Panel Size Calculator

Cradle thin panels (around 6mm and under, e.g. hardboard or 6mm MDF/ply) — they warp readily as paint and ground dry on one face. Larger panels need bracing regardless of thickness: as a working rule a panel much over 300mm (12in) on a side benefits from a perimeter frame, and panels well over that size also want interior cross-braces. The defaults here flag panels 6mm or thinner, or with a longest side over 600mm; adjust both thresholds under Advanced if you prefer the more cautious 300mm trigger used by Golden’s Just Paint guidance.

Cross-bracing is applied in both directions (horizontal and vertical) at roughly 300mm (12in) centres on panels that need it — this is the spacing recommended in Golden Artist Colors’ Just Paint article on plywood substrates. This calculator counts the interior braces needed along each side at your chosen spacing, on top of the perimeter frame. Typical brace stock is nominal 1×2in (about 50×100mm), or rip your own 18×100mm strips from 18mm ply, which resists warping better than softwood batten.

Birch plywood is the preferred choice — stable, smooth and warp-resistant. MDF gives a flat face but is heavy and reacts to moisture, so seal it well. Avoid chipboard and OSB, which have coarse, unstable surfaces.

Yes — seal the back and edges as well as the face, especially for oil. Coating only one side lets the panel take up moisture unevenly and cup; an equal seal (gesso, acrylic medium or size) on both faces and all edges keeps it flat.