---
title: "Calculator Methodology - Modern Stamina"
description: "Learn about the formulas, sources, and limitations used for the calculators at Modern Stamina."
url: "https://www.modernstamina.com/calculator-methodology"
language: "en"
---

# Calculator Methodology

The formulas, sources, and limitations behind the estimators used on Modern Stamina.

## Body Mass Index \(BMI\)

**Formula used:** Weight in kilograms divided by height in meters squared \(kg/m²\).

**Original source:** This is a standard global measurement established mathematically in the 19th century and actively used for population health statistics by the [National Institute of Diabetes and Digestive and Kidney Diseases \(NIDDK\)](https://www.niddk.nih.gov/health-information/health-statistics/overweight-obesity) and the World Health Organization.

### Assumptions and Limitations

BMI is a broad population-level screening tool, not a diagnostic measure of individual health. The formula assumes a standard relationship between height and weight but cannot distinguish between muscle mass and fat mass. It does not account for body-fat distribution, bone density, fitness level, age, or metabolic health. Consequently, it may incorrectly categorise muscular athletes as overweight or underestimate body fat in older adults.

## Total Daily Energy Expenditure \(TDEE\) & Calories

**Formula used:** The Mifflin-St Jeor equation to estimate Basal Metabolic Rate \(BMR\), multiplied by a standard activity factor.

**Original source:** First published by M.D. Mifflin and S.T. St Jeor in 1990. It is widely recognised as a highly reliable predictive equation for resting metabolic rate \([Frankenfield et al., 2005](https://pubmed.ncbi.nlm.nih.gov/15883556/)\).

### Assumptions and Limitations

This formula provides an estimated baseline of the calories your body needs to maintain its current weight. The calculation relies on self-reported activity levels, which are subjective and can be easily overestimated. It does not account for individual metabolic adaptations, specific muscle mass variations, the thermic effect of different foods, or day-to-day fluctuations in non-exercise activity thermogenesis \(NEAT\). The ±500 calorie adjustments for weight loss or gain are generalized starting points, not exact biological guarantees.

## Daily Protein Intake

**Formula used:** Standard weight-based multipliers ranging from 0.8g to 2.2g of protein per kilogram of total body weight, adjusted for activity level and goals.

**Original source:** The baseline 0.8 g/kg represents the general Recommended Dietary Allowance \(RDA\) for sedentary adults. The higher ranges \(1.2 - 2.2 g/kg\) are drawn from position stands on protein and exercise by organizations such as the [International Society of Sports Nutrition \(ISSN\)](https://pmc.ncbi.nlm.nih.gov/articles/PMC2117006/) and the American College of Sports Medicine \(ACSM\).

### Assumptions and Limitations

The calculator uses your total body weight to estimate protein needs. It assumes an average body composition. For individuals with very high or very low body fat percentages, calculating protein based on lean body mass rather than total weight is often more accurate. The formula also assumes normal kidney function and does not account for specific medical conditions or age-related changes in protein metabolism that might alter individual requirements.

## Daily Hydration Baseline

**Formula used:** A baseline estimate of 35ml of water per kilogram of body weight, plus fixed volume additions \(350ml - 700ml\) for increased physical activity and warmer climates.

**Original source:** The baseline calculation of 30–35 ml/kg is a standard fluid estimation rule used in dietetic practice. For context, the [Institute of Medicine \(IOM\)](https://pmc.ncbi.nlm.nih.gov/articles/PMC4207053/) recommends total water intakes of roughly 2.7 to 3.7 liters per day for adult women and men, which encompasses both food and beverage fluid sources.

### Assumptions and Limitations

This calculation yields a general rule of thumb for direct fluid intake. It does not factor in the significant amount of water consumed through water-rich foods \(like fruits and vegetables\) or other beverages \(like tea and coffee\). It also cannot accurately predict acute fluid losses during extreme endurance events, heavy sweating, or high-altitude exposure. Individuals with specific medical conditions affecting fluid balance should rely on professional medical advice rather than this formula.

Last reviewed: August 2026

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