Steel Plate Nesting Yield & Scrap Allowance: Gross vs Net Weight

Nesting yield is the proportion of a purchased plate that ends up in finished parts. It is the ratio that converts net weight, the weight of the parts, into gross weight, the weight of the steel you have to buy:

utilisation % = net part area ÷ purchased plate area × 100
scrap %       = 100 − utilisation %
gross weight  = net weight ÷ utilisation

A fabrication package with 8.0 m² of parts nested at 85% utilisation needs 9.41 m² of plate. In 12 mm carbon steel that is 886.6 kg purchased against 753.6 kg shipped. The 133 kg difference is bought, paid for and mostly thrown away, and it belongs in the quotation.

Gross, net and the three things in between

Four quantities, and the language is used loosely enough that it is worth pinning down.

Gross weight is the weight of the plate as purchased. This is the invoice.

Net weight is the combined weight of the finished parts. This is the delivery.

Skeleton is the web of material left between and around the parts after cutting. It is scrap.

Remnant is a usable piece of plate left over at the edge of a nest, large enough to be racked and used on another job. It is not scrap, provided it actually gets used.

The difference between skeleton and remnant is a policy decision as much as a geometric one. A 600 × 1200 mm offcut is a remnant if the shop has somewhere to put it and a system to find it again, and it is scrap if it does not.

Kerf is the material removed by the cut itself: vaporised, oxidised or blown out of the joint. It is a real loss and a small one.

Calculating utilisation

Utilisation is an area ratio, and working in area rather than weight keeps it simple: thickness and density cancel out because they are the same on both sides.

Known values: a 2000 × 6000 mm plate carrying parts totalling 10.20 m² of net area.

plate area   = 2.0 × 6.0 = 12.00 m2
utilisation  = 10.20 ÷ 12.00 × 100 = 85.0%
scrap        = 100 − 85.0 = 15.0%

Interpretation: 15% of the plate is skeleton and edge. Reverse check: 12.00 − 10.20 = 1.80 m², and 1.80 ÷ 12.00 = 15.0%.

The same plate at three different nests:

Published figures for real fabrication work vary widely. Optimised nests of simple rectangular parts can exceed 90%, complex profiles with large internal cut-outs run much lower, and a typical mixed nest sits somewhere in the seventies or eighties. There is no single correct number, which is the main reason to calculate it rather than assume it.

Converting net weight to gross

This is the calculation that belongs in every plate fabrication quotation.

Known values: 8.0 m² of finished parts in 12 mm carbon steel at 7,850 kg/m³, at three candidate yields.

net weight = 8.0 × 12 × 7.85 = 753.6 kg

Interpretation: the difference between a good nest and a poor one on this package is 228 kg of purchased steel, which is 30% of the weight of the parts themselves. Reverse check: 8.0 ÷ 0.72 = 11.111 m², and 11.111 × 12 × 7.85 = 1,046.7 kg.

Quote the gross figure. Put the net figure on the delivery note. How to estimate steel plate cost from weight covers where the gross weight sits in a full price build-up, including the scrap credit that recovers a fraction of it.

Part orientation changes the answer

Nesting is a packing problem, and small changes in how parts are laid out move the yield more than most people expect.

Known values: 800 × 600 mm rectangular parts on a 2000 × 6000 mm plate, cut in a simple grid with no allowance for kerf or edge trim.

Parts laid 800 across, 600 along:

across = 2000 ÷ 800 = 2 whole parts
along  = 6000 ÷ 600 = 10 whole parts
total  = 20 parts, 9.60 m2
utilisation = 9.60 ÷ 12.00 = 80.0%

Parts rotated, 600 across, 800 along:

across = 2000 ÷ 600 = 3 whole parts
along  = 6000 ÷ 800 = 7 whole parts
total  = 21 parts, 10.08 m2
utilisation = 10.08 ÷ 12.00 = 84.0%

Result: rotating the part gains one part and 4 percentage points of yield from the same plate. Interpretation: the gain comes from the width fitting three times instead of two, which leaves 200 mm of waste across the width instead of 400 mm. Reverse check: 21 × 0.48 m² = 10.08 m².

That is the simplest case there is, two orientations of one rectangle, and it still moves the answer by 4 points. Real nests mix part sizes, allow common-line cutting, respect grain direction where it matters and work around plate defects. Which is the point of the next section.

What a calculated yield cannot do

A utilisation figure worked out by hand is an estimating input. It is not a nest.

Real nesting software accounts for part geometry rather than bounding boxes, rotation in arbitrary increments, common-line cutting where two parts share an edge, lead-in and lead-out geometry, part-to-part and part-to-edge spacing set by the cutting process, thermal distortion management, cut sequencing, remnant generation and machine table limits. Several of those change the achievable yield by more than the difference between two hand estimates.

Use a calculated figure for three things: pricing before the nest exists, comparing plate sizes against each other, and sanity-checking a nest that comes back from software with a surprising number on it. Do not use it as a substitute for running the nest.

Kerf is real and small

Kerf removes a strip of material along every cut path. Its width depends on the process and the machine, and it should come from the equipment rather than from a table.

Known values: 40 parts with a 2.4 m cut perimeter each, 96 m of total cut path, in 12 mm carbon steel.

Interpretation: across a package with 886.6 kg of gross plate, even a wide kerf accounts for about 3% of the purchase and a narrow one for 0.2%. Reverse check: 96 m × 0.003 m = 0.288 m², and 0.288 × 12 × 7.85 = 27.1 kg.

Kerf matters more for part accuracy than for tonnage. Where it does bite on material is indirectly: a wide kerf forces larger part-to-part spacing, and the spacing costs more area than the kerf itself.

Plate size is a yield decision

The choice of stock plate often moves utilisation more than the nesting does, because it changes what fits.

Known values: a job needing 40 m² of 10 mm plate, offered in two stock sizes.

1500 × 3000 mm: 4.50 m2 each, 40 ÷ 4.50 = 8.89 -> 9 plates = 40.50 m2 = 3,179 kg
2000 × 6000 mm: 12.00 m2 each, 40 ÷ 12.00 = 3.33 -> 4 plates = 48.00 m2 = 3,768 kg

Result: the larger plate buys 589 kg more steel, 18.5% more, for the same requirement. Interpretation: big plates only win when the parts fill them. The last plate in a set is where the yield is decided, and a large last plate that is barely touched is expensive. Reverse check: 48.00 ÷ 40.50 = 1.185.

Run both options in the Steel Plate Weight Calculator with a quantity before choosing a stock size. Standard steel plate sizes lists what is available to choose between.

Building the purchasing allowance

A defensible allowance has four parts, and it is worth writing them separately so each can be challenged.

Nesting yield, from the software if a nest exists, from experience with similar work if it does not. This is the big one.

Edge trim, where the plate edge cannot be used because of edge camber, out-of-squareness or surface condition. EN 10029 only guarantees that a rectangle of the ordered size can be inscribed in the delivered plate, so the usable area is the ordered area, not the delivered one.

Rework and rejects, an allowance for parts that have to be recut. It is usually small and it is usually not zero.

Whole-plate rounding, because you cannot buy 3.33 plates. This is frequently the largest single item on a small job and the one most often forgotten.

Those four multiply and add in different ways, so write the calculation out rather than collapsing it into one percentage. A quotation that says “material 886.6 kg gross, from 753.6 kg net at 85% yield, rounded to 2 plates” can be reviewed by someone else. One that says “material 900 kg” cannot.

Where gross and net go wrong

Quoting on net weight. The single most expensive error in plate estimating. The supplier invoices the plate; the calculator, run on part dimensions, returns the parts.

Using one yield percentage for every job. Utilisation depends on part geometry, part mix, plate size and quantity. A figure carried from a job of large rectangles onto a job of small brackets will be far too optimistic.

Counting remnants as recovered at full value. A remnant is only worth its purchase price if it is actually used on another job at that thickness and grade. Racked and forgotten, it is scrap that has been paid to store.

Treating scrap credit as recovering the loss. Scrap sells well below plate price. In a typical build-up the credit returns a small fraction of what the drop material cost to buy, which is why yield matters far more than scrap price.

Forgetting the last plate. Rounding up to whole plates is where small jobs lose their margin.

Nesting yield and scrap FAQs

How do I calculate nesting yield? Divide the total net area of the finished parts by the area of the plate purchased, and multiply by 100. Scrap percentage is 100 minus that figure. Working in area rather than weight keeps thickness and density out of the calculation.

How do I convert net weight to gross weight? Divide the net weight by the utilisation expressed as a decimal. Parts weighing 753.6 kg at 85% utilisation need 886.6 kg of plate.

What is a good plate utilisation percentage? It depends entirely on the parts. Simple rectangles nested well can exceed 90%, complex profiles with large cut-outs run far lower, and a mixed nest commonly lands in the seventies or eighties. Compare against your own history for similar work rather than against a published figure.

Does kerf matter for material estimating? Barely. Across a realistic package a wide kerf accounts for roughly 3% of the plate purchased and a narrow one for a fraction of a percent. It matters more for part accuracy and for the part spacing it forces.

What is the difference between skeleton and remnant? Skeleton is the web of material between and around parts, which is scrap. A remnant is a usable piece large enough to go back on the rack for another job. Whether an offcut is one or the other depends on whether the shop actually re-uses it.

Buy the plate, ship the parts

The gap between those two is not waste in the sense of a mistake. It is an unavoidable cost of cutting shapes out of rectangles, and the only question is whether it appears in the price or comes out of the margin.

Calculate the net weight of the parts, divide by a yield you can justify, round up to whole plates, and quote that. The Steel Plate Weight Calculator gives both figures: run the part dimensions for net, and the stock plate dimensions with a quantity for gross.

Weights in this article are calculated at 7,850 kg/m³, the density adopted in ASTM A6/A6M for rolled steel. Utilisation and kerf figures are worked examples with stated assumptions, not measurements of any particular process; take kerf from your own equipment and yield from your own nesting software.