Easy Wood Calc

How much will an 18 mm MDF shelf sag over 80 cm?

Short answer

An 18 mm MDF shelf spanning 80 cm, 30 cm deep with 25 kilograms of books on it, sags about 3.3 millimetres when first loaded and roughly 9.9 millimetres over time: over seven times the span ÷ 600 guideline of 1.3 millimetres.

How to work it out

  1. I = 300 mm × 18 mm³ ÷ 12 = 145,800 mm⁴
  2. sag = 5/384 × 245 N × 800 mm³ ÷ (3,400 MPa × 145,800 mm⁴) = 3.3 mm · × 3 creep = 9.9 mm vs 1.3 mm target

80 cm is a common flat-pack bookcase width, and MDF (medium-density fibreboard) is one of the most common shelf materials in them. It is much less stiff than solid wood and creeps under constant load, which is why long MDF shelves bow over the years. The table below compares other materials at the same size.

Adjust for your project

cm
The open gap between the sides, pins or brackets — the shelf length if it sits between the sides.
cm
The actual width of the board.
mm
1× boards are ¾ in; ¾ plywood is really 23/32 — type it.
kg
Weigh a sample if you can — dishes, a TV or a stack of records add up fast.
mm
A hardwood strip glued along the front edge, top flush with the shelf and hanging below it
More options: Ends · Load type
Glue and screws only partly lock the ends, so a real fixed shelf lands between the two answers.
One heavy item: enter its weight as the total load.
Sag when first loadedabout 3.3 mm — an obvious dip
Sag after a few years (creep)about 9.9 mm — an obvious dip
Guideline limit (span ÷ 600)
1.3 mm
Verdict
Too much sag — shorten the span, thicken the shelf or add an edge strip
Load that just meets span ÷ 600
3 kg
Load on the shelf
25 kg in total — 31.3 kg per metre
  • MDF and particleboard creep far more than solid wood under a load that never comes off: the long-term figure is 3 × the first-day sag (Eurocode 5’s deformation factor for these boards indoors works out to about 3.25 ×), and a damp room makes it worse.
  • Sag is not strength. On a wall shelf, check the load rating of the brackets and fasten them into studs where you can — drywall anchors have their own limits on the package.
  • Bookcase? Fasten its top to a wall stud with an anti-tip strap or bracket. A loaded bookcase can tip over when a child climbs it.

For this job · affiliate link
Bar clamps (set)The cheapest fix is a hardwood strip glued to the front edge — try 1½ in above. Clamp it every 6–8 in along its length while the glue sets.
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Sag of an 80 cm shelf, 18 mm × 30 cm with 25 kg, by material

Sag of an 80 cm shelf, 18 mm × 30 cm with 25 kg, by material
MaterialSag when first loadedLong-term sag (with creep)Verdict
MDF (approx.)3.3 mm9.9 mmToo much sag — shorten the span, thicken the shelf or add an edge strip
Particleboard (approx.)4 mm12 mmToo much sag — shorten the span, thicken the shelf or add an edge strip
Plywood (approx., along face grain)1.5 mm3 mmToo much sag — shorten the span, thicken the shelf or add an edge strip
Eastern white pine1.3 mm2 mmNoticeable sag likely over time — consider a stiffer shelf
Red oak0.9 mm1.3 mmNoticeable sag likely over time — consider a stiffer shelf

What to know about an 18 mm MDF shelf over 80 cm

Twenty-five kilograms over 80 cm is about 31 kg per metre, a full row of hardbacks. At that load this shelf is well past the guideline, and the estimate says it only stays inside span ÷ 600 with about 3 kg on it: a few ornaments or photo frames. Anything heavier and you should expect a visible bow.

The long-term figure is three times the first-day sag, because MDF creeps far more than solid wood under a load that never comes off; Eurocode 5, the European timber design code, uses a deformation factor that works out to about 3.25 times for MDF in a heated room. In a damp room, or with an unsealed shelf, it can keep going beyond that. Some people flip long MDF shelves every year or so to even out the bow; it helps, but it does not make the shelf stiffer.

Plywood of the same 18 mm thickness sags less than half as much on the first day and creeps less, but it still misses the target at this span and load. The fixes that work are the same as for any shelf: a centre support, a shorter span, or a hardwood strip about 40 × 19 mm glued and screwed under the front edge, which brings this shelf down to about one and a half times the guideline. Add a matching strip under the back edge as well and it passes.

Paint or seal MDF shelves, edges included: the cut edges soak up moisture and finish like a sponge, and How to finish wood covers sealing first. For the same question in inches with solid pine, see Will a 1x12 pine shelf sag over 36 inches.

Different size? Use the full Shelf Sag Calculator.

What you’ll need

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Frequently asked questions

How much does an 18 mm plywood shelf sag over 80 cm with 25 kg?

About 1.5 millimetres at first and 3 millimetres long-term, well under half the MDF figure but still over the guideline of 1.3 millimetres. Plywood can carry about 11 kilograms before crossing it.

How much will an 18 mm MDF shelf sag over 60 cm?

With the same 31 kg per metre (18.75 kg in total), about 1 millimetre at first and 3.1 millimetres long-term against a target of 1 millimetre. Shortening the span from 80 cm to 60 cm cuts the long-term sag by roughly two-thirds.

Does a hardwood front strip stop an 18 mm MDF shelf sagging over 80 cm?

It helps a lot but doesn’t quite get there: a 40 × 19 mm hardwood strip under the front edge brings the long-term sag down to about 1.9 millimetres, against a guideline of 1.3 millimetres, with the full 25 kg on it.

How much does an 18 mm MDF shelf sag over 80 cm with strips front and back?

With a 40 × 19 mm hardwood strip under both long edges, about 1.2 millimetres long-term: inside the guideline of 1.3 millimetres even with the full 25 kg on it.

Other common sizes

Sources