How to use it
- Under Start from, choose Calories and % (grams from a calorie total) or Grams (calories and percentages from grams you already know).
- Type a number in Daily calories. If you don't have one, the TDEE calculator estimates it.
- Set the Protein (%), Carbs (%) and Fat (%) fields. They start at 20, 50 and 30.
- Keep the total at 100%. If it isn't, a note shows the current total, for example "Percentages add up to 90%, not 100%".
- With Grams, type Protein (g), Carbs (g) and Fat (g). Optionally pick Split over meals to divide the day evenly over 2 to 6 meals.
- Read grams per day for each nutrient next to the adult AMDR ranges. Open How this was calculated for the sums, or keep them with Copy link, Save image or Share.
How the calculation works
Each share of calories is divided by the energy one gram of that nutrient provides:
Protein grams = calories × protein % ÷ 4. Carb grams = calories × carbs % ÷ 4. Fat grams = calories × fat % ÷ 9.
Example: 2,000 kcal/day split 30% protein, 40% carbs and 30% fat.
| Nutrient | Calories | Calculation | Grams per day |
|---|---|---|---|
| Protein | 600 | 2,000 × 0.30 ÷ 4 | 150 g |
| Carbs | 800 | 2,000 × 0.40 ÷ 4 | 200 g |
| Fat | 600 | 2,000 × 0.30 ÷ 9 | 66.7 → 67 g |
Going the other way: grams to calories
With Grams, the calculator multiplies each amount by its Atwater factor and adds them up: kcal = protein × 4 + carbs × 4 + fat × 9. Each share is that nutrient's kcal divided by the total.
Example: 150 g protein, 200 g carbs and 67 g fat give 600 + 800 + 603 = 2,003 kcal, a split of 30 / 40 / 30%. The calorie total is not exactly 2,000 because 67 g of fat is a rounded figure (2,000 × 30% ÷ 9 = 66.7 g).
Split over meals simply divides the daily grams by the number of meals: 150 g of protein over 3 meals is 50 g each. It is division, not a recommendation for how to spread food over the day.
How accurate is this?
The grams are exact arithmetic from your calories and percentages, using the Atwater factors of 4, 4 and 9 kcal per gram. Those factors are averages: the energy you actually get from a gram of protein, carbs or fat varies a little by food. The bigger uncertainty is usually the calorie number you start from.
Where 4, 4 and 9 come from
These are the general Atwater factors: 4 kcal (17 kJ) per gram of protein, 4 kcal (17 kJ) per gram of carbohydrate and 9 kcal (37 kJ) per gram of fat. W.O. Atwater and colleagues worked them out at the USDA experiment station in Storrs, Connecticut, at the end of the nineteenth century.
They start from the heat each nutrient gives off when burned, then subtract average losses in digestion and urine. An FAO report on food energy also lists alcohol at 7 kcal per gram. The calculator leaves alcohol out.
The AMDR ranges for adults
The Acceptable Macronutrient Distribution Ranges (AMDR) come from the Dietary Reference Intakes for macronutrients, published in 2002/2005 by the Institute of Medicine (now the National Academies). They are ranges linked to adequate intake of essential nutrients and a lower risk of chronic disease.
The calculator shows your percentages next to them. In the 30/40/30 example, protein and fat sit inside their ranges, while carbs at 40% sit below 45%. The default 20/50/30 sits inside all three.
| Nutrient | AMDR |
|---|---|
| Protein | 10–35% |
| Carbohydrate | 45–65% |
| Fat | 20–35% |
What the numbers do and don't tell you
The grams are a conversion, not a measurement of what any food provides. FAO notes that the general factors ignore real differences between foods. For a mixed US diet they gave values about 5% higher than food-specific factors, and for some vegetables and fruits the gap reached 20–38%.
In the general system, carbohydrate is measured "by difference" and includes fiber, so fiber is counted at 4 kcal per gram. There are no separate factors for fiber or available carbohydrate.
The AMDR ranges shown are the adult ones; the same tables use different ranges for children. They are population ranges, not individual targets, and they are not made for medical diets such as those for kidney disease.
What one percentage point is worth
Because the factors are fixed, each percentage point has a fixed value in grams for a given calorie total. At 2,000 kcal, 1% is 20 kcal. That is 5 g of protein or carbs (20 ÷ 4), or about 2.2 g of fat (20 ÷ 9).
Moving 5 points from carbs to protein at 2,000 kcal therefore adds 25 g of protein and removes 25 g of carbs. Moving 5 points from carbs to fat removes 25 g of carbs and adds about 11 g of fat.
The same split gives different grams at a different total. At 1,800 kcal, the default 20/50/30 works out to 90 g protein (1,800 × 0.20 ÷ 4), 225 g carbs (1,800 × 0.50 ÷ 4) and 60 g fat (1,800 × 0.30 ÷ 9).
Rounding and totals
Grams are rounded to whole numbers for display. Converted back, the rounded grams may not add up to your calorie figure exactly. In the example, 150 × 4 + 200 × 4 + 67 × 9 = 2,003 kcal, because 66.7 g of fat was rounded up.
Questions
- Why is my number marked “under 1,200 kcal a day”?
- Harvard Health Publishing says daily intake should not fall below 1,200 calories for women or 1,500 for men, except under the supervision of a health professional. The macro calculator doesn't ask for sex, so it marks any total under 1,200 kcal. The grams are still calculated; the mark is only a reminder.
- What does 20/50/30 work out to at 2,000 kcal?
- 100 g protein (2,000 × 0.20 ÷ 4), 250 g carbs (2,000 × 0.50 ÷ 4) and 67 g fat (2,000 × 0.30 ÷ 9 = 66.7).
- Why does one gram of fat count more than twice as much as protein?
- Fat releases more energy per gram. The general Atwater factor is 9 kcal per gram for fat and 4 for protein and carbohydrate.
- Can I enter grams instead of percentages?
- Yes. Choose Grams under Start from and type the grams of protein, carbs and fat; the calculator shows the calories and the percentage split. For grams per kilogram of body weight, the protein calculator does that for protein.
- What happens if the percentages add up to 110%?
- The three shares divide one calorie total, so they need to add up to 100%. At 110% the calculator shows "Percentages add up to 110%, not 100%" until one of them comes down.
- Where does the Daily calories number come from?
- You type it. A calorie calculator result is one way to get an estimate, but any number works.
- Do the AMDR ranges apply to athletes?
- The AMDR table gives ranges by age group only, with no separate ranges for athletes. The calculator shows the same adult ranges for everyone.