Why this calculator exists
If you've ever cut four mitered pieces for a picture frame and come up short on the last one, you know the pain of recalculating at the saw. The reason this happens: a miter cut doesn't just trim the corner — it changes the length of the visible piece.
The formula: Long piece length = inside width + (2 × stock width). The miter accounts for one full stock-width of material at each end (the corner joinery).
The math, in plain English
Imagine your frame as a rectangle. The inside edge measures the opening you want. The outside edge is the inside edge plus the molding width on each side. Each mitered piece has to span from one outside corner to the other outside corner — that's the inside dimension plus two stock-widths (one for each end's miter).
What if my frame isn't square?
The math still works for any rectangle. If your frame is 11" × 14" (slightly portrait), your long pieces (top and bottom) are 11" + 2×(stock width) and your short pieces (sides) are 14" + 2×(stock width). For oval or shaped frames, you'd build a separate template or use a different joinery method.
Why cut the long pieces first?
Convention in picture framing: long pieces (top/bottom for a portrait frame, sides for a landscape) get cut from your longest stock. If you make a mistake and waste a piece, you can usually re-cut it as a shorter piece. Cutting the long pieces first also lets you confirm the math is right before committing to all four pieces.
Adding overlap and overshoot
Real woodworkers add 1/16" to 1/8" to each piece length for safety. A blade kerf of 1/8" per cut × 2 cuts per piece = 1/4" lost across the four pieces. The math here already accounts for kerf implicitly because the formula assumes the miter is a "point cut" — but in practice, saws leave a small flat at the corner, so an extra 1/16" per piece is wise.
Rabbet and dust cover
The rabbet (groove) holds the art and any backing material. Standard depth is 1/4" — enough for 1/8" art + 1/8" foam-core backing, or for 1/4" hardboard alone.
For larger frames (over 16" × 20"), add a dust cover — kraft paper taped to the back of the frame over the art. This keeps dust and insects out of the art-glass gap.
Common questions
What about mat boards?
If you're using a mat, measure the mat's window opening (the inner rectangle), not the art's dimensions. Mat window is typically 1/4" larger than the art on each side to allow easy insertion.
Can I cut frames on a table saw?
Yes, with a miter sled. Most production framers use a chop saw for speed, but a table saw with a good sled gives you the ability to handle wide stock that won't fit on a chop saw.
What glue should I use?
Yellow PVA (Titebond II or III) for wood frames. Some framers use a tiny bit of CA (super glue) for quick grab, then PVA for full strength. Avoid epoxy — it's overkill and messy on visible joints.