Frame moulding calculator

Moulding math is where DIY frames go to die. You can cut four pretty 45° mitres and still build a parallelogram if the opposite rails are not the same long point. This calculator reports top/bottom and left/right separately, both as long point (outside) and short point (inside / sight).

The long point of a 45° rail equals the finished outside dimension on that side. The short point equals long point minus twice the face width. If those two disagree with your tape after the cut, either the saw fence moved or you measured to the wrong corner of the moulding. Measuring long point on the back rabbet edge is a classic way to shrink a frame by twice the lip.

Rabbet size is the cavity the package sits in. It should be slightly larger than the glass. Fitting clearance — often 1/16" — is that slack. Too little and a humid week will bulge the package; too much and the stack rattles. The sight opening is what remains after the lip covers the glass. Viewers never see the rabbet. They see the sight.

Kerf is not optional. A 1/8" blade eating 3.2 mm between rails will make two “perfect” 500 mm pieces miss a 1000 mm stick. The optimizer in the studio treats kerf as a real width between segments. This calculator exposes kerf so you stop being surprised at the saw.

Profiles cannot be mixed on one stick. Walnut 1.5" and oak 0.75" are different bins even if a leftover length would otherwise fit. The workshop respects profile IDs. If you only came here for one frame, you still want that habit: label the offcut with the profile name.

Stock allowance is a safety margin you may add to each rail before packing. Some shops add nothing and trust the stop block. Some add 1/8" and trim at the vice. The field is there; the default is zero so the optimizer is not padded twice.

A small shop running four frames for a show can paste every rail into one project and let the 1D packer prefer remnants. That is the difference between a moulding calculator and a workshop. Use this page to learn the lengths, then Build This Frame to see which 96" stick they land on.

Hardwood vs finger-jointed vs polystyrene does not change the formula. Width and rabbet geometry do. Measure face width on the piece you will cut, not the catalogue drawing. Paint and leaf can add a hair; usually it does not matter, but deep cove profiles can fool a ruler if you measure the hollow.

Always dry-fit opposite rails before gluing. The calculator can be perfect and the saw can still wander. If you are 0.5 mm out, take it off the long point of the matching pair, not one random corner.

When you are ready, open the studio. The cut diagram shows the 45° mitres, long points, short points, and kerf on a stick figure of the rail. That drawing is for humans standing at a miter saw, not a marketing claim about CAM.