Why is a rectangular frame always mitered at 45°?

Because a square corner splits evenly between two pieces meeting there — half of 90° is 45° on each. Every rectangular frame takes the same setting regardless of its size, which is why 45° is the one miter angle stamped on the fence of almost every miter saw made.

What does "long point to long point" mean?

The length measured between the two outside corners of a mitered piece — the point of the 45° cut that reaches furthest, as opposed to the short point on the inside edge. It's the number you actually mark and cut to, because the short point is harder to reach with a square and easy to get wrong by the width of the blade kerf.

Why does the top/bottom length equal the outer width?

Because on a rectangle, the long point of each 45° miter lands exactly on the frame's outer corner. Follow the outside edge from one corner to the other and you've walked the full outer width — which is also the long-point-to-long-point length of that piece.

How much extra molding should I buy?

This adds 15% over the four exact lengths, which covers the triangular offcut at every miter and one or two re-cuts if a corner comes out a hair open. Buy a specific board length past the calculator's total when possible — you want any single piece to be long enough for one whole side, not stitched from two shorter offcuts.

Picture frame miter cuts

Enter the opening and the molding width. This gives the four cut lengths, long point to long point, and how much molding to buy, with a true-size miter check below.

Opening width

Opening height

Molding width

How many frames

Cut at 45.0°

Top / bottom

12"

Left / right

14"

Molding to buy

5.0 ft

The cut list

Top and bottom, 2 piecesLong point to long point: 12"
Left and right, 2 piecesLong point to long point: 14"
Outer size: 12" × 14"
Molding to buyFor 1 frame, with corner waste: 5.0 ft

Miter check, at true size

Print at 100%, check the ruler, then hold a corner offcut against the two lines to confirm the saw actually cut 45.0° before trusting it on the good stock.

Check this first

Put a tape on the printed ruler. If it does not read exactly 6 inches, the print was scaled — set the print dialog to 100% or “Actual size” and print it again. Do not cut wood from a template that failed this.

Miter

Printing this properly

In the print dialog, set the scale to 100% or “Actual size” and turn off “Fit to page”. Measure the printed ruler with a tape first — a picture frame shows a gap this small from across the room.

Where to buy this

5.0 ft of moldingHome Depot Lowe's

Feet of stock with the miters allowed for. Buy one length rather than two: a frame made from two sticks of the same profile shows the difference at a corner.

Plain search links, nothing sponsored. Prices and stock are the shop's, and a local lumberyard will usually beat both on solid stock.

Worked example: framing 8" × 10"

A frame around an opening of 8" × 10" in 2" molding is cut at 45.0°: two pieces 12" and two 14", long point to long point.

It comes out 12" × 14" overall, and takes 5.0 ft of molding with the corner waste.

Top and bottom: 12"
Left and right: 14"
Outer size: 12" × 14"
Molding to buy: 5.0 ft

These are the figures the calculator above opens on. Change any field and it works the same problem for your numbers.

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About mitered frames

Why is a rectangular frame always mitered at 45°?: Because a square corner splits evenly between two pieces meeting there — half of 90° is 45° on each. Every rectangular frame takes the same setting regardless of its size, which is why 45° is the one miter angle stamped on the fence of almost every miter saw made.
What does "long point to long point" mean?: The length measured between the two outside corners of a mitered piece — the point of the 45° cut that reaches furthest, as opposed to the short point on the inside edge. It's the number you actually mark and cut to, because the short point is harder to reach with a square and easy to get wrong by the width of the blade kerf.
Why does the top/bottom length equal the outer width?: Because on a rectangle, the long point of each 45° miter lands exactly on the frame's outer corner. Follow the outside edge from one corner to the other and you've walked the full outer width — which is also the long-point-to-long-point length of that piece.
How much extra molding should I buy?: This adds 15% over the four exact lengths, which covers the triangular offcut at every miter and one or two re-cuts if a corner comes out a hair open. Buy a specific board length past the calculator's total when possible — you want any single piece to be long enough for one whole side, not stitched from two shorter offcuts.