Most custom fabrication shops apply one wastage percentage to everything— usually 10%. It's comfortable, simple, and almost always wrong. The actual wastage rate varies dramatically by material: figured timber loses 25%; MDF loses 5%; quartz slabs lose 22% before a single cutout. Apply 10% across the board and you under-order on half your jobs and over-order on the other half — both eat margin.
This guide gives you trade-tested defaults per material, the reasoning behind each, and the change to make in your quoting system to stop bleeding.
Why wastage % is the biggest pricing leak
On a typical custom job, materials are 35–55% of total cost. If your wastage assumption is 8 percentage points off (say, 10% applied where 18% is real), and material is 45% of job value, you've missed about 3.6% of total job cost — silently. Multiplied across a year of work, that's often more than the cost of the software that would have caught it.
An 8-pt wastage gap on one $100,000 job costs 5.4 margin points. Multiply across a year.
Timber & joinery
Solid timber wastage is driven by grade selection and knot avoidance, not just cut layout. The same species can run at very different rates depending on the cut quality and your tolerance for visible defects.
Sheet goods (ply, MDF, particle)
Sheet goods are the easiest to predict because cut layouts are deterministic. The variable is damage / unusable sheets — buy from a reliable supplier and your real wastage drops.
Stone & engineered surface
Stone is the highest-wastage category in custom fabrication. Slab pricing is non-negotiable — you buy the whole slab, even if you only use 70% — so a job's effective material cost is the slab cost divided by usable yield, not by nominal area. Most stone studios under-quote because they miss this.
Glass & glazing
Glass wastage is mostly cut tolerance and breakage. Tempered glass cannot be re-cut once tempered, so a measurement error becomes a full pane write-off. Build a small breakage allowanceinto the quote (1–2% of pane cost) so the loss doesn't come out of your margin.
Aluminium extrusion
Aluminium offcuts are predictable. The trap most fabricators fall into is not allocating offcut value back to a future job — the offcut sits on the rack, then gets cut for an internal repair, and the original job carries the full cost. A real wastage % accounts for unusable end-cuts only; reusable offcuts go back to inventory.
Steel & mild metal
Steel by section (RHS, SHS, plate) at ~6–10%. Stainless higher because of stricter surface tolerance — visible defects mean rejection. Track scrap value separately from wastage; the scrap recovery is real cash and shouldn't be hidden in margin.
How to fix your wastage
- List every material category you use — write them down.
- Set a wastage % per category using these defaults as a starting point.
- For 8 weeks, log actual material consumed per closed job. Compare to quoted.
- Adjust each material's wastage % based on real variance — not vibes.
- Lock the new defaults into your quoting system. Review quarterly.
- For figured / variable materials (timber, stone), consider a wastage band by job type rather than one fixed number.
The shops that do this fix recover 3–6 margin points within two quarters. It's the single highest-leverage pricing change you can make.