Useful Lives, Salvage Value and Depreciation Policy

A company buys a piece of equipment for $10m. It doesn't expense the whole $10m in year one — it spreads that cost over the years the equipment is expected to generate revenue, an allocation called depreciation. But "expected to generate revenue for how long" is an estimate, not a fact, and that single estimate — the useful life — together with the salvage value (what the asset is assumed to be worth when retired) entirely determines how much depreciation expense hits the income statement each year.

The two assumptions that drive the number

Useful life is management's estimate of how many years an asset will remain productive before it's retired or replaced — five years for a fleet of delivery vans, thirty years for a building, and so on. Salvage value is the estimated resale or scrap value at the end of that life. Depreciation expense each year is built from spreading (cost minus salvage value) across the useful life, using a method like straight-line (equal amounts every year) or an accelerated method (larger amounts early on). Neither useful life nor salvage value is observable in advance — both are estimates the company chooses, within limits set by accounting standards, and both estimates directly set the size of the expense that reduces reported net income.

A concrete example

A $10m machine with an assumed 10-year useful life and $0 salvage value depreciates at $1m per year under straight-line. If management instead assumes a 20-year useful life for the identical machine, the annual depreciation expense is only $500,000 — half as much — which mechanically raises reported net income by $500,000 a year with absolutely no change in the machine's actual operation or cash flows. Extending assumed useful lives, all else equal, is one of the more common ways reported earnings can be flattered without touching the business itself.

What this means in practice

Because useful life and salvage value are estimates rather than hard facts, analysts specifically compare a company's depreciation assumptions against industry peers — a company depreciating its trucks over 15 years when competitors use 8 years is either genuinely running a different fleet, or is quietly boosting reported earnings. Changes in useful-life assumptions have to be disclosed, and a sudden lengthening of assumed asset lives, especially right before a period where earnings need a boost, is a classic earnings-quality red flag worth checking in the footnotes before trusting the reported margin.