Skip to main content
FitCalcs

Methodology & Editorial Standards

By Jesse · Last reviewed: August 6, 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

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.

Daily calorie needs (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.

Basal metabolic rate as a standalone figure

The BMR calculator reports Mifflin-St Jeor and the revised Harris-Benedict (Roza-Shizgal) equations side by side, along with their average, because the two disagree by a meaningful margin for some body types and showing only one would hide that spread. Mifflin-St Jeor is the more accurate of the two for most non-obese adults according to a 2005 Academy of Nutrition and Dietetics review, so we label it as the primary figure rather than presenting the average as authoritative. We measured that spread ourselves by running both equations across the full range of adult body sizes: the median gap is near 4%, wider for men than for women, and smaller than the effect of moving one step on the activity ladder. The full analysis shows every figure.

Goal planning over a timeline

The body weight planner uses the standard energy balance model: it converts your goal weight change into a daily calorie adjustment using the approximation that a kilogram of body fat stores about 7,700 kcal, then applies that against your estimated TDEE. This linear approximation is deliberately simple. Real weight change is not linear — expenditure falls as you get lighter, so long timelines drift optimistic. We cap planning at a 1,000 kcal daily deficit and a 500 kcal surplus; these are general guardrails to keep plans sensible, not individualized medical safety thresholds.

Exercise calorie burn

The calorie burn calculator multiplies an activity's MET value (Metabolic Equivalent of Task) by your body weight and duration, using values from the 2011 Compendium of Physical Activities. Two caveats we state on the page: the result is gross burn, meaning it includes the resting calories you would have spent anyway, so it overstates the calories genuinely added by exercising; and MET values are population averages that can be off by roughly 15–20% for an individual, since fitness and movement efficiency vary. Treat the number as a planning estimate, not a measurement.

Returning to maintenance after a diet

The reverse diet calculator is a scheduling tool, and this is the clearest case on the site where the popular claim outruns the evidence. It maps a 50, 100, or 150 kcal weekly increase from your current intake to estimated maintenance. Those increments are convenient planning choices, not clinically validated amounts, and a 2025 preliminary randomized trial found that a gradual ramp did not significantly reduce weight regain compared with returning directly to maintenance. We say so on the page rather than implying the method "repairs" metabolism.

Daily water intake

The water intake calculator uses 32.5 ml per kilogram of body weight, the midpoint of the 30–35 ml/kg range widely cited in clinical nutrition, plus a fixed adjustment for activity level. Hydration needs are genuinely variable — climate, sweat rate, and health conditions all shift them, and authorities including EFSA and the U.S. National Academies publish reference ranges rather than a single formula. The target covers total fluid from all sources, including food, which supplies roughly 20% for most people, so it is not a target for drinking water alone.

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.

For how content is written, fact-checked, and corrected — separate from which formulas we use — see our editorial policy.

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.