Weight Gain Calculator

The Formula

This calculator uses the Mifflin-St Jeor equation to estimate your Basal Metabolic Rate (BMR) — the calories your body burns at rest. BMR is then multiplied by an activity factor to give your Total Daily Energy Expenditure (TDEE). A calorie surplus is added on top to drive weight gain.

Mifflin-St Jeor BMR

Published in 1990 and validated across multiple populations, Mifflin-St Jeor is the most accurate BMR formula for the general population according to the Academy of Nutrition and Dietetics. It accounts for weight, height, age and sex:

Activity Multipliers

BMR reflects calories burned at complete rest. The activity factor scales this up to reflect your actual daily expenditure based on exercise frequency and intensity:

Calorie Surplus for Muscle Gain

A surplus of 200–500 kcal above TDEE is the evidence-based range for lean muscle gain. Hall et al. (2012) and subsequent research confirm that a modest surplus maximises muscle protein synthesis while limiting fat accumulation. Larger surpluses do not build muscle faster — they primarily increase fat storage. The slow pace (200 kcal) suits those prioritising body composition; the fast pace (500 kcal) suits hardgainers or those in a dedicated bulk.

Macronutrient Split

Protein is set at 2.0g/kg bodyweight — within the range supported by Morton et al. (2018) for resistance-trained individuals in a surplus. Fat is set at 25% of total calories (a minimum floor for hormone health). Remaining calories are allocated to carbohydrates, which fuel training performance and glycogen replenishment.