Formula
What this macros calculator does
Computes your daily protein, carb, and fat targets in grams for a specific goal (lose fat, maintain, or gain muscle). Uses the Mifflin-St Jeor formula for BMR, multiplies by an activity factor for TDEE, then derives macros using evidence-based ratios: protein scaled to body weight (most accurate), fat fixed as % of total calories (hormone support), carbs filling the remainder (energy and performance). All in your browser, instant, with no signup. The progress bars show the % of total calories each macro provides, making it easy to compare against your current eating habits.
How the math works
BMR (basal metabolic rate) is how many calories you burn at rest. Mifflin-St Jeor: male BMR = (10 x weight kg) + (6.25 x height cm) - (5 x age) + 5. Female BMR uses the same formula minus 161. TDEE (total daily energy expenditure) = BMR x activity factor. Activity factors: 1.2 sedentary, 1.375 light, 1.55 moderate, 1.725 very active, 1.9 extra active. Goal adjustment: lose = TDEE - 500 (about 1 lb/0.5 kg per week), maintain = TDEE, gain = TDEE + 300 (lean bulk). Then: protein in grams = protein-per-kg setting (1.6 to 2.0) x body weight in kg. Fat in grams = (target calories x fat percentage) / 9. Carbs in grams = (target calories - protein calories - fat calories) / 4.
What each macro does
Protein (4 kcal/g): builds and preserves muscle, especially critical during a calorie deficit. Recommended 1.6 to 2.2 g/kg of body weight depending on goal. Sources: chicken, fish, beef, eggs, dairy, lentils, beans, tofu. Carbs (4 kcal/g): primary fuel for the brain and high-intensity exercise. Quality matters - whole grains, fruits, vegetables, legumes are slow-burning; refined sugar spikes blood glucose. Fat (9 kcal/g): essential for hormone production (testosterone, estrogen), vitamin absorption (A, D, E, K), and cell membrane health. Don't go below 0.5 g/kg of body weight. Sources: olive oil, avocados, nuts, fatty fish, eggs, dairy.
Goal-specific strategies
FAT LOSS: protein high (2.0 g/kg) to preserve muscle during the deficit; carbs moderate to support training intensity; fat moderate. A 500 kcal/day deficit yields ~0.5 kg / 1 lb per week of fat loss. Faster than this risks muscle loss and hormonal disruption. MAINTENANCE: balanced split, protein still 1.6 g/kg for body composition. Useful for body recomposition (slowly losing fat while gaining muscle - works best for beginners and detrained individuals). MUSCLE GAIN: 300 kcal/day surplus is enough for ~0.25-0.5 kg / 0.5-1 lb per month of lean mass; bigger surplus just means more fat gain. Protein 1.8 g/kg supports muscle protein synthesis; carbs high to fuel training; fat moderate.
Tracking and adjusting
Weigh yourself daily at the same time (after using the bathroom, before food/water) and track the 7-day average. Daily fluctuations of 1-2 kg are normal water shifts. Adjust calories every 2-3 weeks based on the weekly trend: losing too fast (more than 1% body weight/week) - eat 200 kcal more; stalled - eat 200 kcal less. Track food using a free app (MyFitnessPal, Lose It, Yazio) for at least 2 weeks to build awareness, then you can transition to estimating. Don't chase perfection - hitting macros within 10-15% is enough. Sleep and stress affect appetite and adherence more than perfect macros.
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