The Biltmore stick is a tool used by foresters to estimate tree trunk diameter at breast height. The tool very often includes a hypsometer scale to estimate height as well. It looks much like an everyday yardstick. With practice a Biltmore stick is considered to be exceptionally accurate, often within half an inch on diameters. Some foresters use the tool regularly, however, many prefer to use more accurate tools such as a diameter tape to measure diameter at breast height (DBH) and a clinometer to measure height. On the other end of the spectrum, some foresters consider the use of a Biltmore stick to be no more accurate than their own visual estimates (based on experience estimating the height and DBH of trees), and make it practice for their surveys to be largely completed in this manner.
The Biltmore stick uses the principle of similar triangles. Similar triangles involve using identical angles but different side lengths.