Calories to grams
Macronutrients are the three nutrients that supply energy in large amounts: carbohydrate, protein and fat. Food labels and diet plans often give a target as a share of daily calories, but you eat grams. The conversion uses energy factors that are standard in nutrition: about 4 kilocalories per gram of carbohydrate, 4 per gram of protein and 9 per gram of fat (general Atwater factors, University of Minnesota Nutrition Coordinating Center). So the grams of a macro equal daily energy × its share ÷ its factor. At 2,000 kcal with 50% from carbohydrate, that is 2,000 × 0.50 ÷ 4 = 250.0 g. The factors are averages, and real foods vary a little around them, which is one reason the figures here are planning arithmetic and not a prescription. The conversion also works backwards: 100 g of protein is 400 kcal, which is 20% of a 2,000 kcal day.
Choosing an energy source
The calculator starts from a daily energy figure, and you choose where it comes from. If you already know your number, from a plan you follow or advice from a professional, pick "I know my calories" and type it in. No height, weight or age is needed in that mode. If you do not, pick "Estimate for me". The page then uses the same resting-energy equations as the BMR calculator and multiplies the result by one of six conventional activity factors from 1.2 to 1.9. For a 30-year-old male of 175 cm and 75 kg, the Mifflin-St Jeor equation times the sedentary factor 1.2 gives 2,039 kcal a day. That factor is a scenario, not a measurement of how active you are, so treat the estimate as a starting guess and not as a promise about what you burn.
An optional illustrative adjustment can raise or lower the starting figure by a percentage between −20% and +20%. A −10% adjustment turns 2,000 kcal into 1,800 kcal. The page does not say how much weight that would change, how long it is safe to follow or whether it suits you. Those questions depend on your health and are beyond a calculator.
Presets and custom percentages
The three shares must add up to exactly 100%, and the calculator checks that to one decimal place so that 99.9% and 100.1% are rejected instead of quietly rounded. Four presets fill the boxes for you: Balanced 50/20/30, Lower fat 55/20/25, Lower carb 45/25/30, Higher protein 45/30/25. They are examples of arithmetic, not diets chosen by a clinician, and picking one does not calculate anything until you press the button. You can also type your own split. As soon as you edit a box, the preset switches to custom. A split of 45% carbohydrate, 30% protein and 25% fat at 2,000 kcal is 225.0 g, 150.0 g and 55.6 g.
Results are compared with the adult ranges published by the Institute of Medicine, now part of the National Academies: 45% to 65% of energy from carbohydrate, 10% to 35% from protein and 20% to 35% from fat (summary in a sports-nutrition review). A split outside them is only flagged. A fat share of 0%, for example, is allowed because it is valid arithmetic, but the page notes that 0% is below the reference range.
Worked 2,000 kcal example
Take 2,000 kcal a day at 50% carbohydrate, 20% protein and 30% fat. Carbohydrate is 2,000 × 0.50 = 1,000 kcal, and 1,000 ÷ 4 = 250.0 g. Protein is 2,000 × 0.20 = 400 kcal, so 100.0 g. Fat is 2,000 × 0.30 = 600 kcal, and 600 ÷ 9 = 66.7 g. Check the total: 250 × 4 + 100 × 4 + 66.7 × 9 = 2,000 kcal. Apply the −10% adjustment and the same split gives 225.0 g, 90.0 g and 60.0 g, which add up to 1,800 kcal. Because fat is 9 kcal per gram and the others are 4, a split that looks lopsided in percentages can look quite different in grams.
Scale is easy to check by eye. Double the energy to 4,000 kcal at the same split and every gram figure doubles, so carbohydrate becomes 500.0 g. Change only the split and the total energy stays put while the grams move between macros.
Per-meal arithmetic and food-quality limits
Choose from 1 to 8 meals and the page divides each daily total equally. With three meals the default example is 83.3 g of carbohydrate, 33.3 g of protein and 22.2 g of fat in each. With five meals the protein is 20.0 g per meal. This is plain division. It is not advice on when to eat, and real meals are rarely equal. Displayed grams are rounded to one decimal place, so a rounded column can differ from the total by a fraction of a gram. The calculation uses the unrounded values.
A macro split says nothing about what the food is. Two people can hit identical numbers with very different quality of food, fiber or micronutrients, and the calculator does not count fiber, added sugar, saturated fat, salt or vitamins. It is also written for healthy adults. Pregnancy, breastfeeding, diabetes, kidney disease, eating disorders and many other situations call for individual guidance from a doctor or registered dietitian. Related calculators are the protein calculator for weight-based references and the Fitness & Health hub.
Macro calculator questions
What are macros?
Macros, short for macronutrients, are carbohydrate, protein and fat, the three nutrients that provide most of the energy in food. Calorie targets are often split between them as percentages, which this calculator turns into grams. On food labels they are measured in grams, and in diet plans they are often discussed as a share of daily calories.
What if I already know my calories?
Choose "I know my calories" and enter the figure. You do not need to give your height, weight or age, because nothing is estimated in that mode. The estimate option is there for people who do not know their figure, and it shows every step it takes.
Do the percentages have to total 100?
Yes, exactly 100.0%. If they total 99.9% or 100.1% the calculator asks you to fix them rather than adjusting one silently. The percentages are checked exactly, with no rounding, so a total that looks right on screen but is slightly off is still caught.
Why don't the rounded grams add up exactly?
The grams shown are rounded to one decimal, while the calculation keeps full precision. A rounded column can therefore be a fraction of a gram away from the total. Each row of the tables is shown to one decimal place for readability.
Does a macro split guarantee weight loss or gain?
No. A split only divides an energy figure. What happens to your weight depends on how much you eat and burn, on your health and on many things this calculator does not model. Talk to a doctor or dietitian before making a large change to what you eat.