Bookshelf Sag Calculator

For fine furniture and bookcases, 2-3mm of sag is generally considered acceptable and barely visible. Workshop shelves may tolerate 5mm. For display shelves, aim for less than 1mm deflection.

Yes, dramatically. A 6mm hardwood lip glued and pinned to the front edge of an 18mm shelf can reduce sag by 40-50% because it effectively increases the thickness of the cross section where bending stress is highest.

Ash and maple have the highest stiffness (12,000-12,600 MPa). Oak is also excellent at 11,000 MPa. Birch plywood at 10,500 MPa is a great sheet material option. Avoid MDF and chipboard for spans over 600mm with heavy loads.

Paperback books weigh about 20-25 kg per metre of shelf. Hardbacks average 30-40 kg/m. Large reference books or records can be 50+ kg/m. A good default for general bookshelves is 30 kg/m.

Yes. The deflection shown here is the initial elastic sag the moment the load goes on. Wood creeps under constant load, so a permanently loaded shelf can sag roughly 50% more than the initial figure over the years (the Sagulator and Hoadley’s Understanding Wood use the same ~50% rule of thumb; EN 1995-1-1 gives a creep factor kdef = 0.6 for solid timber in dry indoor conditions, i.e. total ≈ initial × 1.6). For a permanent bookcase, design so the initial deflection is comfortably under your sag limit — aim for about two-thirds of it — to leave headroom for long-term creep.