How we calculate
Every calculator on Calculators Fit shows its work. We call this an open calculation memory: for each tool you can see the exact formula, the reference ranges that put your result in context, and the source we relied on. Nothing is a black box — you can check the math, understand what it means, and learn something in the process.
1. Recognized formulas, not black boxes
We do not invent equations. Each calculator is built on a formula or guideline that is recognized by health authorities and the scientific literature. Depending on the tool, that includes the World Health Organization (WHO) classification for Body Mass Index (BMI); the CDC and WHO growth charts for children and teens; the Mifflin-St Jeor equation for basal metabolic rate (BMR); the U.S. Navy method and validated skinfold equations for body-fat estimation; standard equations for VO₂max; and established models for heart-rate training zones. When a tool uses a simplification (for example, interpolating between reference points), we say so on the page.
2. The calculation runs in your browser
All the math happens on your own device, in your browser. The numbers you type in are never sent to a server and are never stored by us — they simply feed the formula and produce a result on your screen. Privacy is the default, not an option you have to switch on. You can read the full details in our Privacy Policy.
3. Where the reference parameters come from
The reference values, classifications and cut-off points we use come from recognized health and scientific bodies. The table below summarizes the main sources behind our tools.
| Topic | Reference source |
|---|---|
| BMI, obesity and health ranges | World Health Organization (WHO) |
| Child growth charts & pediatric BMI | CDC Growth Charts |
| Energy expenditure, training & exercise prescription | American College of Sports Medicine (ACSM) |
| Blood pressure, cholesterol, blood glucose | National health authorities and medical societies |
| Pregnancy (gestational age, due date) | Obstetrics & gynecology medical societies |
4. How we keep it up to date
Guidelines change, and so do we. When a health authority updates a classification, a reference table or a recommended equation, we review the affected calculators and adjust them. Where a value depends on a reference year (for example, a growth chart edition), we mark that year on the page so you always know which version a result is based on.
5. An important note
Who maintains this
Calculators Fit is maintained by Vinicius Fonseca, who builds each calculator from recognized formulas and checks the numbers before publishing. He is not a physician — the tools are for estimation and education, not medical advice. Found an error or have a suggestion? Write to contact@calculatorsfit.com.