Shelf Sag Calculator

Start by selecting your shelf material. The calculator includes common options from particleboard and MDF through hardwood plywood and solid hardwoods. Each material has a modulus of elasticity (MOE) value that represents its stiffness. Stiffer materials resist sag better.

Enter the shelf thickness, depth (front to back), and the unsupported span between brackets or supports. Then enter the total weight you expect to place on the shelf, distributed evenly across its length.

Choose the support type: "Resting on Supports" covers shelves sitting on pins, brackets, or cleats. "Fixed / Dados" applies when the shelf ends are captured in grooves cut into the side panels, which dramatically reduces sag (about five times less deflection for the same load).

The results show the expected sag in inches, whether it passes the L/200 visibility threshold, the maximum load this shelf can take before visible sag, and the maximum span for the weight you entered.

Shelf sag (deflection) is governed by the beam deflection formula from structural engineering. Three factors have the biggest impact: material stiffness, thickness, and span length. Span is the most powerful factor because deflection increases with the fourth power of the span. Doubling the span increases sag by 16 times. This is why adding a center bracket is so effective.

Thickness is the second most important factor. Deflection decreases with the cube of thickness, so doubling the thickness reduces sag by eight times. Even a small increase in thickness makes a big difference, which is why adding a face strip to the front edge of a shelf is such an effective reinforcement technique.