Which named equation runs behind each calculator, where the reference numbers come from, and what happens when one of them is wrong.
A calorie calculator is a guess dressed up as a number unless the formula behind it is a real, published one. This page names the exact equations CalorieCalcTools uses, points to the source for every constant that isn't ours to invent, and lays out how often each figure gets rechecked.
The BMR calculator, TDEE calculator and calorie deficit calculator all start from the Mifflin-St Jeor equation, published in 1990 in the American Journal of Clinical Nutrition and still the version most dietitians default to over the older Harris-Benedict formula. For men it reads BMR = 10W + 6.25H - 5A + 5, and for women BMR = 10W + 6.25H - 5A - 161, where W is weight in kilograms, H is height in centimeters and A is age in years. TDEE multiplies that BMR by an activity factor (roughly 1.2 for sedentary up to 1.9 for very active), a convention that predates any single federal standard but is the one nutrition textbooks and the calculators above both use. We do not swap in a different equation to make a number look friendlier.
The body fat calculator uses the circumference-based method developed at the Naval Health Research Center by Hodgdon and Beckett in 1984, built from neck, waist and (for women) hip measurements run through a log-based formula rather than skinfold calipers or a scan. It was designed to estimate body fat percentage for military fitness standards using a tape measure alone, which is exactly why it works well for a browser-based calculator: no lab equipment required, at the cost of some accuracy compared to a DEXA scan.
The BMI calculator divides weight in kilograms by height in meters squared, the same formula the Centers for Disease Control and Prevention and the World Health Organization use to define underweight, healthy, overweight and obese ranges. We show those CDC cutoffs rather than inventing our own. The ideal weight calculator runs several published height-based formulas side by side (Devine, Robinson, Miller and Hamwi) because they disagree by a few pounds and showing one alone would overstate how precise any of them really is.
The protein intake calculator and macro calculator both start from the Dietary Reference Intake set by the National Academies of Sciences in 2005: 0.8 grams of protein per kilogram of body weight, a floor meant to prevent deficiency in a sedentary adult, not to support training. For anyone lifting, running or cutting calories, we cite the International Society of Sports Nutrition's 2017 position stand, which recommends roughly 1.4 to 2.0 grams per kilogram (about 0.7 to 1.0 g per pound), and that range is what the calculator actually targets. Carbohydrate and fat splits fall inside the Acceptable Macronutrient Distribution Ranges published in the 2020-2025 Dietary Guidelines for Americans from USDA and HHS.
The water intake calculator anchors to the National Academies' 2005 Adequate Intake figures, about 3.7 liters of total water a day for men and 2.7 liters for women, including water from food. The popular "half your body weight in ounces" line you'll see repeated across the internet is not a federal guideline; it's a rough mnemonic, and we label it that way on the calculator page rather than presenting it as if the National Academies published it.
A formula can be correct and still miss you personally. BMI cannot see the difference between muscle and fat, so a heavily muscled reader can read as "overweight" by the ratio alone. The Navy body-fat method assumes a fairly typical body shape and drifts for people who carry weight unusually. None of the calculators here diagnose a condition, replace bloodwork, or know your medical history, and every page says so before it says anything else.
Formulas are rechecked whenever the source that defines them is revised, which is not often: the Mifflin-St Jeor equation and the Navy method haven't changed since publication, but we recheck our transcription of each against the original paper roughly once a year. The macro ranges move on a longer, five-year federal cycle tied to the next edition of the Dietary Guidelines for Americans, due in 2030. If a source updates sooner than that, we update the affected calculator page and note the change date at the top of this page.
If you run a calculator's own formula by hand and get a different number than the page shows, that's a bug, not a rounding quirk, and we want to hear about it through the contact form. Genuine formula errors get fixed and dated, not silently patched. Disagreements about which formula is "best" (there's always a competing one) are a judgment call we'll explain rather than treat as an error.
This site is free because it carries display ads through Google AdSense and a small number of affiliate links, Amazon Associates among them. That revenue has no say in which equation a calculator uses or what number it returns; changing a formula to make results look more dramatic would be the fastest way to lose the only thing this site has going for it, which is that the math checks out.
Curious who actually writes and checks this material? The authors page covers who is behind CalorieCalcTools and what gets verified before anything publishes.