Measure the complete package that will sit inside the frame before cutting the molding. That package might include glazing, a mat, artwork and a backing board. Its outside size determines the rabbet opening at the back. The visible opening at the front is smaller because a lip overlaps the package.
Confusing those two openings can leave a carefully cut frame too small. Draw a section through one rail and name each measurement: package width, fitting allowance, rabbet overlap and molding width. Keep depth on a separate drawing. A wide rabbet is not necessarily deep enough to hold the stack.
Work from the back opening
Call the package width W, the total fitting allowance A, the rabbet overlap R and the face width of the molding M. The back opening is W + A. The visible opening is W + A − 2R. For a simple mitered frame with consistent molding, the outside width is W + A − 2R + 2M. Apply the same calculation to the height.
This is geometry, so the formula works in inches or millimeters as long as every input uses the same unit. The calculator uses millimeters. The allowance is the total added across one dimension, not an amount added at every edge. Choose it for the actual framing materials and fitting method; the example allowance is not a conservation framing specification.
A worked example
Suppose a sample package measures 300 × 400 mm. With 2 mm total allowance, 6 mm overlap and 30 mm molding, the back opening becomes 302 × 402 mm. The visible opening becomes 290 × 390 mm. The outside frame measures 350 × 450 mm. Those outside measurements correspond to the long points of simple 45-degree miters.
Draw a rectangle representing the visible opening over a copy or paper outline of the art. Check that the lip does not hide a signature or a border you wanted to show. Change the mat arrangement if necessary before committing to the molding width. A calculator cannot decide how much of an image should remain visible.
Check depth separately
Stack the actual glazing, mat and backing and measure their combined thickness. Add whatever space your chosen retainers require. Then check how much sound wood remains at the front of the rabbet. Rockler's frame guide illustrates this relationship and provides a corresponding rail-length formula. Its sample section belongs to that project; do not copy its cut depth into thinner molding.
Make a short test corner from offcuts. Fit a small piece of the package into it and check both overlap and depth. For valuable or fragile art, use appropriate conservation framing materials and advice; a woodworking dimension check does not establish that adhesives or backing are suitable.
Transfer the dimensions to the saw
Write whether your cutting dimension refers to the inside short point, the rabbet corner or the outside long point. Put that wording directly on the cut list. Mark mating rails as pairs, allow extra stock for test cuts and follow the saw manufacturer's workholding instructions for small pieces. Dry-assemble all four rails around a paper template before gluing. Measure the two diagonals and confirm the package fits without pressure.
Compare the dry frame diagonals using the same approach as the cabinet squareness check before blaming a poor fit on the rabbet dimensions.
| Measurement | Example | Meaning |
|---|---|---|
| Package width | 300 mm | Measure the complete stack outline |
| Total width allowance | 2 mm | Example only; 1 mm each side if centered |
| Rabbet overlap | 6 mm | Subtract twice from back opening |
| Molding face width | 30 mm | Add twice to visible opening |
| Outside frame | 350 × 450 mm | Using a 300 × 400 mm package |
Calculate your dimensions
Values stay in this page. The worked example also works without JavaScript.
Related workshop guides
- Is your cabinet square? Read the diagonals before trimming the door
- Repeat crosscuts with a stop block: control the part and free the offcut
Sources
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