About Calculator.hair

Calculator.hair is a collection of free online calculators for everyday decisions: what a mortgage will cost, how many calories you burn, how to split a bill, or how many days remain until a deadline.

How we build calculators

Every calculator runs entirely in your browser. Nothing you type is sent to a server or stored, and you never need an account. Each page explains the formula it uses, walks through a worked example and links to the official sources behind its assumptions, so you can check the math yourself.

Sources and testing

Formulas come from standard references: lender amortization math, IRS and Social Security publications for tax and retirement figures, and public health guidance such as CDC and WHO for body measurements. Nearly every finance and health page lists the sources it relies on at the bottom of the page.

Every calculator has automated tests that check its default answer and its edge cases against worked examples, so a change that breaks a result is caught before it is published. Rates that change often, like currency rates, are refreshed when the site is rebuilt.

Accuracy

We test every calculator against known reference values. Results are still estimates: tax rules, interest rates and health guidelines vary by person and change over time, so confirm important decisions with a qualified professional.

Keeping pages current

Each calculator page shows the date its content last changed. When tax brackets, contribution limits or health guidelines are updated, we revise the affected calculators and their explanations, and the date moves with them.

Contact

Found a bug or want a new calculator? Email hello@calculator.hair.