By Bailey Richert
Updated Mar 24, 2022
A goniometer is a precision instrument for measuring angles. Unlike a standard protractor, it features two rotating arms that allow the user to capture the full geometry of a joint or structure. Professionals in architecture, geology, and physical therapy rely on goniometers to assess angles with confidence. This guide shows how to construct a functional goniometer at home using a circular protractor and simple yardsticks.
Place a ruler so that its mid‑line aligns with the central bar of a circular protractor. The ruler should sit flush against the protractor’s inner edge.
Position a second ruler on the opposite side of the protractor, mirroring the first. Both rulers must be parallel and separated only by the protractor’s thickness.
Using a drill, create a centered hole that passes through the two rulers and the protractor. The hole should align with the protractor’s center point.
Thread a bolt through the hole and secure it with a nut on the far side. Tighten just enough to hold the components together; leave enough play so the arms can pivot freely.
To measure an angle, place the bolt over the vertex of the shape. Slide the two arms until they touch the sides of the angle, then read the measurement directly from the protractor scale.
Use yardsticks for measuring large angles.
