Macro Calculator

How It Works

The macro calculator runs Mifflin-St Jeor for BMR, multiplies by an activity factor (1.2 sedentary to 1.9 hard daily training), then splits the calorie target into protein, carbs, and fat. The Atwater factors used (4/4/9 kcal per gram) are class averages — actual metabolizable energy from mixed-fiber foods is closer to 3.5 kcal/g for refined carbs and 2.6 kcal/g for almonds (Novotny et al., AJCN 2012). The biggest source of error here is the activity multiplier, not the BMR equation itself: self-reported activity inflates TDEE by 200-400 kcal in roughly half of users (Lichtman et al., NEJM 1992). Pick the multiplier one tier below what feels right and recalibrate from 2-3 weeks of weight trend data.

The Formula

Variables

• BMR — Basal Metabolic Rate — kcal at complete rest, ~60-70% of TDEE for sedentary adults
• TDEE — Total Daily Energy Expenditure — BMR + TEF (~10%) + NEAT + exercise
• weight_kg — Body weight in kilograms
• height_cm — Height in centimeters
• activity_multiplier — 1.2 desk job, 1.375 light 1-3x/wk, 1.55 moderate 3-5x/wk, 1.725 hard 6-7x/wk, 1.9 hard daily + physical job
• calories_per_gram — Atwater factors: protein 4, carbs 4, fat 9, alcohol 7, fiber ~2

Worked Example

75 kg male, 178 cm, 32 yrs, lifts 4x/wk, cutting at -500 kcal/day. BMR = 10(75) + 6.25(178) - 5(32) + 5 = 1,708 kcal. TDEE = 1,708 * 1.55 = 2,647 kcal. Target = 2,147 kcal. Protein at 2.0 g/kg = 150 g (600 kcal). Fat at 0.8 g/kg = 60 g (540 kcal). Carbs = (2,147 - 600 - 540) / 4 = 252 g. Final split: 150P / 252C / 60F.

Practical Tips

• Use a 7-day rolling weight average from morning weigh-ins, post-bathroom, pre-food. Daily readings swing 1-2 kg from sodium, glycogen, and stool weight — the average smooths it out.
• Set protein in g/kg of body weight, not as a percentage of calories. At 1,800 kcal vs 2,800 kcal, a fixed 30% protein gives you 135 g vs 210 g — only one of those preserves muscle in a deficit.
• Mifflin-St Jeor was validated against indirect calorimetry in a sample skewing average BMI. For lean trained individuals it under-predicts BMR by 5-8%; for high-BF individuals it over-predicts by a similar margin (Frankenfield et al., J Am Diet Assoc 2005).
• TEF differs by macro: protein burns 20-30% of its own calories in digestion, carbs 5-10%, fat 0-3%. A 200 g protein day costs roughly 160 extra kcal vs an isocaloric low-protein day.
• If weight is flat for 14 days at the same intake, you found maintenance — not a stalled deficit. Drop intake by 150 kcal or add 1,500 daily steps, then wait another 10-14 days before judging.
• Activity multipliers cap at 1.9 because beyond that, exercise calories are large enough to track separately (TDEE method breaks down). For tour-stage cyclists or 4hr/day endurance athletes, log workouts as discrete kcal additions instead.
• Round protein up, carbs to nearest 5 g, fat to nearest 5 g — exact gram precision is theater given measurement error in food scales (~3%) and label tolerance (FDA allows ±20%).

Last updated: May 04, 2026 · Last reviewed: May 2026 — NutritionCalcs Editorial Team · About our methodology