How to Find Angles Using a Speed Square

A metal speed square isolated on a white background.

The speed square, also called a rafter-angle or triangle square, is an extremely versatile tool originally intended to help carpenters quickly and accurately mark roof rafters. It is a triangle-shaped measuring tool that's invaluable for the DIYer's tool box.

What Is a Speed Square?

Made from plastic, aluminum, or steel, this multi-measuring tool combines a protractor, a Try square, a ruler, a line-scribing tool, a common-width board ripping guide, and a guide for making 90 and 45-degree cuts with a saw. For less than 10 dollars (for the plastic variety), it can be used for measuring, marking, and to make cutting lumber fast and accurate, making many projects quicker and easier. Follow these steps to properly use this handy tool specifically for finding angles.

Step 1 - Orient Yourself

The speed square can basically be used as a protractor because it has degree graduations along the diagonal edge, making reading the angles fairly simple. At the corner of the square there is a pivot point. Because the speed square is a 90-degree angle, each line on the diagonal represents one degree for a total of 90.

Step 2 - Find an Angle

Hold the pivot point against the board and move the square until your desired angle lines up with the edge of the board. It is important to note that the angle you need to find is actually the complement of the angle you need. For example, if you need a 60-degree angle, you need to find the 30-degree mark. This is the complementary angle because the two add up to 90 degrees.

After you have found the correct angle, draw a line along the square. This mark can then be used to make a clean, proper cut as needed. Also, because this tool has a ruler edge as well, you can trim off the appropriate length from a board while getting the correct angle for your edge in a single step, rather than two.

As mentioned previously, making 45 or 90-degree cuts with a speed square can be even easier. Instead of just marking the angle, a thicker plastic square can serve as a shoe for a circular saw to ride along. Note that this is only possible in certain situations, such as when the motor is on the same side of blade as the user's dominant hand.

Now that you have gotten more acquainted with the triangle square, you may find uses for it in your upcoming project. And, it can obviously be used for many purposes, not just finding angles.