Data Desk

The Pass-Through Machine: How London Copper Becomes Your Mill Invoice

Global copper and the price American mills actually charge are two readings on the same metal, and the gap between them is a margin map. Here is how to read the pass-through, and what the markup ratio says about your next copper buy.

Refined copper traded at $13,543/tonne as of Jul 2026, up 38.6% from a year earlier. The producer price index for copper mill shapes, the Bureau of Labor Statistics series tracking what domestic mills charge for the sheet, strip, rod, wire, and tube made from that metal, stood at 542.85index (1982=100) in Jul 2026. Those are two readings on the same atom, taken on opposite sides of an industrial machine that turns exchange contracts in London into mill invoices in the Midwest. Across the 415 overlapping months from January 1992 through July 2026, 34 years of shared history, the two series' monthly moves have run at a correlation of 0.74. That figure is the pass-through machine, measured. What it leaves unexplained is where your margin lives.

One metal, two prices

The London Metal Exchange price is the world's clearing price for refined copper, quoted in dollars per tonne and moved by every smelter outage, grid project, and warehouse drawdown on the planet. The mill-shapes PPI is a different animal: a monthly BLS survey of what U.S. producers actually receive for copper that has been melted, rolled, drawn, and cut into product. One is a commodity. The other is a commodity plus a factory, with the factory's electricity, wages, and pricing power folded into the number. Neither is the price on your purchase order. But your purchase order is built out of both, and the relationship between them is stable enough to be used.

A correlation of 0.74 in monthly percent changes, held across 415 months, is a strong result for economic data. It says the metal does most of the work: when London moves, the mill price tends to move in the same month. But the correlation is well short of perfect, and the residual is fabrication economics running on its own clock: energy and labor costs at the mill, capacity tightness, scrap spreads, tariff policy. The co-movement means copper risk cannot be managed out of a mill-products contract by watching the exchange alone. The residual means there is a second negotiation hiding inside every invoice.

The archived history sketches the scale of the ride. The exchange price has run from $9,531 in May 2025 to $13,552 in June 2026; the mill index from 316.45 in May 2025 to 559.59 in May 2026. The two series carry the same broad story, but they have never carried it identically, and the difference between them is not noise. It is a measurement of who is winning.

The markup ratio

Divide the exchange price by the index level and you get a strange unit, LME dollars per PPI point, useless as a price and valuable as a diagnostic. In January 1992, the first month the two series overlap, the ratio stood at 21.3. Across the full window it has averaged 26.7. With copper at $13,543/tonne over a mill index of 542.85index (1982=100), it now sits at 24.9, below the long-run mark. The reading rule is simple. A falling ratio means the domestic index is rising faster than the metal itself: fabrication is capturing more of the move. A rising ratio means the metal is doing the work and the mill's share of each dollar is thinning. Where the ratio sits against 26.7 tells you which margin regime you are quoting in.

The mill markup ratio: LME dollars per PPI point: 24.9. Stood at 21.3 in January 1992 and averaged 26.7 across the 415 overlapping months through July 2026. A lower reading means mills are capturing more of each move.

Correlation of monthly moves, LME copper vs the copper PPI: 0.74. Computed on monthly percent changes over the 415 overlapping months from January 1992 through July 2026 (LME, BLS).

The correlation says the metal moves your invoice. The ratio says who kept the difference.

What the wedge costs a copper shop

Put numbers on it. A busbar or transformer shop converting 20 tonnes of copper a year holds about $270,856 of metal at the current exchange price before a mill ever touches it. Now suppose that shop buys $500,000 of copper mill products annually, priced off the domestic index, and that its supplier's pricing tracks the PPI, which the 0.74 correlation says it broadly does. With the ratio at 24.9 against the long-run average of 26.7, roughly 6.6% of that invoice, about $32,819 a year, is fabrication priced above the historical norm, the premium implied by holding the metal price fixed and letting the ratio revert to its average. A metal-only escalator clause will never claw that back, because the metal did not cause it. This arithmetic is a model, not an audit; alloy, gauge, order size, and freight all move the number for any particular shop. But the direction comes straight from the ratio, and the direction is what you take into the meeting.

What to do with the number

Treat the two series as two line items, because that is what they are. First, split any copper-bearing contract into a metal escalator and a conversion price: index the metal to the exchange, where both sides can watch it, and negotiate fabrication as its own number with its own justification. Second, time the conversion conversation with the ratio. When it sits below the long-run average, mills are capturing an unusually large share of each move, and fabrication adders are where the recoverable money is at renewal. When it sits above, lock conversion pricing while the regime favors you and let the escalator carry the metal.

Third, audit what already happened. Run your last year of copper invoices through the material price variance calculator, splitting the metal's move from the total change; what remains is the mill's share, the part the exchange never explains. That residual is the negotiable portion of your copper bill. Re-quote anything priced before the last big move in either series, and let the ratio, not the headline, tell you which side of the invoice to push on.

Feed a real invoice into the material price variance calculator to separate what the exchange did from what your supplier did, and see which share grew. Split the metal from the mill

Published 2026-08-18.