An 18 mm particleboard shelf holds 15-20 kg across a 60 cm span, 10-15 kg across 80 cm, and 5-8 kg beyond 90 cm before it sags permanently. A metre of paperbacks weighs 10-12 kg and a metre of hardbacks 15-20 kg, so a 60 cm shelf of hardbacks sits at 9-12 kg — comfortably inside the limit. The failure starts at the 80-90 cm spans that open shelving units love.
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Span is the only number that matters
Shelf capacity is not about thickness, it is about the distance between supports. The engineering limit is deflection of span/300: a 60 cm shelf may bend 2 mm under load, an 80 cm shelf 2.7 mm, a 120 cm shelf 4 mm. Because deflection rises with the cube of span, doubling the span makes the shelf eight times more flexible. That is why the same 18 mm board holds 15-20 kg at 60 cm and only 5-8 kg at 100 cm.
| Board | Span | Safe load per shelf | What that means |
|---|---|---|---|
| 18 mm particleboard | 60 cm | 15-20 kg | 40-50 paperbacks, or 25 hardbacks |
| 18 mm particleboard | 80 cm | 10-15 kg | 30-40 paperbacks, visible bow after 2 years |
| 18 mm particleboard | 90-100 cm | 5-8 kg | Paperbacks only, add a centre leg |
| 18 mm MDF | 60 cm | 20-25 kg | Denser, stiffer, heavier unit |
| 25 mm solid pine | 80 cm | 25-30 kg | Will bow but recovers |
| 25 mm plywood | 90 cm | 30-35 kg | Best span-to-load ratio in flat-pack |
Those figures assume the load is spread across the shelf and the unit is standing on a level floor. A 10 kg dumbbell in the middle of an 80 cm shelf does more damage than 20 kg of books spread along it, because the point load is what starts the permanent bow.
Weighing your books
- Paperback, 3 cm spine: 250-350 g. One metre holds about 33 books, so 8-12 kg per metre.
- Hardback, 4 cm spine: 500-800 g. One metre holds about 25 books, so 15-20 kg per metre.
- Art, cookery, reference: 1.5-2.5 kg each. Ten of these on one shelf is 15-25 kg — over the limit on an 80 cm particleboard shelf.
- Quick estimate: 30 cm of shelf holds 10 paperbacks (3 kg) or 7 hardbacks (4.5 kg). Multiply up for your shelf length and stay under 70% of the rating.
Loading a tall unit so it does not fail
Heavy at the bottom, light at the top. On a 180 cm unit the bottom two shelves can take the full rating, the middle shelves 70%, and the top shelf nothing heavier than 5 kg. Load the ends of a shelf rather than the middle where you can — supports are at the ends, and the centre is the weakest point.
Every open unit over 120 cm tall needs a wall anchor, loaded or not. A 180 cm bookcase with 60 kg of books tips at about 12 degrees of lean, which a child climbing the bottom shelf can produce. The wall anchor weight guide has the fixing ratings, and the shelf spacing guide covers how much gap each book size actually needs.
Fixing a shelf that has already bowed
Flip the shelf over — a bowed board will slowly straighten under its own weight if the load is removed for a month. If it does not, add a 2 x 2 cm batten screwed under the front edge at mid-span, or move that shelf to the bottom of the unit where the load is lowest. A bow of more than 5 mm over 80 cm will not come back; replace the board rather than shimming it.
Keep reading
More dimension guides are in the collection, and the full index is on the furnolio journal.
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