Guides · 23 October 2026

Material cost for machined parts: weight, kerf, yield and stock lengths

Material is the easiest cost on a quote to get right, and still often wrong: the finished part's weight instead of the blank's, no allowance for the saw, no extra blanks for scrap, and bar that turns out to be one length short. Here is a method that avoids all four.

Start from the blank, not the part

You buy and cut blanks, not finished parts. Price the blank you will actually saw off: the stock size that contains the part plus a few millimetres to clean up each face, and the length plus the saw kerf and facing allowance.

Weight is cross-section area × length × density:

• Plate or block: width × thickness
• Round bar: π ÷ 4 × diameter²
• Hex bar: √3 ÷ 2 × across-flats²
• Tube: π ÷ 4 × (outside diameter² − inside diameter²)

With millimetres and density in kg/m³, divide by 10⁹ to get kilograms. Typical densities: aluminium 6061 about 2,700 kg/m³, mild steel 7,850, 304 stainless 8,000, brass 8,500.

A worked example: plate

A part machined from 6061 plate, 80 × 25 mm section, 120 mm long, with 3 mm for kerf and facing. Plate costs A$9.50/kg delivered and the supplier charges A$1.20 to cut each blank. The customer wants 25 good parts and you plan for 97% yield.

That is A$7.81 per good part, not the A$6.31 a part-weight estimate would give. On a part with a thin margin, the difference matters.

Bar stock: count the lengths

For bar, also check how many blanks you get from one stock length. Ø40 mm 6061 round bar at 123 mm per blank gives 24 blanks from a 3 m length, so 26 blanks need two lengths. If the offcut from the second length has no other use, price it into this job.

Yield is a cost, not a hope

If one part in thirty usually ends up scrap, from a bad first-off, a tool breaking or a dimension out of tolerance, you start more blanks than the order needs. Round up: 25 good parts at 97% means 26 blanks, and 100 parts means 104. The extra blanks need cycle time as well as material, so put the yield into the whole quote, not just the material line.

Keep prices current

Metal prices move. Write down the date of each price you use and refresh it before it is a month or two old, sooner when the market is moving. A quote valid for 30 days, built on a price from last year, can lose its margin before the job starts.