Methodology
How We Calculate Nutrition
Every number on this site comes from somewhere, and you should be able to check it. This page lists the data source and formula behind each figure, and — just as importantly — where each one stops being reliable.
Where the food data comes from
Per-ingredient calories and macronutrients come from USDA FoodData Central, the food composition database maintained by the US Department of Agriculture. A recipe's totals are the sum of its ingredients, divided by the number of servings stated on the page.
That process is arithmetic, so it is only as good as its inputs. Three things move the real number away from the printed one:
- Portion size — the largest single source of error. Self-reported servings are commonly off by 20–30%.
- Cooking — fat absorbed during frying, fat drained from meat, and water lost or gained all change the finished dish.
- Brand variation — two supermarket versions of the same packaged ingredient can differ by 15% or more.
A realistic accuracy for recipe nutrition here is ±10–15% for a typical home-cooked portion. That is enough to make decisions with, and not enough to audit. If you need clinical precision — for managing diabetes or kidney disease, say — weigh your own ingredients and work from the packaging.
What each calculator actually does
Formula, source, and the point at which it stops being trustworthy.
- Method
- Mifflin-St Jeor BMR — (10 × kg) + (6.25 × cm) − (5 × age) + 5 for men, − 161 for women — multiplied by an activity factor of 1.2 to 1.9.
- Where it breaks down
- Validated on healthy adults. Less reliable at the extremes of body composition — very lean, very heavy, or highly muscular people should expect a wider error margin.
- Method
- Calories from Mifflin-St Jeor as above, then protein set per kilogram of bodyweight by goal, fat at 25% of total calories, and carbohydrate as the remainder.
- Where it breaks down
- Fat at 25% is a practical default, not a physiological requirement. Anything above roughly 0.5 g/kg of fat is compatible with health; the split between fat and carbohydrate is mostly preference and training style.
- Method
- 1.6 g per kg of bodyweight for maintenance, 2.0 g/kg for fat loss, 2.2 g/kg for muscle gain.
- Where it breaks down
- Based on bodyweight, not lean mass. At high body-fat percentages this overestimates need — using target or lean bodyweight is more appropriate there.
- Method
- 35 ml per kg of bodyweight when sedentary, rising to 38, 40 and 45 ml/kg as training volume increases.
- Where it breaks down
- Roughly 20–30% of total water intake comes from food, which this figure does not subtract. Thirst and pale-yellow urine remain better day-to-day guides than any formula.
- Method
- Weight in kilograms divided by height in metres squared.
- Where it breaks down
- BMI cannot distinguish muscle from fat and was designed to describe populations, not individuals. A muscular person can register as overweight while carrying little fat. Waist circumference and body-fat percentage track metabolic risk better.
- Method
- US Navy circumference method, using neck, waist and (for women) hip measurements together with height.
- Source
- Hodgdon JA, Beckett MB. Prediction of percent body fat for U.S. Navy men and women from body circumferences and height. Naval Health Research Center, Reports 84-11 and 84-29, 1984.
- Where it breaks down
- Accurate to roughly ±3–4 percentage points against DEXA, and highly sensitive to measuring-tape technique. Measure at the same points, at the same time of day, and treat the trend as more meaningful than any single reading.
What these numbers are not
They are not medical advice, and they are not measurements of you. Every equation on this site is a regression fitted to a study population — it cannot account for your thyroid, your medication, your sleep, or how much you fidget. Two people given the same target can respond quite differently.
The useful way to treat a calculated number is as a hypothesis. Pick it, eat to it consistently for two to four weeks, and let the scale and the mirror tell you whether it was right. Adjust from evidence about you, not from a more precise formula.
If you are pregnant, managing a health condition, taking medication, or have a history of disordered eating, speak to a doctor or a registered dietitian before acting on anything here.
Found an error?
Corrections are welcome and get made. If a number here looks wrong, or a formula has been superseded, get in touch with the page and the specific figure. Nutrition science moves, and pages that never change are usually pages nobody is checking.