Our macro calculator returns three calorie figures, three macro gram targets, and one water line. Most articles explain what macros are. This one documents the arithmetic our component runs, including one place where our own page copy and our code disagree, and one label in the result chart that reads wrong.
Step one, BMR from Harris-Benedict
The component computes basal metabolic rate with the revised Harris-Benedict equation:
male: 88.362 + 13.397 x weight + 4.799 x height - 5.677 x age
female: 447.593 + 9.247 x weight + 3.098 x height - 4.330 x age
Weight enters in kilograms, height in centimeters, age in years. A 75 kg, 175 cm, 28 year old male scores 1774 kcal. A 60 kg, 165 cm, 30 year old female scores 1384 kcal.
Here is the mismatch worth knowing. The tool page copy in our SEO file claims the Mifflin-St Jeor formula. The component ships Harris-Benedict. When you compare our output against another site, the formula behind the number decides whether the comparison means anything. The code is the truth of this article, and we flagged the copy for correction.
Step two, the five activity multipliers
TDEE is BMR times one of five constants:
1.2 sedentary, desk job, no exercise
1.375 lightly active, exercise 1 to 3 days per week
1.55 moderately active, exercise 3 to 5 days per week
1.725 very active, hard exercise 6 to 7 days per week
1.9 extra active, athlete level
The tool defaults to 1.55, the middle band. Our male example lands at 1774 x 1.55, which is 2749.7 and displays as 2750 kcal. The female example at 1.2 gives 1660 kcal.
Most people overestimate here. The bands count training days, so a desk job with three gym sessions is lightly active at 1.375, not moderately active. Pick the band that matches an ordinary week, and pick the lower one when two bands both look plausible.
Step three, the goal offset
Three buttons adjust TDEE. Maintain keeps it. Lose subtracts a flat 500 kcal per day. Gain adds a flat 300. Our male example reads 2750, 2250, and 3050 across the three settings.
The offsets are fixed constants, not percentages, so they hit a small eater harder than a large one. The same minus 500 produces a 2250 target for our example and a far lower target for a small, older person, and both deserve a sanity check against hunger and training quality.
The split ratio, fixed for every goal
This is the part the article title names. Whatever the goal, the component applies one ratio:
protein grams = calories x 0.30 / 4
carb grams = calories x 0.45 / 4
fat grams = calories x 0.25 / 9
The divisors are the energy densities, 4 kcal per gram for protein and carbs, 9 for fat. At the 2750 kcal maintain target the tool reports 206 g protein, 309 g carbs, 76 g fat. At the 2250 lose target it reports 169, 253, and 63. At the 3050 gain target it reports 229, 343, and 85.
The ratio never changes with the goal. Cutting calories under a fixed 30/45/25 split trims protein along with everything else, which is the opposite of what most cutting diets prescribe. If you want protein held while dieting, compute your own protein floor, subtract its calories, and split the remainder between carbs and fat yourself.
The water line
Water uses its own formula:
liters = weight x 0.033 + (activity multiplier - 1.2) x 0.5
At 75 kg and multiplier 1.55 the tool shows 2.7 L, which it converts to cups at 4 cups per liter, giving 11 cups. The activity term has a quirk worth knowing. It scales with the raw gap between your multiplier and 1.2, so it adds about 0.09 L for lightly active and tops out at 0.35 L for the athlete band. Two adjacent bands can differ by less than a tenth of a liter, and the one decimal display rounds that difference away.
The label we know reads wrong
The donut chart sums the three gram figures and prints the total in its center with the word kcal underneath. For the 2750 kcal example, the center shows 591, because 206 plus 309 plus 76 is 591 grams. The number is a gram total wearing a kcal label. Read your calories from the three stat tiles above the chart, and use the donut only for the proportion of each macro.
Checklist before you trust the output
- Inputs are metric, kilograms and centimeters, because the tool has no imperial branch.
- The activity band matches an ordinary week, and you took the lower band when unsure.
- You remember the goal offsets are flat, minus 500 and plus 300.
- You set your own protein floor when cutting, since the split will not hold it for you.
- You ignored the donut center number and read the stat tiles.
The formula is a population estimate for typical bodies. It knows nothing about body composition, medical conditions, or pregnancy. Track your weight trend for two to three weeks against the target, then adjust by 100 to 200 kcal, instead of re-reading the calculator after every meal.
Run your own numbers through the
[macro and calorie calculator](https://webrecast.com/en/macro-calculator) and check the
split by hand with the 4/4/9 rule. If your measured maintenance sits far from the TDEE it
reports, that gap is exactly the data we want.
