tangled up with anxiety, skip the calculator entirely; the gentler starting points are what to eat when your appetite is lowand how to increase your appetite, and if you've been losing weight without trying, see your GP before any of this.

When the number is bigger than your appetite

For a lot of people the calculator's hardest output isn't the maths, it's the gap between the target and what they can actually face eating. That gap is the entire reason Phoenix Bars exist: up to 557 calories in a small, soft 120g bar, vegan and gluten-free with up to 66g of carbohydrates and 19g of protein, so a single bar quietly closes a 500 calorie shortfall that three extra plates of food never would. Pair your target with the foods in high calorie low volume foods and a daily gainer shake, and most numbers on this calculator stop being frightening. The Starter Bundle covers nearly two weeks of that daily top-up.

Frequently asked questions

How many calories do I need to gain weight? Your maintenance plus 300 to 500 a day for most adults, which typically lands between 2,500 and 3,500 depending on size, sex and activity. The calculator above gives your personal estimate.

How accurate is a weight gain calculator? Within roughly 10 percent for most adults, which is why results here show as a range. Your scale over two to three weeks is the real answer; adjust by about 200 calories based on what it says.

How fast should I gain weight? Around 0.25 to 0.5 kg a week. Faster than that and the additional gain is almost entirely fat, and the eating burden climbs steeply.

Why am I not gaining on my calculated number? Usually one of three things: the estimate ran low for your body, you're burning more than the activity setting assumes, or intake is quietly falling short of the target on busy days. Add 200 and hold it for two more weeks before concluding anything.

Do I need a protein target too? If you're training for muscle, around 1.6 to 2 grams per kilogram of body weight a day is the widely used range, spread across your meals. For weight gain generally, the calorie target leads and protein supports.

Written by James Frost, founder of Flaming Phoenix. This tool gives general estimates for healthy adults, not medical advice. Last updated July 2026.