Calculators that show their working
Percentages, tips, BMI, loans, salary, calories, grades and age. Every tool comes with the formula, worked examples and the source it was checked against. Nothing you type leaves your browser.
Calculators
Free, no sign-up, metric and imperial where it matters.
The formula is on the page
Each calculator is followed by the equation it uses, two or three worked examples with real numbers, and the mistakes people actually make.
Checked against a source
Health and money calculators cite WHO, CDC, published equations and consumer-finance guidance, and show the reference case each formula was tested on.
Runs on your device
The maths happens in your browser. Weights, salaries and loan amounts are never sent to a server, logged or stored.
Free, fast calculators that run entirely in your browser. Nothing you type is sent to a server, stored, or shared — every figure below is worked out on your own device.
Calculators
- Percentage Calculator — Three calculators in one: find a percentage of a number, work out what percentage one number is of another, and measure percentage increase or decrease between two values.
- Tip Calculator — Enter the bill, pick a tip percentage, and split the total across your party.
- BMI Calculator — Calculate body mass index from height and weight in either metric or imperial units, and see which category the result falls into.
- Loan & Mortgage Calculator — Work out the monthly payment on any amortising loan, along with the total interest you will pay over its life.
- Salary Calculator — Convert pay between hourly, daily, weekly, monthly and annual figures, with control over hours per week and paid weeks per year.
- TDEE Calculator — Estimate the calories you burn in a day, from your basal metabolic rate up through your activity level.
- Grade Calculator — Add each assignment with its score and weight to see your current grade, then find out what you need on the final.
- Age Calculator — Enter a date of birth to get an exact age broken down into years, months and days, plus the countdown to the next birthday.
How these differ from most calculator sites
Every tool here comes with the formula written out, worked examples using real numbers, and an honest account of where the method breaks down. A BMI figure without the caveats about muscle mass, or a loan payment without the note that it excludes tax and insurance, is a number that can mislead you. We would rather explain than just display.
All calculations happen in your browser using plain JavaScript. There is no account to create, no data collected about what you calculate, and nothing transmitted anywhere. Closing the tab discards everything.