Methodology & Editorial Standards
Last reviewed: July 15, 2026
FitCalcs exists to make trustworthy fitness and nutrition math easy to use. Because these topics affect real decisions about health and training, we hold ourselves to a clear set of standards for how we choose formulas, cite sources, and describe their limits. This page explains that process so you can judge the numbers for yourself.
The principles we follow
- Published formulas only. Every calculator implements an equation from peer-reviewed research or a recognized health authority. We do not invent our own formulas or tune numbers to look impressive.
- Show the math. Each calculator displays the exact formula it uses, defines every variable, and links to the original source. Nothing is a black box.
- State the limitations. Estimates have error bars. We tell you the typical margin of error and when a formula stops being reliable, rather than presenting a single number as absolute truth.
- Estimates, not medical advice. Our tools are for general education by healthy adults. They do not replace a doctor or registered dietitian, and we say so on every calculator.
- Correct mistakes openly. If a formula or source is wrong or out of date, we fix it. Reader corrections are welcome and taken seriously.
The formulas we use and why
Below is a summary of the core methods behind our calculators and their known limitations. Each calculator page also carries its own "Formula used" and "References" sections with the full citation.
Calorie and energy needs (BMR / TDEE)
We estimate resting metabolism with the Mifflin-St Jeor equation, which research has found to be one of the most accurate predictive equations for the general population, then apply standard activity multipliers to reach Total Daily Energy Expenditure. Like all prediction equations, it carries a margin of error of roughly ±10% for an individual, because it cannot account for your exact body composition, genetics, or non-exercise movement. Used by the TDEE, calorie deficit, and macro calculators.
Body Mass Index (BMI)
Our BMI calculator uses the standard World Health Organization definition (weight in kilograms divided by height in meters squared) and WHO adult categories. BMI is a population-level screening tool, not a measure of body fat or health on its own — it does not distinguish muscle from fat, so very muscular people can read as "overweight" despite low body fat. We present it as a starting reference, not a diagnosis.
Protein and macronutrients
Protein targets in the protein and macro calculators are based on the International Society of Sports Nutrition position stand, which supports roughly 1.4–2.0 g of protein per kilogram of body weight per day for most exercising adults, with higher intakes useful during calorie restriction. These are evidence-based ranges, not a single prescription.
Body fat percentage
The body fat calculator uses the U.S. Navy circumference method. It estimates body fat within roughly 3–4 percentage points for most people — good enough to track a trend, but not a substitute for a DEXA scan or hydrostatic weighing. Measurement technique affects the result, so we explain how to measure consistently.
Ideal weight
The ideal weight calculator shows the classic Robinson, Miller, Devine, and Hamwi formulas alongside your healthy BMI range. These formulas predate modern body-composition science and give a single reference figure, so we present the healthy weight range as the more practical target rather than any one "ideal" number.
Pace and strength (training tools)
The pace calculator applies the plain time-distance-pace identity and projects finish times at a constant pace, which we clearly label as optimistic for longer races. The one-rep max calculator averages four established rep-max formulas (Epley, Brzycki, Lombardi, and Lander) because each is biased differently across rep ranges; all such estimates lose accuracy above about 10 reps. These are performance-math tools, not health estimates.
How we keep this current
We periodically review the formulas and sources behind each calculator to confirm they still reflect current, recognized guidance, and we note the last review date at the top of this page. When we add a new calculator, it must ship with its formula, variable definitions, and a real published source before it goes live.
FitCalcs provides general estimates for healthy adults and is not medical advice. Always consult a qualified professional for decisions about your health, diet, or exercise — especially if you have a medical condition, are pregnant, or have a history of disordered eating.