Line Cost & Quoting

What Drives Cost Per Unit on a Production Line and How to Quote It

How cost per unit is really built on an assembly or conveyor line: direct labor spread over throughput, machine and conveyor burden, balance loss, scrap, and overhead, with the numbers that make a quote hold.

Cost per unit on a line starts with direct labor divided by throughput, and the divisor is where money leaks. If 8 operators cost a fully loaded 42 dollars per hour and the line ships 120 good units per hour, labor is 8 x 42 / 120 = 2.80 dollars per unit. Quote against the good rate, not the nameplate rate: if real throughput is 102 units per hour after downtime, the same crew costs 8 x 42 / 102 = 3.29 dollars, a 49 cent per unit miss that vanishes on a full order. Pull the crew count from the line balance and the good rate from actual throughput, not the equipment brochure.

Balance loss is a labor cost driver that estimators routinely forget to price. A line balanced at 87.5% efficiency wastes 12.5% of every paid station hour on idle time waiting for the bottleneck. On that same 8 operator, 42 dollar crew, the idle fraction is 8 x 42 x 0.125 = 42 dollars per hour of pure waste, or 0.35 dollars per unit at 120 per hour. Quoting as if all stations run at 100% understates labor by exactly the balance loss. Use the Line Balance efficiency figure to load the crew cost honestly, and note that adding a ninth station to relieve the bottleneck may cost less per unit than the idle time it removes.

Machine and conveyor time carries its own burden rate, separate from labor. A machine rate bundles depreciation, energy, maintenance, and floor space into an hourly figure, commonly 15 to 60 dollars per hour for conveyor and assembly equipment. At a 35 dollar per hour line burden and 120 units per hour, machine cost is 35 / 120 = 0.29 dollars per unit. Energy alone on a 15 kW conveyor and drive train at 0.12 dollars per kWh is 15 x 0.12 = 1.80 dollars per hour, or 1.5 cents per unit. Spread the burden over the good rate from Throughput, and confirm the equipment is actually loaded using Machine Utilization before you assume the hourly rate is fully absorbed.

Scrap and rework compound every upstream cost. A part scrapped at final station has absorbed all the labor and machine cost of the line, so a 3% scrap rate on a 6.00 dollar unit does not cost 0.18 dollars, it costs the full loaded value of what was thrown away plus the replacement's cost to reach the same point. Price it as unit cost divided by (1 minus scrap rate): 6.00 / (1 minus 0.03) = 6.19 dollars, a 19 cent uplift. Drop scrap to 1% and the uplift falls to 6 cents. Track first pass yield per station so you scrap early and cheap rather than late and fully loaded.

Overhead and non-productive time sit on top of the direct numbers. Changeover between SKUs is unbilled line time: a 25 minute changeover on a line worth 8 x 42 plus 35 machine burden, roughly 371 dollars per hour, costs 371 x 25 / 60 = 155 dollars, spread across the batch. On a 900 unit batch that is 0.17 dollars per unit; on a 200 unit batch it balloons to 0.77 dollars. Small orders carry setup badly, which is why quote logic must divide fixed setup by the order quantity. Add plant overhead as a percentage of direct cost, commonly 20 to 40%, applied after labor, machine, and scrap are summed.

Build the quote as a stack so each driver is visible and defensible. Take direct labor 2.80, balance loss 0.35, machine burden 0.29, energy 0.02, scrap uplift 0.19, and setup amortization 0.17, summing to 3.82 dollars of direct cost per unit. Apply 30% overhead: 3.82 x 1.30 = 4.97 dollars. Add target margin of 15%: 4.97 / (1 minus 0.15) = 5.85 dollars quoted. Every line ties to a source: labor to crew and Throughput, balance loss to Line Balance, burden to Machine Utilization, setup to batch size. A buyer who challenges the price can see exactly which driver moves it.

Estimates go wrong in predictable places, and each costs real money. Quoting at nameplate speed instead of the bottleneck rate is the most common error: if Bottleneck Capacity caps the line at 102 units per hour but the quote assumed 120, every per-unit cost is understated by 18%. Ignoring ramp, where a new line runs at 60 to 70% of steady rate for the first weeks, understates early cost by a third. Forgetting that scrap absorbs full upstream cost, not material cost, understates yield loss by 3 to 5x. And spreading setup over an assumed large batch when the real order is small can turn a profitable quote into a loss.

Sensitivity matters more than precision on a line quote. The two variables that swing cost per unit hardest are good throughput and scrap rate, because both are divisors or near divisors. A 10% drop in good throughput raises labor and burden cost per unit by about 11%; a scrap rate moving from 1% to 5% adds roughly 4% to loaded unit cost. Setup amortization dominates only on small batches, becoming negligible above roughly 1,000 units. Quote a rate you can hold in steady state, verify it against Cycle Time and Throughput before signing, and carry a contingency of 5 to 10% for the ramp period rather than burying it invisibly in margin.

Published 2026-07-01.