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The formulas, data sources and limitations behind every tool on FytFree
FytFree provides free nutrition and fitness calculators designed to help you make more informed decisions about your diet and exercise. This page explains exactly how each tool works, where our data comes from, why results are estimates rather than exact prescriptions, and when you should seek professional guidance instead of relying on an online calculator.
We believe in transparency. You should understand the methodology behind any number that influences what you eat or how you train. Every formula referenced below is publicly available and widely used in nutrition science and clinical practice.
Our Calorie Calculator uses the Mifflin-St Jeor equation, published in 1990 by Mifflin, St Jeor and colleagues in the American Journal of Clinical Nutrition. It estimates Basal Metabolic Rate (BMR) — the energy your body uses at complete rest — based on four variables: weight, height, age and biological sex.
| Sex | Formula |
|---|---|
| Male | BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) + 5 |
| Female | BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) − 161 |
Your BMR is then multiplied by an activity factor to estimate Total Daily Energy Expenditure (TDEE). Activity multipliers range from 1.2 (sedentary — desk job, minimal exercise) to 1.9 (very active — physical job plus daily intense training). We chose Mifflin-St Jeor because comparative studies, including a 2005 review by Frankenfield et al. in the Journal of the American Dietetic Association, found it to be the most accurate predictive equation for estimating BMR across diverse populations.
The Macro Calculator divides your TDEE into protein, carbohydrate and fat targets using evidence-based percentage ranges. Protein is calculated first (typically 1.2–2.2 g per kg of body weight, depending on goal and activity level), then the remaining calories are split between carbohydrates and fats according to the selected goal profile. These ranges align with position statements from the International Society of Sports Nutrition (ISSN) and the Academy of Nutrition and Dietetics.
The Protein Calculator uses gram-per-kilogram ranges based on published research. For sedentary adults, the baseline is 0.8 g/kg (the WHO/FAO Recommended Dietary Allowance). For active individuals, we use 1.2–2.2 g/kg depending on training type and goal, consistent with the ISSN position stand on protein and exercise (Jäger et al., 2017).
The Water Intake Calculator uses a baseline of approximately 33 ml per kilogram of body weight, adjusted for activity level, climate and exercise duration. This aligns with the European Food Safety Authority (EFSA) adequate intake recommendations of 2.0 L/day for women and 2.5 L/day for men, scaled to individual body weight for more personalised estimates.
The Calorie Deficit Calculator uses the widely referenced energy balance principle: approximately 7,700 kcal of deficit corresponds to roughly 1 kg of body fat loss. This figure comes from the energy density of adipose tissue and is used as a practical planning estimate. In reality, metabolic adaptation means weight loss is not perfectly linear, but the 7,700 kcal/kg figure is a useful approximation for setting realistic timelines.
The Steps to Calories Calculator estimates calories burned using MET (Metabolic Equivalent of Task) values from the Compendium of Physical Activities, maintained by Arizona State University. Walking at different speeds corresponds to MET values between 2.0 and 5.0, which are multiplied by body weight and duration to estimate energy expenditure.
The Glycemic Index Food Finder uses values sourced from the International Tables of Glycemic Index and Glycemic Load Values, compiled by researchers at the University of Sydney and published in peer-reviewed journals (most recently updated by Atkinson et al., 2021). GI values represent the average blood glucose response in healthy subjects consuming 50 g of available carbohydrate from a given food, compared to 50 g of glucose.
Every calculator on this site produces an estimate, not a diagnosis. Formulas like Mifflin-St Jeor are derived from population-level data — they represent the average metabolic rate for a person of a given weight, height, age and sex. Your individual metabolism may differ by 10–20% from any formula's prediction due to factors that no online calculator can measure.
This does not mean the calculator is broken. It means real bodies vary. Two people with the same age, height and weight can have noticeably different calorie needs because of the factors below.
Because of these variables, we consistently recommend treating calculator results as a starting point, not a final answer. Use the number for 2–3 weeks, monitor your real-world results (weight trends, energy levels, performance, sleep, hunger), then adjust by 100–200 kcal as needed. The goal is not to find the "true" number on day one. The goal is to find a target that produces consistent progress in your actual life.
If your results differ significantly from what the calculator predicts after several weeks of honest tracking, see Why Weight Loss Stalls Even in a Calorie Deficit for a structured way to troubleshoot.
We use publicly available, peer-reviewed sources for all data and formulas on this site:
Online calculators are useful tools when used correctly. Here is how to get the most reliable results:
Being transparent about limitations is as important as explaining capabilities. Our calculators cannot:
We encourage you to consult a registered dietitian, physician or qualified healthcare professional in any of the following situations:
Our tools are designed to support healthy adults making general dietary and fitness decisions. They complement — but do not replace — professional healthcare guidance.
All calculator inputs (your weight, height, age, sex and activity level) are processed locally in your browser using JavaScript. No personal health data is sent to our servers or stored in any database. Your inputs are never logged, tracked or shared with third parties. Basic technical data (such as page visits) may be processed by our hosting and advertising partners as described in our Privacy Policy.
The calorie calculator uses the Mifflin-St Jeor equation, published in 1990 in the American Journal of Clinical Nutrition. It estimates BMR from weight (kg), height (cm), age and biological sex, then multiplies by an activity factor to produce your TDEE. We chose this formula because comparative reviews have consistently found it to be among the most accurate for healthy adult populations.
All formulas produce estimates based on population averages. Individual metabolic rates can vary by ±10–20% from any prediction. Factors like body composition, genetics, medications and metabolic adaptation are not captured by any online formula. Treat results as informed starting points and adjust based on 2–3 weeks of real-world observation.
GI values are sourced from the International Tables of Glycemic Index and Glycemic Load Values, compiled by researchers at the University of Sydney and published in peer-reviewed journals. The most recent comprehensive update was published by Atkinson et al. in 2021. Values represent average blood glucose responses measured in healthy subjects under standardised conditions.
No. These tools provide general educational estimates for healthy adults. They cannot account for medical conditions, medications, metabolic disorders, food allergies or individual physiological differences. If you have a health condition that affects your diet, or if you are under medical treatment, professional guidance from a registered dietitian or physician is essential.
No. All calculations are performed locally in your browser using JavaScript. Your weight, height, age, sex and other inputs are never sent to our servers, stored in a database or shared with third parties. You can verify this using your browser's developer tools (Network tab) while using any calculator.
Different calculators use different underlying equations (Harris-Benedict, Mifflin-St Jeor, Katch-McArdle, Cunningham) and different activity multiplier scales. Variations of 100–300 kcal between calculators are normal and expected. This is one of the reasons we recommend treating any calculator result as a starting point rather than an absolute target. The Mifflin-St Jeor equation we use has the strongest validation data for general healthy adult populations.