Understanding macronutrient splits
4 / 9 / 4 — the constants behind the splits.
How a kcal target becomes grams of protein, fat and carbs, the ratios that actually matter, and the ones that don't.
The energy density of each macro.
Protein: 4 kcal/g. Carbs: 4 kcal/g. Fat: 9 kcal/g. Alcohol: 7 kcal/g (relevant for honest accounting). To convert a calorie target to grams of each macro, decide the protein and fat first, fill the rest with carbs. The math is just g = kcal / density per macro.
Protein is fixed by body weight.
For body composition goals (muscle gain or fat loss with muscle preservation), 1.6-2.2 g of protein per kg of body weight per day is the sweet spot across modern research. Lower limits muscle synthesis; higher doesn't help beyond a point. For sedentary adults the RDA is much lower (~0.8 g/kg) — fine for maintenance, suboptimal for any body-comp goal. Take your weight in kg, multiply by 1.6-2.2, that's your protein gram target.
Fat has a floor, not a target.
0.5-1.0 g/kg/day of fat is the minimum for hormonal health (oestrogen, testosterone, thyroid all need dietary fat). Above that, fat is just an energy source — eat as much or as little within your calorie budget as you prefer. Low-fat (20 % of calories) and low-carb (60 % fat) both work for fat loss when calories are matched. The choice is preference and satiety, not biological imperative.
Carbs fill the rest.
After protein and fat targets, carbs absorb the remaining calorie budget. For a 2100 kcal target with 150 g protein (600 kcal) and 70 g fat (630 kcal), carbs = (2100 − 600 − 630) / 4 = 217 g. The ratio comes out automatically; no need to start from "I'll eat 40 % carbs". Higher carbs help training performance (especially endurance and high-volume strength); lower carbs help glucose control for some people. Within the calorie budget, the trade is preference.
A worked split.
A 75 kg moderate exerciser, 2100 kcal target for a moderate cut. Protein at 1.8 g/kg: 75 × 1.8 = 135 g → 540 kcal. Fat at 0.8 g/kg: 75 × 0.8 = 60 g → 540 kcal. Carbs: (2100 − 540 − 540) / 4 = 255 g → 1020 kcal. Final split: 135 P / 60 F / 255 C. Ratios: 26 % / 26 % / 48 %. Reasonable, with no carb-loaded sport in the picture; for an endurance athlete, pull fat down and carbs up.
Body-weight-driven split
P 1.8/kg, F 0.8/kg, C fills the rest
Two anchors, one residual.
135 P + 60 F (1080 kcal) → 255 g C (1020 kcal)
= 135 / 60 / 255 g
Tracking doesn't have to be perfect.
Hitting macros within ±10 % daily is fine for results; obsessing about ±2 g of protein is wasteful. Weekly averages matter more than daily ones — a day at 110 g protein followed by a day at 160 g averages to the target. Apps that demand precise daily compliance burn out users faster than ones that allow flexibility within a weekly window.