Every calculator on Trendy String is built on an established, publicly documented formula — never a guess. Here's how we approach building and maintaining each tool.
For each calculator, we start by identifying the most widely accepted, evidence-based formula for that measurement — generally the one recommended by major public health bodies (WHO, CDC, NIH) or used most often in clinical and dietetics practice. Where more than one formula is in common use, we favor the one considered more accurate for the general population, and we note this in the calculator's own page.
Every tool on this site gives an estimate based on population-level formulas. They are not diagnostic tools, and they can't account for every individual factor (medication use, underlying conditions, unusual body composition, and so on). We design each calculator's result page to explain not just the number, but what it does and doesn't tell you — see our Journal for deeper explainers on several of our most-used tools.
Public health guidance evolves. When a major body like the WHO or CDC updates a relevant guideline, we review our calculators to make sure they still reflect current best practice.
Each calculator page includes a short explanation of the formula it uses, along with an FAQ addressing common questions. If something's still unclear, feel free to reach out.