MDF and plywood thicknesses
Thickness decides more of a job than it seems: the weight you carry, how far a shelf can span, the inside width of every carcass and whether a back fits its groove. These are the thicknesses UK stockists commonly list, and what each one is for.
Common thicknesses in the UK
| Material | Commonly listed, mm |
|---|---|
| MDF | 3, 6, 9, 12, 15, 18, 22, 25, 30 |
| Birch plywood | 4, 6.5, 9, 12, 15, 18, 21, 24 |
| Softwood / hardwood plywood | 5.5, 9, 12, 18, 25 |
| MFC (melamine-faced chipboard) | 18 as standard; some decors also 8, 12, 15, 22, 25 |
Not every thickness comes in every size, grade or decor. Thin and very thick boards are often stocked only as 8x4, and most MFC decors only in 18 mm. Check the exact board with your supplier before you plan around it.
What each thickness is used for
| Thickness | Typical use |
|---|---|
| 3–4 mm | Templates, backs of small boxes, drawer bottoms in light drawers |
| 6–6.5 mm | Cabinet backs set in a groove, drawer bottoms |
| 9 mm | Backs that carry fixings or brackets, small boxes |
| 12 mm | Drawer sides and backs, light shelving |
| 15 mm | Drawer boxes, light carcasses and doors |
| 18 mm | The standard: carcasses, doors, shelves and most furniture |
| 22–25 mm | Long or heavily loaded shelves, desk and table tops |
| 30 mm | Heavy shelving and thick tops where 25 mm is not enough |
These are workshop habits, not rules. A drawing, a hinge or a runner may call for a different thickness, and it wins.
18 mm is not three-quarters of an inch
Plans drawn in inchesA plan from the US built in “3/4 plywood” assumes about 19 mm. Built in 18 mm board, a carcass with two sides is 2 mm wider inside than drawn, and a groove cut for 3/4 is loose on 18 mm. Convert the plan to the board you will actually use.
Plywood often measures a little under its nominal thickness, and the difference varies between products. Measure a sheet with callipers before you cut grooves or set a housing to it.
Choosing a thickness for shelves
Sag grows with the cube of thinness: a shelf half as thick sags eight times as much under the same load. The table above shows, for each thickness, the longest span that keeps sag below span/600 — the point where it stops being noticed. For a 300 mm deep shelf loaded with books (about 25 kg per metre):
Many shelves are built longer than this and look fine empty. Loaded, they bow visibly, and MDF and chipboard keep creeping under a constant load. For long spans use thicker board, a stiffer material, a support in the middle or a lipping under the front edge; the shelf sag calculator shows the sag for your exact shelf.
Thickness changes the cut list
Inside widths
A 600 mm carcass with 18 mm sides has a 564 mm top and bottom; with 15 mm sides, 570 mm. Change the board and every part between the sides changes.
One board per thickness
Parts of different thickness never share a sheet. Add each thickness as its own board, so the layout and the order count them apart.
Weight
An 8x4 of 18 mm MDF is about 40 kg, 25 mm about 55 kg. The sheet weight calculator does any size and material.
Edges
Edge banding is sold by width: it must be wider than the board is thick, so a 25 mm board needs wider tape than an 18 mm one.
Picking a thickness in four steps
- Start with what the job carriesShelves and tops under load decide the thickness; carcass sides rarely do.
- Check the hardwareHinges, runners and connectors are made for a range of board thicknesses. Check the range before you change the board.
- Check what is stockedThe thickness has to exist in the material, size and finish you want.
- Plan with the real figureEnter the board and recalculate the inside parts, then count the sheets.
Questions
What thickness of MDF is best for shelves?
For ordinary bookshelves, 18 mm MDF keeps sag below span/600 up to about 525 mm at 300 mm deep; for longer shelves use 25 mm, plywood, or add a support. The comparison above works it out for your depth and load.
Is 18 mm the same as 3/4 inch?
No. 3/4 inch is 19.05 mm; 18 mm is about 11/16 inch. The 1 mm difference matters for grooves, housings and plans drawn in inches.
What thickness is used for cabinet backs?
Usually 6 mm set in a groove, or 9 mm where the back carries fixings. Thin backs keep the cabinet square; they are not meant to carry load.
How heavy is an 18 mm sheet of MDF?
About 40 kg for a 2440 × 1220 mm sheet at a typical 740 kg/m³; birch plywood of the same size is about 36 kg.
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