Splitting your calories three ways

The three macronutrients

Macronutrients are the three components of food that carry energy, and each carries a different amount per gram:

Fat carrying more than twice what the other two carry is the single most practically useful fact here. It is why an unmeasured tablespoon of cooking oil is 120 kcal, and why fat is where under-logging does most of its damage.

Be clear about what macros do and do not decide. Total calories determine whether you lose weight. Macros influence what you lose — how much of it is fat rather than muscle — and how tolerable the process feels. Both are worth getting roughly right, and neither overrides the first.

Protein: the one worth getting right

Protein is the macro where the number genuinely matters during a deficit, because it is the one that decides how much of your loss comes from muscle.

The RDA is the intake below which a sedentary adult starts losing lean mass. It is a floor, and it is the default in this calculator for one specific reason: it is the only figure defensible without knowing anything about you. Higher intakes are well supported while dieting, but starting a first-time visitor at a bodybuilding target and presenting it as fixed asks them to plan around an amount of protein most people will not eat.

If you are lifting and in a deficit, somewhere between 1.6 and 2.2 g/kg is the range most of the evidence points at. Two side benefits are worth knowing: protein is the most satiating of the three per calorie, and it carries the highest thermic effect — roughly 20-30% of its energy is spent on digestion, against 5-10% for carbohydrate and 0-3% for fat.

Splitting the remainder

Once protein is set, the rest of your calories divide between fat and carbohydrate. For fat loss specifically, the ratio between those two matters far less than most diet marketing implies — controlled studies matching calories and protein find little difference in fat loss between low-carb and low-fat approaches.

There are floors worth respecting rather than ratios worth chasing. Fat below roughly 0.5 g per kg of bodyweight starts to interfere with hormone production and fat-soluble vitamin absorption; 0.8 to 1.0 g/kg is a comfortable region. Carbohydrate has no strict requirement — the body can manufacture glucose — but it is what fuels hard training, and cutting it low while trying to lift heavy is a choice with an obvious cost.

This calculator's default sends 35% of the non-protein calories to fat and the remainder to carbohydrate, which lands in the conventional middle. If you have a reason to prefer a different split, change it; if you do not, this one is unremarkable and fine.

A worked split

An 80 kg person on a 2,000 kcal target, at 1.6 g/kg of protein:

1. Protein: 1.6 × 80 = 128 g, which is 512 kcal.
2. Remainder: 2,000 − 512 = 1,488 kcal.
3. Fat at 35% of the remainder: 520.8 kcal ÷ 9 = 58 g.
4. Carbohydrate takes what is left: 2,000 − 512 − 522 = 966 kcal ÷ 4 = 241.5 g.

That last line is a problem, and it is worth showing rather than hiding. You cannot eat half a gram meaningfully, and rounding it either way leaves the three slices adding up to something other than 2,000 — a donut chart that visibly disagrees with the target printed above it.

Fat is the only slice whose 9 kcal per gram can shift the leftover off a multiple of 4, so fat absorbs the adjustment: 56 g of fat leaves 984 kcal, which is exactly 246 g of carbohydrate. The final split is 128 g protein, 56 g fat, 246 g carbohydrate — 2,000 kcal exactly. Two grams of fat is nutritionally nothing next to a total that fails to add up.

Why nothing here is locked

An earlier version of this tool pinned protein at 2.2 g/kg and let fat and carbohydrate absorb every change. It made a tidy chart and a bad plan.

Presenting one aggressive figure as though it were a physiological constant is the first problem. Protein needs vary with training, age, and how much of the deficit someone is trying to hold lean mass through — none of which a calculator knows about you. The second problem is arithmetic: on a genuine cut, 176 g of protein against a 1,600 kcal target eats 44% of the budget, and the other two slices collapse into something nobody would eat.

So all three slices are editable, from either direction: percentages of the target, or exact grams if you are working from a plan you already have. Grams are what the tool stores underneath, because grams are what you weigh out, and a percentage of a moving target is not a stable thing to keep.

What actually matters, in order

Macro tracking absorbs a disproportionate share of attention relative to its effect. Ranked by how much each actually influences the outcome:

1. Total calories. Decides whether you lose weight at all. Everything below is a refinement on top of this.
2. Protein intake. Decides how much of the loss is fat rather than muscle. The one number worth measuring properly.
3. Consistency. Hitting rough targets most days beats hitting exact ones for a fortnight and abandoning it.
4. Fibre and micronutrients. Not macros, but they are what make a deficit tolerable. Aim for 25-30 g of fibre.
5. The fat-to-carbohydrate ratio. Set it to whatever you find easier to sustain. Genuinely.
6. Meal timing and nutrient timing. Effectively irrelevant for fat loss outside of competitive athletics.

If you take one thing from this page: get your calories roughly right, eat enough protein, and stop optimising the rest. The calorie deficit guide covers the first, and it is where the leverage is.

Sources

The research this guide draws on, and the claim each one supports. Figures that come from the calculator itself — the NEAT penalty, the danger thresholds, the break model — are documented with their own sources on the methodology page.

1. The 0.8 g/kg protein RDA and the energy content of each macronutrientInstitute of Medicine (2005). Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids. The National Academies Press.
2. The benefit of extra protein levelling off around 1.6 g/kgMorton RW, Murphy KT, et al. (2018). A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength in healthy adults. British Journal of Sports Medicine 52(6):376–384.
3. Higher protein intakes for lean, resistance-trained people in a deficitHelms ER, Zinn C, et al. (2014). A systematic review of dietary protein during caloric restriction in resistance trained lean athletes: a case for higher intakes. International Journal of Sport Nutrition and Exercise Metabolism 24(2):127–138.
4. The thermic effect of protein, carbohydrate and fatWesterterp KR (2004). Diet induced thermogenesis. Nutrition & Metabolism 1(1):5.
5. Controlled studies finding little fat-loss difference between low-carb and low-fat dietsHall KD, Guo J (2017). Obesity energetics: body weight regulation and the effects of diet composition. Gastroenterology 152(7):1718–1727.
6. A 12-month trial finding the same for low-fat and low-carbohydrate dietsGardner CD, Trepanowski JF, et al. (2018). Effect of low-fat vs low-carbohydrate diet on 12-month weight loss in overweight adults: the DIETFITS randomized clinical trial. JAMA 319(7):667–679.