Macro Calculator

Enter the number of calories you plan to eat in a day, choose how to divide them, and the calculator converts percentages into food: grams of protein, carbohydrate and fat per day. The protein figure sits on top because it is the number people track most closely, and the table beneath it lays out all three macronutrients with the grams, the calories each one claims and its share of the total. The input that matters most arrives from outside: the calorie target itself. A maintenance estimate from the calorie calculator, built on the Mifflin-St Jeor equation and an activity multiplier, is the natural starting point — take that figure, subtract for a deficit or add for a surplus, and enter the result here.

How the calculation works

Each macronutrient supplies a roughly fixed amount of energy per gram, so once a share of the day's calories has been assigned to it, the grams follow by division:

grams = calories × share ÷ 100 ÷ energy per gram

The energy-per-gram figures are the Atwater general factors: 4 kcal per gram of carbohydrate, 4 kcal per gram of protein and 9 kcal per gram of fat. Fat's factor is more than double the other two, which has a practical consequence explored further down the page: percentage points moved into or out of fat change the gram count far less than the same points moved between carbohydrate and protein.

Three preset splits are offered, written as carbohydrate / protein / fat:

• Balanced — 50/30/20
• Lower carb — 25/40/35
• High protein — 40/40/20

Choosing Custom opens three percentage fields instead. The three shares must add up to exactly 100, because each one is a slice of the same calorie total; if they sum to 90 or 110 the calculator reports the actual total and asks for an adjustment rather than guessing which field to trust. Any single share may sit anywhere from 0% to 100% inclusive, so a zero-carbohydrate or zero-fat split computes without complaint even though few people would want to eat one.

A worked example from the defaults

The defaults are 2,200 kcal on the balanced 50/30/20 split. Protein gets 30% of the day: 2,200 × 0.30 = 660 kcal, and at 4 kcal per gram that is 660 ÷ 4 = 165 g. Carbohydrate takes half: 2,200 × 0.50 = 1,100 kcal, or 1,100 ÷ 4 = 275 g. Fat gets the remaining 20%: 2,200 × 0.20 = 440 kcal, and dividing by 9 gives 48.9 g, shown as 49 g. The three calorie figures reassemble the original day exactly — 660 + 1,100 + 440 = 2,200 — while the gram figures are rounded to whole numbers for display, which is why multiplying the rounded grams back by 4, 4 and 9 can land a few calories off the target.

Prout's three classes and Atwater's factors

The idea that food reduces to three great components is nearly two centuries old. William Prout (1785–1850), an English physician and chemist, set it out in a paper printed in the Philosophical Transactions of the Royal Society for 1827, "On the ultimate composition of simple alimentary substances", which sorted the nourishing parts of food, water aside, into three classes — the saccharinous, the oleaginous and the albuminous, the last named after egg white. The scheme maps almost directly onto today's carbohydrate, fat and protein. Prout received the Royal Society's Copley Medal that same year, though nothing in the record consulted here ties the medal to this particular paper, and the chemistry of what each class actually contains was worked out over the following decades by many hands. His contribution was the sorting, not the discovery.

Attaching a calorie value to each class took most of the following century. Wilbur Olin Atwater (1844–1907), professor of chemistry at Wesleyan University from 1873 until his death, spent the later part of his career measuring the energy in food and in people. With colleagues at the Storrs Agricultural Experiment Station in Connecticut he burned weighed samples to find their heat of combustion, and at Wesleyan, working with the physicist Edward Bennett Rosa, he built a room-sized respiration calorimeter in which a subject could live for days while everything eaten, exhaled and excreted was weighed and measured; the first of close to 500 such experiments ran in 1896. The correction mattered, because a food's furnace value overstates what a body extracts — digestion and absorption are incomplete, and the nitrogen in protein leaves as urea, carrying energy out unburned. Out of those corrected measurements came the rounded working averages that Atwater and Woods set out in 1896: 4 kcal per gram for carbohydrate, 4 for protein and 9 for fat, with alcohol assigned 7 in 1902. In Atwater's original scheme carbohydrate was whatever remained once the other components had been measured, so fibre was counted in. Those Atwater general factors are the same three constants this calculator uses. Their modern reference is a USDA handbook by Merrill and Watt, last revised in 1973, and US labeling regulations name the 4, 4 and 9 general factors as one of several permitted ways to compute the calorie line on a Nutrition Facts panel, alongside food-specific factors and a variant that subtracts non-digestible carbohydrate — which is why label arithmetic does not always match the general factors.

The reference ranges behind the presets

Percentage-based targets have an official reference point. Between 2002 and 2005 the Institute of Medicine, working under the US National Academies, published Dietary Reference Intakes for macronutrients that include Acceptable Macronutrient Distribution Ranges: for adults, 45 to 65 percent of calories from carbohydrate, 10 to 35 percent from protein and 20 to 35 percent from fat. A 2024 National Academies letter report that re-examined the concept still tabulated those same adult ranges. The presets here are editorial choices made for this page, not recommendations from any scientific body. The balanced split sits inside all three ranges, with fat resting exactly on the 20 percent floor. The lower-carb and high-protein presets deliberately step outside them — both place protein at 40 percent, five points above the top of the protein range, and carbohydrate at 25 or 40 percent, below the 45 percent floor, while the lower-carb preset's 35 percent fat sits exactly on the fat ceiling — because that is what those eating patterns look like in practice, and a preset should describe the pattern honestly rather than sand it down until it fits a reference band.

What moving the percentages changes

Start with what the split does not change: the calorie total. Every option assigns the same day of energy, so switching presets is not a way to eat less, only a way to eat differently. What changes is grams. At 2,200 kcal, protein is 165 g on the balanced split and 220 g at 40 percent — the difference between a day with protein at most meals and a day built around it at every sitting. The arithmetic of a swing is worth internalizing: moving 10 percentage points at 2,200 kcal moves 220 kcal, which is 55 g when it travels between carbohydrate and protein but only about 24 g when it moves into or out of fat, because each fat gram absorbs 9 kcal rather than 4. Fat percentages are therefore deceptively quiet. A split that trims fat from 35 to 20 percent of a 2,200 kcal day removes only about 37 g of fat while freeing 330 kcal for the other two macronutrients, and the reverse move adds those calories back just as invisibly.

Assumptions and limits

The constants 4, 4 and 9 are conventions, not measurements of the food on any particular plate; they are rounded averages applied uniformly to every food in the day, and in kilojoules the same conventions are 17, 17 and 37 per gram. Alcohol is not modeled at all — the Atwater system carries it at about 7 kcal per gram, a fourth energy source this three-way split ignores, so the grams describe the food portion of a day with drinks only if the calorie input already excludes the alcohol. Grams and calories in the table are rounded to whole numbers and the headline protein figure is rounded the same way. The 6,000 kcal ceiling is enforced by the calculation itself, which also requires a target above zero; the 800 kcal floor is only the form field's minimum, so the browser flags a smaller entry in the form while the calculation behind it would still split any positive target. Custom shares are expected as whole percentages, though decimals are accepted as long as the three still total exactly 100, and each share must be between 0 and 100 inclusive. Finally, the percentages describe calories, not weight: 30 percent protein means 30 percent of the day's energy, not that a third of the food on the plate is protein.

A macro split is arithmetic on a convention, not nutrition advice for any individual; personal targets belong with a clinician or registered dietitian. See the site disclaimer.

Frequently asked questions

How do I calculate my macros from calories?

Multiply your daily calories by each macronutrient's percentage share, then divide by the calories in a gram: 4 for carbohydrate, 4 for protein and 9 for fat. On 2,200 kcal with a 50/30/20 carb/protein/fat split that works out to 275 g of carbohydrate, 165 g of protein and about 49 g of fat per day.

What is the best macro split for weight loss?

No single split is best, and total calories, not the split, decide whether weight changes. The split only decides how those calories are divided among protein, carbohydrate and fat. The Institute of Medicine's acceptable ranges for adults are 45 to 65 percent of calories from carbohydrate, 10 to 35 percent from protein and 20 to 35 percent from fat, and the presets on this page are editorial starting points, not recommendations.

Why does fat have 9 calories per gram when carbs and protein have 4?

The figures are the Atwater general factors, rounded averages that came out of Wilbur Atwater's calorimetry work at Wesleyan University and the Storrs experiment station in Connecticut in the 1890s. Burning foods to measure their heat and correcting for what the body actually retains gave about 4 kcal per gram for carbohydrate and protein and 9 kcal per gram for fat, and US labeling rules still list the same constants as one permitted way to compute the calorie figure on food labels.

How many grams of protein is 30 percent of 2200 calories?

165 grams. Thirty percent of 2,200 kcal is 660 kcal, and protein carries about 4 kcal per gram, so 660 divided by 4 gives 165 g. The same two steps work for any target: multiply the calories by the share, then divide by 4 for protein or carbohydrate and by 9 for fat.

Do macro percentages have to add up to 100?

Yes. Each percentage is a share of the same daily calorie total, so shares that sum to 90 leave a tenth of the day's calories unassigned and shares that sum to 110 count some calories twice. This calculator reports the actual sum and asks for an adjustment instead of silently rescaling the numbers.

How do I know how many calories to enter?

Start from an estimate of your maintenance calories, such as the one this site's calorie calculator produces from the Mifflin-St Jeor equation and an activity multiplier, then adjust it down for a planned deficit or up for a surplus. The macro calculator only divides whatever total it is given; it does not judge whether that total suits you.