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These are the everyday workhorses. The calculators you reach for without thinking.

Geometry answers questions about shape and size; trigonometry answers questions about angles and the lengths tied to them. Together they cover almost everything measured on a building site, a workshop bench, a survey traverse or a drawing board.

Dates look simple and behave badly. Months change length, leap years appear on a three-tier rule, clocks jump twice a year, week numbering has competing standards, and payroll wants minutes expressed as decimal fractions of an hour.

A thermometer only tells part of the story. Humidity decides whether sweat can cool you, wind decides how fast heat leaves exposed skin, and dew point decides whether the air feels sticky and whether condensation will form on a cold surface.

Finance calculators turn interest rates, terms and contribution amounts into figures you can act on: a monthly payment, a maturity balance, a tax bill.

These calculators translate body measurements and training numbers into targets you can train and eat to: a daily calorie figure, a protein target, a heart rate zone, a realistic one-rep max. They are used by people starting a weight change, lifters planning a program, and runners pacing a race.

Math calculators here cover the operations people actually reach for outside a classroom: percentage changes, averages and spread, probability and combinatorics, equation solving and unit conversion.

Engineering calculators here handle the routine sizing and checking work that sits behind a design: current and voltage relationships, cable losses over distance, fastener torque, and the load a simple beam carries.

Education calculators answer the questions students ask around results day and exam week: what my average currently is, what I need on the last paper to reach a target, and how a raw score converts to a scaled one. Teachers and parents use them to sanity-check a report card before it is filed.

Lifestyle calculators cover the small everyday sums that are easy to get wrong under pressure: an exact age in years and days, a fair tip split between friends, a sale price after two discounts, or a due date counted forward from a known start.

Business calculators here focus on what is left after costs: the fee a marketplace or payment processor takes, the margin on a product once shipping and returns are counted, and the hourly rate a freelancer needs to hit a target income.

Tech calculators cover the sizing and budgeting questions that come up when running software: how much bandwidth a traffic level consumes, what an uptime promise allows in downtime minutes, what a cloud bill will look like, and whether a colour pair meets accessibility contrast rules.

Construction calculators convert the dimensions of a job into a shopping list: cubic metres of concrete, tonnes of asphalt, boxes of tile, litres of paint.

Automotive calculators cover the two sides of car ownership. What it costs and what it does. Fuel economy, finance payments and lease-versus-buy comparisons on one side; tyre sizing, horsepower estimates and quarter-mile times on the other.

Unit conversion looks like the most trivial arithmetic on the internet, and it is responsible for a remarkable share of expensive mistakes. Lost spacecraft, mis-dosed patients, under-ordered concrete and ruined cakes.

These are the core relationships of school and undergraduate science, implemented so that the units are unambiguous and the constants are the current internationally fixed values rather than textbook approximations.

Statistics is the discipline of saying how much a limited set of observations can support. These calculators cover the two halves of that job: descriptive tools that summarise the data in front of you, and inferential tools that quantify how far you can generalise beyond it.