Shelf Sag Calculator
A sagging shelf is a shelf nobody trusts. Enter the span, the wood and the load, and get the deflection, the maximum safe span and the minimum thickness you need.
Workshop AI assistant
Describe the piece you want to build and get a complete plan: dimensions, cut list, materials, cost, steps, tools and time.
Workshop assistant
Describe the piece you want to build and get a complete plan: dimensions, cut list, materials, cost, steps, tools and time.
AI-generated results are indicative. Verify dimensions, loads and local codes before building.
How it works
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1
Enter the shelf span (the clear distance between supports).
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2
Set the shelf depth and thickness, and choose the wood (each species has its own stiffness).
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3
Enter the total load in kg and whether it is spread out or concentrated.
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4
Calculate: the tool estimates the deflection and the deflection ratio.
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5
Read the recommended maximum span and minimum thickness for a standard L/240 limit.
Formula
Deflection δ = 5·W·L³ / (384·E·I) for a distributed load, or δ = W·L³ / (48·E·I) for a concentrated load. I = b·h³/12 (moment of inertia). E is the modulus of elasticity of the wood (N/mm²). Acceptable limits: L/180 (heavy), L/240 (standard), L/360 (brittle panels).
Practical tips
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Books are heavy: a full bookshelf can easily exceed 50 kg per metre of shelf.
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The thickness matters more than the width — doubling the thickness makes the shelf 8 times stiffer.
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Solid oak and beech are stiffer than MDF of the same thickness.
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Support long shelves in the middle to split the span and dramatically reduce sag.
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Add a 5 mm apron or edge strip to turn a shelf into a stiffer channel.
Common mistakes
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Using the external width of the carcase instead of the clear span between supports.
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Ignoring the actual load — decoration weight is not book weight.
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Assuming MDF is as stiff as solid wood at the same thickness.
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Loading the centre of a long shelf and wondering why it dips.
Disclaimer
These results are indicative only. Always verify against local building codes, standards and professional advice before building.
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