IIFYM Calculator (If It Fits Your Macros)

Macros are the three main nutrients your body uses for energy: protein, carbohydrates, and fat. Protein (4 kcal/g) builds and repairs muscle. Carbohydrates (4 kcal/g) fuel activity and brain function. Fat (9 kcal/g) supports hormones, cell membranes, and vitamin absorption. Tracking macros rather than just calories gives you a more complete picture of your nutrition and helps you hit body composition goals more precisely.

IIFYM is a flexible dieting approach that focuses on hitting daily protein, carb, and fat targets rather than restricting specific foods. As long as your food choices fit within your macro budget, you can include any food you enjoy. This makes it far more sustainable than traditional restrictive diets while still producing consistent fat loss or muscle gain results.

The calculator first estimates your Basal Metabolic Rate (BMR) — the calories your body burns at rest — using the Mifflin-St Jeor formula (or Katch-McArdle if you provide body fat %). Your BMR is then multiplied by an activity factor to get your TDEE (Total Daily Energy Expenditure). Finally, a calorie adjustment is applied based on your goal (e.g. −20% for fat loss), and that target is split into protein, carb, and fat grams using evidence-based ratios.

For fat loss, a common starting point is 40% protein, 30% carbs, and 30% fat. Higher protein intake (around 0.8–1g per lb of body weight) helps preserve lean muscle during a calorie deficit. The exact ratio matters less than consistently hitting your total calorie target — this is why percentage-based diets alone are less effective than tracking actual gram targets.

Use your current body weight for the most accurate calorie and macro targets. The calculator applies a deficit or surplus to your TDEE, which is based on your current size and activity. As your weight changes over time (every 4–6 weeks), recalculate your macros to keep them aligned with your new body weight and metabolism.

Yes — if you enter your body fat percentage, the calculator uses the Katch-McArdle formula, which bases BMR on lean mass rather than total weight. This tends to be more accurate for people who are very lean or who carry more body fat, since muscle and fat tissue burn calories at different rates. If you don't know your body fat, the Mifflin-St Jeor formula still provides a reliable estimate.

A 20% calorie deficit (the default fat loss setting) typically produces a loss of 0.5–1 lb per week, which is considered a sustainable and healthy rate. Aggressive deficits (−25%) may speed this up slightly but carry a higher risk of muscle loss and energy crashes. Consistency with your daily macro targets over weeks and months is the most reliable path to lasting results.

Recalculate every 4–6 weeks, or whenever your weight changes by more than 5–10 lbs. As you lose fat or gain muscle, your TDEE shifts and your macro targets need updating to keep producing results. Plateaus are often a sign that your macros no longer match your current body weight and activity level.