Units of Production

The Mathematics Behind Usage-Based Depreciation

The units-of-production method anchors depreciation expense to actual asset utilisation rather than the calendar. The core formula: per-unit depreciation rate = (Cost − Salvage Value) ÷ Estimated Total Lifetime Units. Annual depreciation = per-unit rate × Units Produced That Year. For a mining shovel costing $4.8 million with a $200,000 salvage value and estimated lifetime capacity of 9,200,000 tonnes, the per-tonne rate is ($4.8M − $200K) ÷ 9,200,000 = $0.50 per tonne. In a year where the shovel moves 650,000 tonnes, depreciation is exactly $325,000. If output falls to 300,000 tonnes the following year due to planned maintenance, depreciation drops proportionally to $150,000 — automatically matching expense to production revenue. Total accumulated depreciation can never exceed the depreciable amount regardless of actual output; if actual lifetime production exceeds the original estimate, the asset stops depreciating at the ceiling.

Real-World Applications: Mining, Transport, and Printing

The units-of-production method has deep roots in capital-intensive industries. In mining, draglines, haul trucks, and processing mills are depreciated per tonne extracted or processed — aligning the deduction with mineral sales revenue. Both US GAAP (ASC 360-10) and IFRS (IAS 16) explicitly permit this approach for qualifying assets. In long-haul trucking, owner-operators calculate depreciation per mile driven: a semi-truck with a 750,000-mile expected life provides a fixed cost per mile that feeds directly into freight rate pricing models. Printing presses are commonly depreciated per thousand impressions, enabling print shops to set per-job costs with precision. Oil and gas companies routinely use units-of-production for depletion of proven reserves, where the "units" are barrels of oil or MCF of gas, and the reserve estimates are updated annually based on new geological data.

Capacity Estimation and Prospective Revision

The accuracy of units-of-production depreciation depends entirely on the quality of the initial capacity estimate. Manufacturers typically provide expected lifetime output in technical specifications — a furnace rated for 20,000 operating hours, a press brake for 2 million cycles. These assume manufacturer-recommended maintenance under standard operating conditions. When material differences emerge between actual and estimated capacity, both FASB (ASC 250) and IASB (IAS 8) require prospective revision: the remaining undepreciated balance is spread over revised remaining capacity from the revision date. Companies that systematically over-estimate capacity understate annual depreciation expense, inflating reported profits and carrying overstated asset values — a specific risk that auditors test through asset life and utilisation reviews, particularly for oil and gas reserve depletion where reserve estimates can change substantially between annual assessments.