Why percentages alone don't tell you grams
A calorie percentage (like "30% of calories from fat") only tells you how much of your total calorie budget that macronutrient represents — converting it into an actual gram amount requires dividing by that specific macronutrient's calories-per-gram value. Protein and carbohydrates each provide about 4 kcal per gram; fat provides about 9 kcal per gram — more than double. These are the standard Atwater factors used across nutrition science and food labeling.
The formula
Grams = (total daily calories × macronutrient percentage ÷ 100) ÷ calories per gram for that macronutrient. The percentage and total calories determine how many calories are allocated to that macronutrient; dividing by the calories-per-gram value converts that calorie allocation into an actual gram target.
A worked example showing why fat needs fewer grams
A 2,000-calorie daily target split as 30% protein, 40% carbs, 30% fat: protein works out to 150 grams (2,000 × 0.30 ÷ 4), carbs to 200 grams (2,000 × 0.40 ÷ 4), and fat to only 67 grams (2,000 × 0.30 ÷ 9). Protein and fat are both allocated the identical 30% of total calories, yet protein's gram target (150g) is more than double fat's (67g) — purely because fat packs more than twice the calories into each gram.
Why this matters for meal planning
Someone tracking macros by weighing or measuring food needs the gram targets, not the percentages, to actually plan meals — and misunderstanding that fat's gram target will be much smaller than an equal-percentage protein or carb target (because of its higher caloric density) is a common source of confusion when someone expects "30% fat" to mean roughly the same portion size as "30% protein."
Why the three percentages must add to 100%
Since protein, carbs, and fat together account for the entirety of a diet's caloric content (alcohol aside, which most macro-tracking approaches don't separately account for), their three percentages must sum to exactly 100% of total daily calories — a macro split that doesn't add up to 100% doesn't correspond to a complete, internally consistent calorie target, and needs to be adjusted before the resulting gram amounts mean anything useful.