Everyday Maths, Fuel and Electricity Calculators

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Percentages, forwards and backwards

Most percentage mistakes come from direction. "What is 20% of 80" and "80 is 20% of what" are different questions with different answers, and "what percentage is 16 of 80" is a third. The percentage calculator lays all three out so you pick the one you mean, and the discount calculator handles the common retail case — a price, a percentage off, and the amount you actually pay — including stacked reductions where the second discount applies to the already-reduced figure rather than the original.

Margin and markup are not the same number

A 25% markup on a £100 cost gives a £125 price. The margin on that sale is £25 out of £125, which is 20%, not 25%. The two calculators here are deliberately separate so the convention is never ambiguous: the margin calculator works from the selling price downwards, the markup calculator from the cost upwards, and each shows the other figure alongside so you can see the relationship. If you price by markup but report by margin, the gap between the two is where profit quietly disappears.

Units and lengths

The length converter covers metric and imperial in both directions and accepts imperial input the way people say it — 5'10" is understood as five feet ten inches without converting to decimal feet first. Results are shown to a sensible precision for the unit rather than a fixed number of decimal places, so millimetres do not come back with six decimals and miles do not get rounded to nothing.

Fuel and electricity

The fuel cost calculator takes a distance, your car's consumption in L/100km, km/L or miles per gallon, and the pump price, and returns the litres and dollars for the trip with a per-person split. The fuel economy converter moves between those units in every direction, keeping US and imperial gallons apart. The electricity cost calculator turns kilowatt-hours and a tariff into a bill, with the daily supply charge that Australian bills carry, and the appliance running cost calculator prices a single fridge, heater or dryer per hour, day and year from its wattage. Tariff examples are Australian; the arithmetic is the same anywhere.

Clothing, shoe and ring sizes

The clothing size converter reads a women's dress size or a men's suit or collar size across US, UK, Australian, European, Italian and Japanese labels, the shoe size converter does the same for men's, women's and children's shoes in US, UK, EU and foot length in centimetres, and the ring size converter moves between US numbers, UK letters, EU circumference, Japanese sizes and the inside diameter in millimetres. Each page shows the full chart and says plainly that brands, lasts and band widths vary, so the result is where to start, not a guarantee of fit.

When the form is not enough

The scientific calculator is there for everything the single-purpose tools do not cover. It evaluates a typed expression rather than emulating a row of buttons, so you can paste a formula, edit one number and re-run it, and it handles trigonometry, logarithms, powers and roots with the operator precedence you would expect from a maths textbook.

Everything runs on your device

None of these calculators sends anything to a server. Results appear as you type, the pages keep working if your connection drops, and nothing you enter is stored.