Which calculator for which situation
- Proportion. Rule of three handles two quantities; compound rule of three handles three or more. Ratio and proportion and scale cover plans, models and maps.
- Percentage. Percentage does the five essential calculations; reverse percentage finds the value before a discount or increase — the one where you divide and never subtract.
- Geometry. Area of shapes, circle, triangle and volume of solids for measurement; Pythagorean theorem and distance between points for diagonals and coordinates; slope for ramps and roofs.
- Algebra and analysis. Linear system, logarithm, sequences, complex numbers and derivative, plus a full scientific calculator.
- Numbers. Factorial, prime numbers, scientific notation, fraction to decimal and Roman numerals.
The most common error: units and order of magnitude
Almost every absurd result in applied maths comes from two causes, and both announce themselves if you know where to look.
Mixed units. Measuring in inches and multiplying directly gives square inches, not square feet — a factor of 144. The safe rule is to convert everything before setting up the calculation, never during it.
Wrong unit in the answer. Area comes out squared, volume cubed, length on its own. If your area answer is not in squared units, you calculated something else — most likely the perimeter.
There is a subtler third case: scale is not linear in area. In a 1:50 model, length divides by 50, area by 2,500 and volume by 125,000.
Why every page shows the formula
Each calculator here states the formula and works an example with real numbers. The reason is practical: an answer you cannot reproduce on paper is an answer you cannot check — and checking is exactly what separates using a tool from depending on it.
Frequently asked questions
Is this useful for homework and exams? For studying and checking, yes: every page shows the steps, which is what an exam actually grades. Where calculators are not allowed, the value is in rehearsing the method, not in having the answer.
Which decimal separator do the fields accept? Both. Inputs are interpreted according to the page language, and the English pages use a decimal point with commas for thousands.
How do I round without accumulating error? Round only at the final result. Rounding at each step propagates the deviation: one third rounded to 0.33 and multiplied by 300 gives 99, not 100.