Grams from percentages
Your calories come from the TDEE calculation (Mifflin-St Jeor × activity), adjusted for the goal. The split divides them between the three macronutrients, and grams follow from their energy: 4 kcal per gram of protein or carbohydrate, 9 kcal per gram of fat.
Common questions
Why is fat calculated differently — divided by 9, not 4?
Because fat actually contains more energy per gram — about 9 kcal/g versus 4 kcal/g for protein and carbs. That's a physical property of the macronutrient, not a calculator choice.
Which split should I pick if I don't have a strong preference?
Balanced (30/40/30) is a reasonable default that works for most goals — the more extreme splits (keto, low-carb) are worth trying only if you have a specific reason to, since they're harder to sustain long-term for most people.
For general information only — not medical advice. Talk to a doctor about anything that concerns you.