A macro split is a calorie target divided three ways, and the conversion back to grams is where the asymmetry shows: protein and carbohydrate pay 4 calories a gram and fat pays 9, so equal percentages buy very unequal weights. A 30/40/30 split of 2,400 calories is 180 g of protein, 240 g of carbohydrate and only 80 g of fat. The deeper problem is that only one of those three has a target with evidence behind it — and a percentage split will usually put protein below it.
That is the practical order of operations: set protein by body weight, set fat to a floor, and let carbohydrate take whatever remains.
How much protein?
The 0.8 g per kilogram figure on nutrition labels is a minimum to prevent deficiency rather than an optimum for anyone training. The bodies that publish position stands on sports nutrition settle somewhere between 1.4 and 2.0 g per kg for anyone doing resistance work, and push the upper end higher during a deficit — not to build anything, but because protein is what stops the weight coming off as muscle.
| Situation | Grams per kg | For a 75 kg adult |
|---|---|---|
| Sedentary minimum | 0.8 | 60 g |
| General activity | 1.2 | 90 g |
| Strength training | 1.6 | 120 g |
| Training in a deficit | 2.0–2.2 | 150–165 g |
A 75 kg lifter therefore wants around 120 g a day, which at 4 calories a gram is 480 calories: 20 per cent of a 2,400 target. A 30 per cent protein split would give 180 g, which is more than needed and not harmful; a 15 per cent split would give 90 g, which is short.
How should the rest divide?
Largely by preference, within limits. Once protein is set and fat has a floor of roughly 0.6 to 0.8 g per kg for hormonal function and fat-soluble vitamin absorption, the remaining calories can go either way with little difference to body composition.
What the ratio does affect is adherence and training. Higher carbohydrate suits high-volume or high-intensity training because it refills glycogen; higher fat suits people who find it more satiating. Neither is better in the abstract, and the one you can sustain is the one that works.
This is why a macro split is a starting configuration rather than a prescription. The calorie total drives the weight change; the split drives how comfortable getting there is.
How much water?
A common clinical baseline is 35 ml per kilogram of body weight, so a 70 kg adult needs about 2.45 litres a day in temperate conditions; with roughly 400 to 800 ml more per hour of exercise, and more again in heat.
Nobody has ever produced the study behind the eight-glasses rule, and its bigger problem is structural rather than evidential: it is a single number offered to people who differ by a factor of two in body mass and by more than that in how much they sweat. A weight-based figure is better and still approximate, because sweat rates vary enormously between people.
Food contributes too, typically 20 to 30 per cent of total intake, which is why the fluid figure and the drinking figure are not the same number. The most reliable everyday indicator remains urine colour rather than any calculation.
Does the split change in a deficit?
Protein goes up and everything else absorbs the reduction. In a deficit protein is doing a job it does not have to do at maintenance. Protecting lean tissue from being used for fuel, which is why the recommended range rises to 2.0 or 2.2 g per kilogram when calories are restricted and training continues.
For a 75 kg adult that is 150 to 165 g, or 600 to 660 calories. On a 1,800 calorie cutting target that is a third of the total before anything else is decided, which is a substantially different-looking plan from the same person at maintenance.
Do the numbers need to be exact?
No, and treating them as if they do is the most common way a plan becomes unsustainable. Food labels carry a tolerance, portion estimation is imprecise, and the calorie target itself came from a population equation with a 10 per cent spread.
Getting protein close and the calorie total close is most of the benefit. Chasing the last few grams of carbohydrate against a target derived from an estimate is precision applied to the wrong end of the calculation.
Questions people ask
Is there a best macro split? Not for body composition at a fixed calorie total and adequate protein. There are better and worse splits for a specific sport, a specific medical condition or a specific appetite, which is a different question.
Why is fat 9 calories a gram? Because of its chemistry. Fat is more reduced than carbohydrate and releases more energy per gram when oxidised. It also means fat calories arrive in a smaller volume of food, which cuts both ways for satiety.
Does alcohol count? At 7 calories a gram, and it fits into none of the three categories. Where it is a regular part of intake it has to come out of the total somewhere.
Should I hit protein on a rest day? Yes. Muscle protein synthesis runs for a day or more after training, and intake is easier to keep consistent than to vary by day.
Do I need to hit the water figure exactly? No. Thirst and urine colour regulate this well for most people in temperate conditions, and the calculation is most useful where those signals are unreliable: during illness, in heat, or at altitude.
Set protein by weight, put a floor under fat, and let the rest be preference. The macro calculator splits a calorie target by percentage, the protein calculator works from body weight and training, and the water intake calculator adds the fluid side with an allowance for exercise and heat.