Stretcher Bar Calculator

Suppliers disagree on the exact trigger: Fitzroy Stretches recommends a single brace once a side reaches 44 in (about 1100mm), UCS Art suggests bracing bars over 34 in, and gallery-wrap (deep-edge) bars often bow noticeably by 40 in. This calculator defaults to a conservative 900mm (about 35 in) so smaller canvases are still supported, but you can raise the threshold under Advanced Options if you are using deeper, stiffer bars. Beyond 1500mm a second brace stops the long bars bending inward under canvas tension.

Corner keys (also called canvas wedges) are small wooden or plastic wedges tapped into slots at each mitred corner. Driving them in spreads the bars apart very slightly, re-tensioning a slack canvas. The standard convention is 2 keys per corner, so a plain four-bar canvas takes 8 keys; each cross-brace junction adds another 2.

Most commercial stretcher bars are kiln-dried pine or fir with a raised front lip that holds the canvas off the flat back of the bar (so the edge does not print through the painting). Premium bars use poplar or basswood for lighter weight and less warping. Always measure and order by the outer (finished) dimensions of the canvas.