How to Miter Corners

Co-authored by Sarah Fogle

Last Updated: August 25, 2022 References

If you want to hide the end grain of a board, a miter joint is the joint to make. Miter joints are decorative joints used for picture frames, door and window trim, and around openings. Miter joints are weak joints but useful when you want something to look nice without needing it to support a lot of weight. Just get started with Step 1 below to make your own.

Part 1

Part 1 of 3:

Setting Up for Success

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1
Get the right cutting and measuring tools. There are a couple of different tools that you can use to miter corners and what you can use will depend on what you're cutting and how it needs to be cut. Basic, standard cuts can be done on simple materials using a miter box and a hand saw. More complex cuts will need a circular or table saw. These can be rented from your local big box hardware store and they can advise on exactly which type of machine you need.^{[1]
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- For many interior molding situations such as baseboard and crown, etc., you should not miter the inside corners. It won't fit right and miters tend to separate in these situations. For inside corners, you should learn a technique called "coping." This is not difficult (do a search for instructions).
Measure the complete angle change. Divide that by the number of pieces that will make the turn. The resulting number is the angle to which you will need to set your miter. The most common miter angle is 45 degrees.^{[2]
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Since the cuts are angled, you'll have one side of the wood longer than the other. You'll need to keep track of which side will be which and make your measurements correctly. For example, on an inside corner piece, the measurement of the wall will need to equal the longer, back side of the wood. For an outside corner piece, the inside will need to be equal to the wall but will b the shorter distance.
4
Measure for your angle gap. If you want to make use of as much wood as possible, you'll want to be able to use scraps whenever you can. Because the piece will need to be longer on one side then what might actually appear (such as on inside corners), you'll need to be able to calculate how much extra material you'll need at the end. If your mitered corner is at the standard 45 degree angle((360/2)/corners), then you'll need a gap at the end equal to the depth of the piece of wood.
- If you're using a non-45 angle, you'll need to do some math. Don't worry: it's easier than it sounds. Use a calculator or the interactive circle on this page to find the sin and cos for the angle of the cut you're making. Next, take your calculator and divide the sin by the cos. Take the resulting number and multiply it by the depth of your wood. The resulting number is how much extra space you'll need.

Part 2

Part 2 of 3:

Cutting the Material

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Set the miter saw to the angle it will need to cut at. Use the included instructions for your machine, as each is different.
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Line up the wood. Place the wood on the machine so that it is upside down, when possible. Always make sure you know where the short and long sides of your wood need to be. Make use of a laser guide if your machine has one (most newer machines do). If you're unsure about how a piece of wood is lined up, keep scrap wood handy to test the cut on. Testing is important.^{[3]
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- When cutting a picture frame style cut, place the flat side on the cutting surface.
- When cutting a box style cut, place the flat side against the fence (or back piece of the cutting surface).
If you are not using a piece long enough to hold and have your hands well out of the way of the saw, you should use clamps to hold the wood in place. Seriously, a lot of people lose fingers this way. Be careful.^{[4]
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Usually there are a series of buttons and actions you have to take to start and lower the saw. Follow the instructions that came with the machine you have. When you lower the saw, keep your hands away and do not press down very hard. Simply guide it to where it needs to go and let the saw do the cutting.
Once it's done, you can unclamp and remove your piece.

Part 3

Part 3 of 3:

Joining the Material

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No matter what method of joining you use, using clamps to keep the joint steady while it sets in important. There are lots of different styles of clamps depending on what project you're working on. See your local hardware store for help choosing what's right for you.^{[5]
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The most basic way to join the pieces is to glue the angles, fit them together, and then nail them into place using a pneumatic brad nailer. This works best if the pieces can be nailed onto something else, like a door frame, because the ends will not hold together well. Set the nails if they do not go in far enough with a nail setter, then fill the holes with wood putty and stain or paint the putty to match the wood.^{[6]
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You can drill dowel holes and place dowels in the joined ends of the wood if it is thick enough. This can add support and make the joint much stronger. Simply drill the hole, cover a dowel in wood glue, and join the pieces. Use a dowel size appropriate to the thickness of your wood.^{[7]
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Once the ends are joined, you can also try a notch joint. Using a thick circular saw, cut a notch in the corner of the joint. Then, fill the notch with glue and fit in a triangular shim, cut to the correct size and shape. This is a more advanced method but can be beautiful if done correctly.