Measuring & Marking Trim

Get Miter Angles Right From a Digital Angle Finder: Subtract From 180, Then Halve

▶ Watch the instagram video by @thehomeimprovementschanneluk

The Problem

A digital angle finder tells you the truth about a corner, and then most people do the wrong thing with it. The instinct is to take the reading and divide by two for the miter cut — but the finder reads the full included angle of the corner, not the angle your saw thinks in. Divide a 154-degree reading by two and you'll set the saw to 77, which isn't even on the scale of the mistake you're about to make in a 90-degree corner that secretly reads 91.

The Technique

Open the angle finder into the corner and take the reading — say 154 degrees.
Subtract the reading from 180: 180 − 154 = 26.
Divide that by two: 13 degrees. That's your miter saw setting for each of the two pieces.

The formula matters most on corners that look square. A corner that reads 91 degrees needs each piece cut at (180 − 91) ÷ 2 = 44.5 degrees. The divide-the-reading instinct — or just cutting 45 and caulking the grief — is how paneling and trim end up with corners that never quite close.

Sanity check: a true 90-degree corner gives (180 − 90) ÷ 2 = 45, exactly the cut you'd expect.

Pro tip: Trust the finder over your eye — the video's "90-degree" demo corner actually read a degree off true, which is typical of real walls. For paneling and trim work, measure every corner and cut to its number; the half-degree adjustments are invisible at the saw and very visible in the joint.