perimeter

The perimeter is the distance around the outside of a shape, found by adding the lengths of all its sides. For a rectangle that means 2(length + width), for a square 4 × side, and for any triangle the sum of its three sides. It is a length, so the answer carries a unit such as feet or meters — not a squared unit.

What distinguishes the formulas is which shapes need a shortcut. Straight-sided shapes are just addition. A circle has no sides to add, so its perimeter is the circumference, 2πr. Irregular shapes — an L, a T, a shape described in a diagram — are handled by walking the outline and adding each edge, which is why a question often gives more side lengths than the formula would need.

The perimeter is usually confused with the area, and the difference is worth stating plainly: perimeter is the fence, area is the paint. They use the same measurements and are wrong for each other, and a question that gives a running track, a picture frame or a piece of fencing is asking for the distance around, not the space inside. Perimeter questions on the ASVAB are usually one or two steps of addition once the shape is set up correctly.