IIFYM Calculator (If It Fits Your Macros)

What is IIFYM?

IIFYM (If It Fits Your Macros) is a flexible dieting approach that focuses on hitting daily targets for protein, carbohydrates and fat rather than following a rigid meal plan or avoiding specific foods. Because no food is outright banned, adherents can include virtually any food they enjoy as long as it fits within their macro budget for the day. Research supports flexible restraint over rigid restraint: people who practise flexible dieting tend to have better long-term compliance, lower rates of binge eating and lower body weight compared with those who follow stricter exclusion diets.

How the calculation works

The calculator runs in three stages. First it estimates your Basal Metabolic Rate (BMR), which is the number of calories your body burns at complete rest. When no body fat percentage is provided, it uses the Mifflin-St Jeor equation (the most accurate formula for the general population in controlled studies), which adjusts for sex, weight, height and age. If you enter a body fat percentage, it switches to the Katch-McArdle formula, which works from lean body mass and is considered more precise for people who are very muscular or very lean. Second, your BMR is multiplied by an activity factor to produce your TDEE (Total Daily Energy Expenditure), the number of calories you burn on a typical day. Third, a deficit or surplus is applied based on your goal: roughly 10-20% below TDEE for fat loss, nothing for maintenance, or 10-15% above TDEE for muscle gain. Macro targets are then distributed using a protein-first approach: protein is set at roughly 1.8-2.2 g per kg of lean body mass (higher during a deficit to spare muscle), fat is floored at 20% of total calories to support hormone production, and carbohydrates fill the remaining calories.

Setting your activity level honestly

The activity multiplier is the single biggest source of error in TDEE estimates, and most people overestimate it. A sedentary multiplier of 1.2 covers someone who sits most of the day and does very little intentional exercise. Light (1.375) covers 1-2 genuine 45-minute gym sessions per week, with the rest of the day still largely sedentary. Moderate (1.55) is appropriate for 3-4 hard training sessions per week alongside a desk job. Hard (1.725) suits people training 5-6 days per week or doing a physically demanding job. Very hard (1.9) is reserved for elite athletes or people doing two-a-day sessions. If in doubt, select one level lower than you think you need, track your actual weight for 2-3 weeks, and adjust upward if you are losing weight faster than intended.

Protein, fat and carbohydrates: priorities and flexibility

Protein is the highest priority macro for anyone with body composition goals. Each gram provides 4 kcal, carries a thermic effect of 20-30% (meaning roughly a quarter of its calories are spent on digestion), and provides the amino acids needed for muscle repair and growth. Fat supplies 9 kcal per gram and should not drop below roughly 40-50 g/day to preserve testosterone, oestrogen and fat-soluble vitamin absorption. Once protein and fat floors are set, carbohydrates fill the remaining budget. Many people find carbs the easiest macro to cycle: eating more on hard training days and fewer on rest days is a simple form of carb cycling that can improve workout performance and manage hunger without changing weekly totals.