About Calculators
Most calculations people look up online are not hard — they are just easy to get subtly wrong. Removing a percentage is a division rather than a subtraction. Two stacked discounts multiply instead of adding. A tip on a bill split four ways needs rounding decided before anyone reaches for their card. The arithmetic is trivial; remembering which way round it goes, under time pressure, in a shop or at a till, is not.
The calculators here each answer one of those questions and show the working underneath, so you can check the result rather than trust it blindly. Every one runs in your browser as you type — there is no submit button, no waiting and no account. Nothing you enter is sent anywhere.
If you are comparing prices, the discount and VAT calculators are the ones to start with. For anything proportional — scaling a recipe, converting a map distance, pricing five of something when you know the cost of three — the rule of three calculator handles it in one step.