How many grams of carbs and fat should a day contain?
Quick answer
The IOM publishes ranges as shares of energy, not grams: protein 10 to 35 percent, carbohydrate 45 to 65, fat 20 to 35. At 2,500 kcal a day that is 63 to 219 g of protein, 281 to 406 g of carbohydrate and 56 to 97 g of fat.
The conversion uses the general Atwater factors published by Merrill and Watt: 4 kcal per gram of protein, 4 per gram of total carbohydrate and 9 per gram of total fat. Multiply your energy figure by the percentage, divide by the factor, and you have grams. There is nothing more to a macro calculator than that.
What matters is the status of the percentages. The AMDR is a range published by an IOM committee for populations, and the committee set no single optimum inside it. A split that sits inside the range is not a score, and one that sits outside it is not forbidden - it is simply outside what the IOM range covers, which is what this site's tool says rather than grading anyone.
The ranges are also wide enough to contain almost every popular way of eating, which is itself informative. Protein from 10 to 35 percent of energy is a factor of three and a half. If a calculator hands you one split to the gram, the precision came from whoever built the preset, not from the IOM.
One practical note: the energy figure you multiply is usually an estimate with a few hundred kcal of uncertainty around it, so the grams inherit that. Protein is the exception worth knowing about, because its published ranges are set per kilogram of body weight rather than as a share of energy - see how much protein do I need per day.
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