What does this carb calculator actually work out?

The figure it prints is a daily calorie estimate, not grams of carbohydrate. The script works out your resting energy expenditure with the Mifflin–St Jeor equation, multiplies it by the activity factor you pick, rounds it, and shows that number. The panel that opens under the result then describes the steps that would turn calories into carb grams — a goal adjustment, a macronutrient split, and 4 calories per gram — but the script does not carry those steps out for you.

In Metric mode it reads height in centimeters and weight in kilograms, which is what the equation was built for: for men, 10 × weight + 6.25 × height − 5 × age + 5; for women, the same with − 161 at the end. Then the multiplier:

How many carbs a day do the reference figures allow?

Two numbers come from a named source. The National Academies (Institute of Medicine) set the RDA for carbohydrate at 130 grams a day for adults — a floor based on the minimum needed to supply glucose to the brain, not a target. The same report puts the acceptable range at 45 to 65 percent of calories from carbohydrate, alongside 10 to 35 percent from protein and 20 to 35 percent from fat.

The UK framing is by volume rather than percentage: the NHS Eatwell Guide says starchy foods “should make up just over a third of the food we eat,” and steers toward higher-fiber and wholegrain versions. Neither source tells you a single correct number — both give a band.

What this number does not tell you

• It is a prediction, not a measurement. Mifflin–St Jeor came from 498 healthy adults aged 19 to 78, measured by indirect calorimetry, and explained 71% of the variation between them. Nearly a third of the difference between two people the same size stays unexplained.
• The activity multiplier is you guessing. Picking Extra active instead of Sedentary returns 58% more calories for the same body, and the choice comes from a five-item menu with no definitions attached.
• Select Metric before you type. The page loads with Standard already selected.
• It does not know what kind of carbohydrate. Grams count oats and soda alike — what changes is fiber, and the rest of the meal. That is where the useful reading is: the fiber and cholesterol evidence in our oatmeal every day breakdown, or the pulse-and-grain build of a chickpea and quinoa bowl.
• It is built for a healthy adult. Pregnancy, breastfeeding, growth, illness, and kidney or heart conditions all move the figure, and none of them are inputs here.

This calculator is an estimate, not a diagnosis. It is not medical advice — talk to a doctor or a registered dietitian before making changes based on a number you get here.

Sources

• Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO. “A new predictive equation for resting energy expenditure in healthy individuals.” Am J Clin Nutr. 1990;51(2):241–7 — PMID 2305711
• Institute of Medicine, Food and Nutrition Board. Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids (2005) — carbohydrate RDA 130 g per day; 45–65% of calories from carbohydrate — nationalacademies.org
• NHS, The Eatwell Guide — starchy foods and fiber — nhs.uk