Geometry Calculators

Calculating the Area, Perimeter, and Interior Angle of a Regular Decagon

A regular decagon's perimeter, area, and interior angle all fall out of just two inputs — the number of sides and the side length — and this article works through the real numbers for one.

Finding a Missing Dimension: Combining the Equation Solver and Area Calculator

Any area formula that multiplies a known dimension by an unknown one — length × width, or base × height ÷ 2 — can be rearranged into a linear equation and solved for the missing dimension, then checked by plugging the answer back into the original area formula.

How a Rectangle's Diagonal Is Calculated

A rectangle's diagonal is the square root of its length squared plus its width squared — the Pythagorean theorem applied to the right triangle formed by two adjacent sides and the diagonal itself.

How the Pythagorean Theorem Finds a Right Triangle's Hypotenuse

The Pythagorean theorem says the hypotenuse of a right triangle is the square root of the sum of its two legs squared: c = √(a² + b²) — it only works for right triangles, where the two legs meet at exactly 90°.

How to Calculate Area for Four Common Shapes

A circle, rectangle, square, and triangle each use a different area formula built from their own dimensions, but all four reduce to the same idea — multiplying two lengths together to get a result measured in square units.