A book depreciation useful life is management’s estimate of how long a tangible asset will produce economic benefit for your business, and under U.S. GAAP you spread the asset’s cost across that period so expense lands in the same periods as the revenue the asset helps generate. The life you assign is not the asset’s total physical lifespan. It’s how long you expect to use it before retirement, replacement, or disposal, and it drives every dollar of depreciation expense that follows.
What Counts as a Depreciable Asset
Before you assign a life, confirm the item belongs on the balance sheet. GAAP requires capitalization when a purchase is material to the financial statements and has a useful life longer than one year. Companies set their own dollar thresholds. A small business might expense anything under $500; a large corporation might draw the line at $5,000 or $10,000. Pick a threshold and apply it consistently to every purchase.
Land is the one major exclusion. Because it doesn’t wear out or become obsolete, GAAP prohibits depreciating it. When you buy property that includes both a building and the ground under it, you allocate the purchase price between the two and depreciate only the building. Land stays on your books at original cost indefinitely.
How to Estimate Useful Life
Three factors shape the estimate. The one that dominates depends on the asset.
Physical deterioration. Wear and tear from daily use, exposure to the elements, and operating intensity all shorten the service period. A company running manufacturing equipment around the clock will assign a much shorter life than one running the same machine on a single daytime shift.
Functional obsolescence. This often overrides physical durability, especially for technology. Server hardware might run fine for a decade, but if newer systems deliver meaningfully better performance at lower cost within three years, the older equipment becomes economically impractical well before it breaks. Any asset tied to rapidly evolving technology should be evaluated with obsolescence as the primary constraint.
Contractual and legal limits. These cap the life regardless of physical condition. If you lease equipment under a five-year agreement requiring return at the end of the term, the useful life to your company can’t exceed five years. Leasehold improvements follow a similar rule: depreciate them over the shorter of the improvement’s own useful life or the remaining lease term. The exception is when the lease transfers ownership or you’re reasonably certain to exercise a purchase option, in which case you can depreciate the improvement over its full useful life.
A single asset sometimes contains parts with meaningfully different lives. A commercial building’s roof might last 20 years while its HVAC system needs replacement in 12. GAAP permits, but does not require, component depreciation, where each identifiable part with a distinct life is depreciated separately. It produces more accurate expense recognition and more recordkeeping, so most companies reserve it for high-value assets with clearly divergent components.
Typical Book Depreciation Life Ranges
Useful life is always a company-specific judgment, but industry practice clusters around predictable ranges. Use these as a starting point, then adjust for your own operating conditions.
- Buildings and structures: 20 to 40 years for most commercial buildings, with fire-resistant construction pushing toward 50 or more. Lightweight structures like sheds or temporary facilities might warrant 10 to 15 years.
- Machinery and production equipment: 5 to 15 years, with heavy industrial equipment trending longer and technology-dependent manufacturing systems trending shorter.
- Office furniture and fixtures: 7 to 10 years, balancing physical durability against the likelihood of replacement for aesthetic or functional reasons.
- Computer hardware and software: 3 to 5 years for desktops, laptops, and standard software licenses. Enterprise systems and mainframes sometimes extend to 7 years.
- Vehicles: 4 to 8 years, depending on expected mileage and replacement policy. Light vehicles used for high-mileage sales routes land at the short end; heavier trucks with lower annual miles land at the long end.
Salvage Value and the Depreciable Base
Salvage value is what you expect to recover at the end of the useful life through resale, trade-in, or scrap. Subtract it from original cost to get the depreciable base, which is the total you’ll expense over the life.
Factor disposal costs into the estimate. If selling old equipment costs $2,000 in removal and transport and you’d receive $8,000 from the buyer, net salvage value is $6,000. Salvage value can never go below zero, even if disposal costs exceed recovery.
Many companies set salvage value at zero for simplicity, particularly for assets they intend to use until the end of their economic life. Technology that will be worthless by retirement justifies a zero estimate. But for vehicles and heavy machinery with established resale markets, ignoring salvage value overstates depreciation expense and understates the asset’s carrying value.
Choosing a Depreciation Method
The life you assign works together with the method you pick. GAAP doesn’t mandate a specific method, only that the one you choose be systematic and rational, meaning it reflects how the asset delivers its economic benefit over time.
Straight-Line
The most widely used method in financial reporting. Subtract salvage value from cost, divide by useful life in years, and expense the same amount every period. A $50,000 machine with $5,000 salvage value and a 10-year life produces $4,500 in depreciation each year. Straightforward to forecast, and appropriate for assets that deliver roughly consistent value across their lives.
Double Declining Balance
An accelerated method that front-loads expense. Apply twice the straight-line rate to the asset’s remaining book value each year. For a five-year asset, the straight-line rate is 20%, so the DDB rate is 40%.
You don’t subtract salvage value before calculating each year’s charge. Instead, salvage acts as a floor: depreciation stops once book value reaches the salvage amount. In practice, the declining balance eventually produces smaller annual charges than straight-line would for the remaining life, so most companies plan from the outset to switch to straight-line at the crossover point. Because the switch is part of the original plan, it isn’t treated as a change in method.
Sum-of-the-Years’ Digits
Another accelerated method, less common than DDB. For a five-year asset, add the year numbers (5 + 4 + 3 + 2 + 1 = 15). Year one depreciates 5/15 of the depreciable base, year two 4/15, and so on. The fraction shrinks each year. Well suited to assets like vehicles that lose disproportionate value early but still contribute meaningfully later.
Units of Production
Ties depreciation to actual usage rather than the calendar. Divide the depreciable base by total estimated lifetime output for a per-unit rate, then multiply by the units produced each period. A press rated for 500,000 impressions with a $100,000 depreciable base costs $0.20 per impression; a year of 80,000 impressions produces $16,000 in expense. Ideal for manufacturing equipment, mining assets, or anything where wear correlates more closely with output than with time. Expense fluctuates with volume, which can complicate period-to-period comparisons.
When the Clock Starts and Stops
Depreciation begins when the asset is available for its intended use, meaning it’s in the location and condition necessary to operate the way management intends. You don’t wait for actual production. If a machine is installed, tested, and ready to run on March 15 but you don’t start using it until April, depreciation still begins in March.
It ends when the asset is fully depreciated to salvage value, disposed of, or reclassified as held for sale, whichever comes first. An asset that remains physically present but is fully depreciated stays on the balance sheet at salvage value (often zero) with accumulated depreciation equal to the depreciable base. It doesn’t come off the books until actual retirement or disposal.
Assets rarely arrive on the first day of a fiscal year, so companies adopt conventions for partial periods. The half-year convention assumes every asset is placed in service at the midpoint of the year and gives half a year’s depreciation in both the first and last year. The mid-month convention assumes placement in the middle of the actual month of acquisition. Some companies prorate to the exact day. GAAP doesn’t prescribe a convention, but whatever you choose must be applied consistently across similar assets.
Revising the Useful Life Later
Circumstances change. A machine you expected to last 10 years might show accelerated wear at year four, or a technology shift might extend an asset’s relevance beyond your original estimate. When that happens, you revise the useful life or salvage value as a change in accounting estimate.
You do not go back and restate prior financial statements. The original estimate was reasonable when made, and GAAP treats it as correct for those earlier periods. Take the current book value, subtract any revised salvage value, and spread the remaining depreciable amount over the new remaining useful life. Only current and future periods are affected.
Changing the method itself is a higher bar. GAAP treats a method change as inseparable from a change in estimate, but you must justify that the new method is preferable because it better reflects the pattern of economic benefits. You can’t switch from accelerated to straight-line simply because it produces a more favorable expense number. Like a life revision, the change applies prospectively.
Book Life Is Not Tax Life
Book depreciation reflects management’s estimate of an asset’s actual economic life. Tax depreciation follows the Modified Accelerated Cost Recovery System, which assigns assets to fixed recovery period classes under the Internal Revenue Code and often accelerates deductions to encourage capital investment.1Office of the Law Revision Counsel. 26 U.S. Code 168 – Accelerated Cost Recovery System
The two systems almost always produce different numbers for the same asset. A company might depreciate equipment over 10 years on its books using straight-line while deducting it over 7 years on its tax return, or faster with bonus depreciation. Total depreciation over the asset’s full life is the same under both systems, but the timing gap creates temporary differences and deferred tax liabilities on the balance sheet. When you’re setting a book useful life, set it based on economic reality for your business. The tax recovery period is a separate calculation.