How To Bisect Angles for Cutting Miters

Perfectly mitered trim work can elevate your space, but getting those tight-fitting corners is tricky—especially when angles aren’t exact. Thankfully, there’s a straightforward way to bisect angles accurately with just a few basic tools. Ask This Old House general contractor Tom Silva walks you through his favorite way to cut precise miters in the video above. Here’s how to master the skill, whether you’re tackling baseboards, crown molding, or custom trim.

What Is Angle Bisection?

In simple terms, angle bisection is splitting an angle into two equal parts, which allows for perfectly matched cuts. Though there are tools designed specifically for this, you don’t need anything fancy to get clean results. Let’s dive into why this technique is key for trim work and what you’ll need to get started.

Why Does Angle Bisection Matter?

Bisecting angles accurately makes a difference in the fit and finish of your project. When done well, it helps to:

• Create professional-looking trim work and tight-fitting joints
• Reduced waste from incorrect cuts
• Improve the overall look of your finished projects

Tools and Materials Needed for Angle Bisection

Gather these items before you start:

• Miter saw
• Pencil
• Steel rule
• Two 16-inch-long 1-by-4 boards (make sure they’re the same width for accurate results)

Step-by-Step Guide to Bisecting Angles

Follow these steps to create an angle-marking guide and bisect angles for perfect miters:

1. Position one 1-by-4 board flat against the wall where you need to cut the miter.
2. Place the second 1-by-4 board on top of the first, pressing it tightly against the adjoining wall.
3. Use a sharp pencil to draw lines along each edge of the top board, marking the bottom board.
4. Remove the top board and use a steel rule to draw a line connecting the ends of the two previously drawn lines. This new line represents the bisected angle.
5. Stack the two 1-by-4 boards on top of each other and bring them to the miter saw.
6. Adjust the blade angle to match the bisected angle line you drew.
7. Cut through both boards at the same time.
8. Test the fit by holding the cut boards against the wall.
9. Make any necessary minor adjustments to the blade angle and recut the 1-by-4 boards until you have a tight-fitting joint.
10. Once you’re satisfied with the miter angle, use the same setup to cut your actual trim.

Tips for Precise Angle Cuts

Here are a few ways to make your angle bisection process easier and more precise:

• Use a sharp pencil for clear markings.
• Double-check that the 1-by-4 boards are exactly the same width.
• Press the boards firmly against the walls to capture the true angle.
• Take your time when drawing the bisecting line.

Troubleshooting Common Angle Bisection Challenges

Even with careful preparation, you may face some challenges when bisecting angles for miters. Here are some common issues and their solutions.

Uneven Walls

If your walls aren’t perfectly straight, it can affect the accuracy of your angle bisection. To address this:

• Take measurements at different points along the wall.
• Use the average of these measurements as the angle for your calculations.
• Use flexible caulk to fill small gaps in the final installation.

Compound Angles

Some trim work requires compound angles, which involve both a miter cut and a bevel cut. In these cases:

• Bisect the miter angle, then set the saw’s bevel angle to match the trim profile.
• Make your test cuts on scrap wood first to confirm accuracy.

Working With Crown Molding

Crown molding presents unique challenges because of its angled profile. When bisecting angles for crown molding:

• Cut the molding “upside-down and backward” on the saw for accurate miters.
• Create a jig to hold the molding at the correct angle.
• Try using a digital angle finder for increased precision.

Outside Corners

Here’s how to handle projects with outside corners:

1. Measure the corner angle carefully—any irregularities can affect the fit.
2. First, create a test cut with a scrap piece to verify the angle.
3. Adjust your saw and cutting angles as necessary to accommodate the outside corner.