How to Cut Perfect 45 Degree Angles For Picture Frame Corners

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Two old sayings came to mind prior to writing this article:

  • “There’s more than one way to skin a cat,” and
  • “Everything’s math.”

Stay with this piece for just a few moments, and you’ll see what we mean.

What, you may ask, does either have to do with woodworking or, in particular, with making a frame for your picture or painting?  Actually, a lot.  The process of making your frame, the angled corners required to make the frame square, the tools in your shop all will impact how you approach your project.

We’ll begin with some of the basic math first.

The Miter Angle

We all remember from our high school math classes that a square corner is a 90-degree angle.  Four 90-degree angles make a square.  The picture or painting we’re framing is square.  So, we need a square frame.

Simple, so far.

The theory, then, is if we join two pieces of wood that have been cut at a 45-degree angle, and since 45 times 2 is 90 when they are joined, we have a square joint.  This miter angle is what we want to create.

It just so happens this is what a miter saw’s purpose is: to cut wood at carefully measured angles to create our 90-degree picture frame joined corners.  If the measurements we make are accurate, the results are perfect miters.

More in a moment.

The Picture Frame Cutting Formula

The math part is pretty easy so far.  But more is necessary.  The rails, or sides, of the frame you are making, must be of the same lengths opposite each other, or else the joints will not be square, and the frame will be off-kilter.

And, you need to know how much stock of the framing material to purchase.  Fortunately, there is a formula to help you determine that purchase:

  1. First, measure the piece you are framing, height, and width.
  2. Add these two measurements together, H + W.
  3. Then, double them, 2(H + W).  This will be the length of stock you will need to build your picture frame.

If the painting you are framing is 13” x 17”, it looks like this:  2(13 + 17) = 60”. You will need 60 inches of stock.

Here are some common picture frame sizes and the total length of wood needed:

Frame Size (Height x Width)Total Length of Stock Required
8″ x 10″2(8 + 10) = 36″
12″ x 16″2(12 + 16) = 56″
13″ x 17″2(13 + 17) = 60″
20″ x 24″2(20 + 24) = 88″
24″ x 36″2(24 + 36) = 120″

How To Measure Miter Cuts for Picture Frames

We know the opposite rails on a frame must be exactly the same length to have a square frame.  We also know that we’re cutting a 45-degree angle at both ends of those rails.  This means each rail will have a longer outer measurement than the inner measurement, with the internal measurement being the same as the piece being framed and the glass cover of the piece, if a photo.

When using a miter saw for your cuts, a stop block is necessary to butt the already-cut end of the rail against when making the second cut on the rail’s other end.  The stop block must itself be a perfect 45-degree block and measured from the blade on the inner dimension of the rail.

Using the above example, we’ve calculated 60” of stock for the project.  The two stiles will be 13” on the inside, and the two rails will be 17” again on the inside.

But we also know the outside of the rail will be longer than the inside.  This means 60” isn’t going to cut it (pun intended), and we’ll run a bit short.   It will depend on the width of the molding you’re using for your frame.

The formula is pretty common sense:

  • There will be 8 ends (4 rails x 2 ends).
  • You’ll need an additional stock measurement to compensate for the outside rail measurement, and that’s based on the width of the molding.
  • If the molding width is 3 inches, you’ll lose 3 inches on a 45-degree miter cut. Another way of saying this is the outside of the rail will be 3” longer than the inside of the rail.
  • The formula, then, is 8 ends x 3” molding width, or 24 inches. You’ll need 84” of stock for your frame.

This will give you enough stock to accommodate all the miter cuts and the 8 ends of the rails and stiles.

Rails and Stiles

Miter Cuts

If you have a miter saw, there are a few steps to consider before you begin making cuts.  Taking just a moment for them will give you an excellent result.

You want your cuts to be precisely 45 degrees.  We’ve already gone over the math for this:  45 + 45 = 90.

Your miter saw will have a plate to help you set the angle at 45 degrees.  But go the extra step with a carpenter’s square to double-check its accuracy.  From the teeth of the blade to the fence, make sure it confirms a 45-degree angle; if it does not, adjust the blade accordingly. These extra steps and adjustments will ensure a good end result.

We’ve already talked about using a stop block with a true 45-degree angle and measuring the inside length of each rail you are cutting.  The result of the extra step ensures a perfect miter cut, and the math will tell you the pieces of each opposite side of the frame are uniform in length.

Table Saws

But, what if you only have a table saw?  No problem.  Anything is possible with the right jig, and perfect miter cuts can be achieved with the right jig.

A lot of words could explain this, but you’ll get a better education with a 20-minute video on that right jig for the perfect 45-degree cut.  We happened to find one for you, and you can watch it here:

It even has the perfect title:  “Perfect Miters Every Time.” In fact, we made the exact jig and can attest it’s fantastic!  Anyone that is serious about making picture frames should consider making one themselves.

Bottom of Picture Frame Jig

Hand Saw and Miter Box

Perhaps you’re old school and want to do everything by hand. Perhaps you’re just starting out, and Dad or a favorite uncle has left you his hand tools.  Perhaps you’re in the midst of a power outage and can’t use your power tools.

No problem, as old school will work just fine as well.  The same math applies; the only change is the means of the cuttings.

A hand saw, and miter box will do the trick for you.  A miter box consists of a hand saw with teeth made for cross-cutting – cutting against the grain of the molding for your frame, and a box with slots on both sides that guide the saw accurately for 45-degree cuts.

The same measurement considerations apply to your stock to ensure opposite rails are precisely the same lengths.  If you have measured accurately and follow the marks for those measurements on the wood, you’ll get the same results with a miter box as you will with the power tools.

Power or hand, the results will be a frame you’ll be proud to hang.  You see, there is more than one way to skin a cat.  And the math is pretty straightforward and mostly common sense, with no calculator required.

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