What is the maximum span for a 18mm MDF shelf?

An 18mm MDF shelf with 200mm depth supporting 30 kg/m will start to noticeably sag at around 500-600mm. MDF has a relatively low modulus of elasticity (about 3,000 MPa) compared to solid timber, making it prone to sagging over long spans. Adding a front lip or using a thicker board significantly improves performance.

How do I stop shelves from sagging?

Several methods reduce shelf sag: increase the thickness of the shelf, use a stiffer material (oak instead of MDF), add a hardwood lipping to the front edge which effectively increases the thickness, reduce the span by adding a centre support, or reduce the load. A 6mm hardwood lip glued to the front edge can increase stiffness dramatically.

What is an acceptable amount of shelf sag?

For bookshelves and display shelves, 2-3mm of deflection over the span is generally considered acceptable and barely noticeable. For workshop shelves holding heavy items, up to 5mm may be tolerable. For fine furniture, aim for less than 1-2mm of deflection.

Does the depth of a shelf affect how much it sags?

Yes. The depth of a shelf (front to back) directly affects sag because it forms the breadth of the cross section in the moment of inertia calculation (I = b x h³ / 12). A deeper shelf is stiffer because the cross section has more material resisting bending. Doubling the shelf depth halves the deflection for the same load.

Which timber is best for long shelf spans?

Ash and oak have the highest stiffness among common UK timbers, with a modulus of elasticity around 11,000-12,000 MPa. Birch plywood is also excellent at about 10,500 MPa. For the longest spans without visible sag, use 25mm solid ash or oak. Softwood pine at 9,000 MPa is a good budget option.

WoodCalcs

Shelf Span Calculator — Maximum Shelf Length Before Sagging

Calculate the maximum unsupported shelf span before sagging based on material, thickness, and load.

Shelf Material

Shelf Thickness

Height of the cross section

Shelf Depth

Front to back dimension

Load per Metre

Weight the shelf must support per metre

Max Acceptable Sag

Maximum deflection at centre

How We Calculate This

This calculator uses beam deflection theory to determine the maximum unsupported span for a shelf before it sags beyond your acceptable limit.

The formula

For a uniformly distributed load on a simply supported beam:

Deflection = (5 x w x L⁴) / (384 x E x I)

Where w = load per unit length (N/mm), L = span (mm), E = modulus of elasticity (MPa), and I = moment of inertia = (b x h³) / 12 for a rectangular cross section.

The calculator rearranges this formula to solve for L (maximum span) given an acceptable deflection, then applies a 10% safety factor for the recommended span.

Frequently Asked Questions

Related Calculators

Drawer Slide Calculator

Work out drawer and slide dimensions for smooth-running drawers.

Table Leg Calculator

Calculate table leg dimensions and taper angles for stable tables.

Chair Seat Height Calculator

Determine ergonomic chair dimensions based on user height.

Bookcase Calculator

Calculate shelf spacing, material quantities, and dimensions for bookcases.

Cabinet Door Calculator

Work out cabinet door dimensions and overlay calculations.

Last updated: February 2026

All calculations are estimates. Always verify measurements and material quantities before cutting or purchasing.