Protein Calories per Gram: 4 kcal, Plus Carbs and Fat

Protein has 4 calories per gram. Carbohydrate has 4 as well, and fat has 9. Those are the Atwater general factors. The UK version of EU food-labeling law writes them as 17 kJ/g for protein and carbohydrate and 37 kJ/g for fat. Alcohol adds a fourth, at 7 kcal per gram. Food databases use slightly different numbers, and in the records below the gap reaches 22%.

How many calories are in a gram of protein?

Alcohol sits in the same FAO list at 7 kcal or 29 kJ a gram, although it isn’t a macronutrient, so don’t add it to your protein, fat or carbohydrate totals. What counts as a macro is covered in what are macros.

Here is the same rule as a lookup table. Find your grams, read your calories.

Grams	Protein or carbohydrate	Fat	Alcohol
10 g	40 kcal	90 kcal	70 kcal
25 g	100 kcal	225 kcal	175 kcal
50 g	200 kcal	450 kcal	350 kcal
100 g	400 kcal	900 kcal	700 kcal

For US readers, an ounce is 28.35 g. An ounce of protein is about 113 kcal, and an ounce of fat is about 255 kcal. These figures are for the pure nutrient, and food is mostly something else. A 100 g portion of braised chicken breast isn’t 100 g of protein. It has 32.1 g of protein and 166 kcal, not 400. Raw and cooked chicken differ again, as how many grams of protein in a chicken breast shows.

Where do 4, 4 and 9 come from?

From a USDA experiment station in Storrs, Connecticut, at the end of the nineteenth century. According to the FAO, W.O. Atwater and his colleagues based the factors on the heat protein, fat and carbohydrate release when burned. They corrected it for losses in digestion, absorption and urea in urine. One factor per nutrient, whatever the food it sits in.

The FAO says the system was widely used in part because it is simple. Three numbers cover every food. What you give up is precision, since a single factor treats the protein in an egg and the protein in a lentil as the same thing.

Why do food records show different calories per gram?

Because later work gave each food its own factor. The FAO reports that, in the Atwater specific system, protein runs from 2.44 kcal/g for some vegetable proteins to 4.36 kcal/g for eggs. Fat runs from 8.37 to 9.02 kcal/g, and total carbohydrate from 2.70 kcal/g in lemon and lime juices to 4.16 kcal/g in polished rice.

The FAO’s own comparison found that general factors gave values about 5 percent higher on average than specific ones for the US diet. For a few foods, such as snap beans and cabbage, the gap was 20 to 38 percent.

I checked it on eight foods. The table sums protein, fat and carbohydrate at 4, 9 and 4, then sets the result next to the energy each USDA record lists.

Food (per 100 g)	Protein	Fat	Carbs	4-9-4 sum	Record energy	Gap
Banana, raw	0.7 g	0.3 g	23.0 g	98 kcal	88 kcal	−10%
Almonds, raw	21.5 g	51.1 g	20.0 g	626 kcal	584 kcal	−7%
Peanut butter, creamy	24.0 g	49.4 g	22.7 g	632 kcal	589 kcal	−7%
Broccoli, raw	2.6 g	0.3 g	6.3 g	38 kcal	32 kcal	−17%
Baby spinach, raw	2.9 g	0.6 g	2.4 g	27 kcal	21 kcal	−22%
Chicken breast, raw	22.5 g	1.9 g	0.0 g	108 kcal	112 kcal	+4%
Egg, whole, large	12.4 g	10.0 g	1.0 g	143 kcal	147 kcal	+3%
Atlantic salmon, farmed, raw	20.3 g	13.1 g	0.0 g	199 kcal	203 kcal	+2%

Raw spinach is the extreme row. The record lists 21 kcal where the 4-9-4 sum says 27. The vegetables, the banana and the nuts all come in under the sum, by 7% to 22%. Meat, eggs and fish come in a little over, by 2% to 4%.

The gap comes from how the food was measured and says nothing about your logging. Use the listed energy. Use 4, 9 and 4 only when all you have is three gram figures.

Do fiber and alcohol have calories per gram too?

Alcohol does, 7 kcal per gram. Both the FAO and the UK version of EU food-labeling law list it.

Fiber is where the systems split. In the original Atwater system, carbohydrate is found by difference. Fiber sits inside it and counts 4 kcal per gram. The FAO notes that a separate 2 kcal/g factor for fiber has been recommended but not implemented. UK law already applies it, with fiber at 8 kJ/g (2 kcal/g), polyols at 2.4 kcal/g and organic acids at 3 kcal/g.

So the same gram of fiber is 4 kcal in a simple sum and 2 kcal on a UK pack. The difference matters little in one meal. At 30 g of fiber a day, the two methods differ by 60 kcal.

One more case is protein powder. The whey powder record has 78.1 g of protein per 100 g and lists 352 kcal. That’s 3.5 kcal per gram of powder, because about a fifth of the powder isn’t protein and adds only about 40 kcal. The protein itself still counts 4 kcal a gram, while the powder as a whole comes out lower.

How do I use calories per gram when I log food?

Weigh the food, then multiply grams by the right factor, or let the database do it. A check helps. Take a packaged food and multiply the label’s grams of protein, fat and carbohydrate by 4, 9 and 4. Compare the sum with the printed calories. If the two are far apart, recheck the figures you typed in. Labels follow their own country’s rules, so a small gap is not a mistake. A full day of this arithmetic is worked in how to count macros.

Take the Greek yogurt record. It lists 10.3 g of protein, 0.4 g of fat and 3.6 g of carbohydrate per 100 g, and 61 kcal. The sum is 59 kcal. For a 156 g tub, that is 95 kcal listed and 92 kcal summed. I’d ignore a 3 kcal difference like that.

Say a bar’s label lists 20 g of protein, 8 g of fat and 25 g of carbohydrate but no calories. You’d work out 80 + 72 + 100 = 252 kcal. That’s a usable estimate. Turning a calorie goal into a daily split is the job of what your macros should be.

To turn your own calorie goal into grams, use the macro calculator. It converts a goal into grams of protein, fat and carbohydrate.

I keep to one rule when logging. I weigh the food in the state the record describes, because cooked and raw versions of the same food carry different calories per gram, and I read every calorie figure as an estimate, since the factors can’t see how well you absorb a given meal.

Calsa puts the source and the sum, “per 100 g × grams,” on each entry, so a number you doubt can be traced to its record.