Weight Gain Calculator

Gaining weight sounds like the easy direction, but for people who struggle to eat enough it is genuinely hard work. The same principle applies in reverse: you need a sustained calorie surplus, and the body will happily store the extra as either muscle or fat depending on the training and protein that come with it.

A surplus of about 300 calories a day — a large glass of milk and a handful of nuts — produces roughly a quarter of a kilogram a week, which is a productive rate for adding muscle without much fat. If you are underweight and simply need to regain weight, a larger surplus is fine and will get you there faster. The calculator shows the intake you would need against your own maintenance requirement, so the number is tailored rather than copied from a magazine.

What you eat during the surplus changes the result. Protein should be high, around 1.6 to 2.2 grams per kilogram of body weight, because muscle is built from amino acids and you cannot store them in advance. Resistance training is the other half of the equation: without it, a surplus simply becomes fat. The combination of a modest surplus, plenty of protein and progressive strength training is the only reliable way to gain lean mass rather than waistline.

If you find it hard to eat enough, the problem is usually volume, not appetite. Liquid calories are the classic workaround — milk, smoothies, olive oil on everything, nut butters — because they add hundreds of calories without filling you up the way solid food does. Eat by the clock rather than by hunger, and track your intake for a week or two so you know whether the surplus is real or imagined. Gaining weight slowly, at a rate you can measure, is far easier to sustain than trying to force it.

This calculator returns estimates for general information only. It is not medical advice, diagnosis or treatment, and it cannot replace an assessment by a qualified health professional. Individual results vary, and figures such as calorie needs, body-fat percentage and heart-rate zones are approximations based on population averages rather than measurements of you.