What straight-line depreciation actually measures
Straight-line depreciation answers one question: how much of a long-lived asset did you consume this year? When you buy a $50,000 delivery van you do not have a $50,000 expense — you have swapped one asset (cash) for another (a van). The expense arrives gradually, as the van wears out. Depreciation is the accounting mechanism that moves cost off the balance sheet and onto the income statement in step with the benefit you receive.
The straight-line method assumes that benefit arrives evenly. Each year of a five-year life consumes exactly one fifth of the asset. That assumption is crude, but it is defensible for assets whose output does not decline with age — office fit-outs, furniture, buildings, software licences, most leasehold improvements — and its simplicity is worth a great deal when you have several hundred assets on a fixed-asset register.
Three numbers define the entire schedule. Cost is everything you capitalised: invoice price, freight, non-refundable taxes, installation, testing, and any professional fees required to bring the asset to working condition. Salvage value is what you expect to recover at retirement, net of the cost of removing and selling it. Useful life is how long you will use it, which is often much shorter than how long it could physically last — a laptop with a seven-year physical life gets a three-year book life because that is your replacement cycle.
Under US GAAP, ASC 360 requires that the depreciable amount of property, plant and equipment be allocated systematically over its useful life; IAS 16 imposes the same requirement under IFRS and additionally requires that you review residual value and useful life at least at each annual reporting date. Neither standard mandates straight-line — they mandate that your method reflect the pattern in which the asset's economic benefits are consumed, and require you to disclose which method you chose.
The formula, one variable at a time
Annual depreciation is the depreciable base divided by the useful life: D = (C − S) / n. Everything interesting is in the numerator.
Why subtract salvage. Depreciation charges the portion of cost you will never get back. If you buy a $50,000 van and expect to sell it for $5,000 in five years, the van costs you $45,000 to use, not $50,000. Charging the full $50,000 would overstate expense during the life and then force a $5,000 gain on disposal at the end — moving real operating cost into a one-off gain line. Subtracting salvage keeps the expense where it belongs. Note the asymmetry with tax depreciation: MACRS ignores salvage value entirely and always depreciates the full basis to zero, which is one of several reasons your book and tax schedules will not agree.
Why divide by years and not by output. Dividing by n is the assertion that time, not usage, drives consumption. If an asset's value is destroyed by usage rather than by the calendar — a die, a haul truck, an aircraft engine — a units-of-production method fits better, and if obsolescence front-loads the loss, the double declining balance method fits better.
Why the first year is usually not a full year. Assets rarely go into service on the first day of the fiscal year. Most companies prorate by whole months from the in-service date, which pushes the tail of the charge into an extra fiscal year: a five-year asset placed in service in September of a calendar-year company runs across six fiscal years. Some companies use a half-year convention instead and charge six months in the first year and six in the last, which keeps the paperwork simpler at the cost of a small timing difference. Set the months field to 6 to reproduce that convention.
Once you have D, the rest is bookkeeping. Accumulated depreciation is the running total of every charge posted so far; it is a contra-asset account, so it appears on the balance sheet as a deduction from gross PP&E. Book value, also called carrying amount or net book value, is cost minus accumulated depreciation. Straight-line book value falls in a perfect line from cost to salvage, which is where the method's name comes from.
Worked example: a $50,000 van with $5,000 salvage over five years
You buy a delivery van. The invoice is $47,200, delivery costs $1,300, and the shelving you install to make it usable costs $1,500. You expect to run it for five years and sell it for about $5,000 net of the cost of listing and cleaning it. The van goes into service on 1 January, so the first year is a full year.
- Capitalise the full cost. C = 47,200 + 1,300 + 1,500 = $50,000. The shelving is capitalised because it is required to put the van into the condition needed for its intended use; the fuel you put in it on day one is not.
- Find the depreciable base. C − S = 50,000 − 5,000 = $45,000. That is the total that will ever hit the income statement.
- Divide by useful life. D = 45,000 ÷ 5 = $9,000 per year.
- Convert to a monthly posting. 9,000 ÷ 12 = $750 per month. This is the number your accounting system actually books, as a monthly journal entry debiting depreciation expense and crediting accumulated depreciation.
- Roll the schedule forward. End of year 1: accumulated 9,000, book value 41,000. Year 2: accumulated 18,000, book value 32,000. Year 3: accumulated 27,000, book value 23,000. Year 4: accumulated 36,000, book value 14,000. Year 5: accumulated 45,000, book value $5,000 — exactly the salvage estimate, as it must be.
Now change one thing: the van goes into service on 1 September instead, leaving four months in the first fiscal year. Year 1 charges 9,000 × 4 ÷ 12 = $3,000. Years 2 through 5 charge the full $9,000 each, totalling 3,000 + 36,000 = $39,000 after five fiscal years. The remaining 45,000 − 39,000 = $6,000 falls into a sixth fiscal year, which is the eight months of charge that the first year did not take. The total is unchanged at $45,000; only the timing moved.
Finally, test the disposal. If you actually sell the van for $6,200 at the end of year 5, book value is $5,000, so you record a $1,200 gain on disposal. If you sell it for $3,000, you record a $2,000 loss. That gain or loss is the correction for an imperfect salvage estimate, and it is why a large disposal gain is evidence that your useful lives were too short.
How to read the result
Compare the annual charge against the cash you expect the asset to generate, not against your intuition about its price. An asset that depreciates $9,000 a year needs to contribute more than $9,000 a year of operating margin before it is earning its keep in accounting terms — and more than that in cash terms, because the purchase consumed cash up front while depreciation only allocates it afterwards. That gap between the expense and the cash timing is why depreciation is added back in the free cash flow bridge and why capital expenditure is subtracted separately.
Watch the ratio of the annual charge to gross PP&E. That ratio is your composite depreciation rate, and its reciprocal is the implied average life of the fixed-asset base: a charge equal to 10% of gross PP&E implies an average ten-year life. If that figure drifts down over several years while the asset mix has not changed, someone has been lengthening useful lives, which flatters operating income today at the cost of larger write-offs later. Auditors and analysts both check this, and it is one of the cheapest earnings-quality tests there is.
Book value is not market value and is not intended to be. A five-year-old server with $200 of book value may be worth nothing; a fully depreciated building may be worth more than it cost. Depreciation is a cost allocation, not a valuation. When an asset's recoverable amount falls below its carrying amount, the correct response is an impairment test under ASC 360-10 or IAS 36, not an adjustment to the depreciation rate. Depreciation also flatters return on assets as an asset base ages: accumulated depreciation shrinks the denominator every year, so the ratio drifts upward on an ageing asset base with no improvement in the business behind it.
One number to sanity-check every time: the salvage estimate. Set it too high and you under-depreciate and take a loss on disposal; set it to zero out of habit and you overstate expense for assets with real resale markets, such as vehicles and construction plant. If salvage is more than roughly half of cost, the asset is barely depreciating at all, and this calculator flags it.
Annual depreciation rate and charge by useful life
| Useful life (years) | Annual rate | Annual charge per $10,000 of base | Typical assets |
|---|---|---|---|
| 3 | 33.333% | $3,333.33 | Laptops, tablets, tooling |
| 5 | 20.000% | $2,000.00 | Vehicles, servers, office equipment |
| 7 | 14.286% | $1,428.57 | Furniture, general machinery |
| 10 | 10.000% | $1,000.00 | Heavy plant, fit-outs |
| 15 | 6.667% | $666.67 | Land improvements, site works |
| 20 | 5.000% | $500.00 | Utility and distribution assets |
| 27.5 | 3.636% | $363.64 | Residential rental buildings (the MACRS life) |
| 39 | 2.564% | $256.41 | Non-residential buildings (the MACRS life) |
| 40 | 2.500% | $250.00 | Long-lived structures under book policy |
Rates are 1 ÷ life, rounded to three decimals; charges are the exact rate applied to $10,000. The 27.5-year and 39-year rows are the recovery periods US tax law assigns to residential and non-residential real property, which many companies also adopt for book purposes.
Mistakes that make a straight-line schedule wrong
- Depreciating land. Land has an indefinite life and is never depreciated. When you buy a building, you must split the purchase price between land and structure — usually on the property tax assessor's ratio — and depreciate only the structure.
- Expensing costs that belong in the asset. Freight, installation, testing and site preparation are part of cost. Training your staff to use the machine is not. Getting this boundary wrong changes both the expense in year one and every year after.
- Forgetting the partial first year. An asset placed in service in November has two months of service, so a full year of charge overstates year-one expense by ten twelfths of the annual amount — 83% of it — and the overstatement does not reverse, because the schedule simply ends a year early relative to the asset's actual service.
- Using the tax life for the books. MACRS recovery periods are statutory, ignore salvage value, and are deliberately shorter than economic life. Using them for financial reporting is common in small businesses but is not GAAP unless the difference is immaterial. See the MACRS calculator for the tax schedule.
- Never revisiting the estimate. IAS 16 requires an annual review of useful life and residual value, and ASC 250 treats a change in either as a change in accounting estimate: you spread the remaining book value less the new salvage over the remaining life prospectively, and you never restate prior years.
- Depreciating past salvage. Once book value reaches the residual estimate, the charge stops even if the asset is still in use. A fully depreciated asset stays on the register at cost with equal accumulated depreciation until it is disposed of.
When another method fits better
Choose straight-line when the asset delivers roughly constant service each year and you cannot measure its output directly. Choose an accelerated method when the asset loses value fastest at the start, or when repair costs rise steeply with age — combining an accelerated charge with rising maintenance produces a flatter total cost of ownership than straight-line does. The double declining balance calculator builds that schedule at 200% or 150% of the straight-line rate and switches back to straight-line at the optimal year.
Choose units of production when output is measurable and variable: charge per mile driven, per hour run, or per tonne extracted. The formula becomes (C − S) × units this period ÷ total expected units, which tracks the economics of a mine or a fleet far better than a calendar ever will.
For US tax returns, you generally have no choice at all. MACRS is mandatory for most tangible property placed in service after 1986, and it is not straight-line: it is 200% or 150% declining balance switching to straight-line, over statutory recovery periods, with salvage value ignored and a half-year or mid-quarter convention applied. The gap between your book schedule and your MACRS schedule creates a temporary difference and a deferred tax liability that unwinds over the asset's life. If you need the tax numbers, use the MACRS depreciation calculator; if you want a longer worked schedule with month-level detail, the straight-line depreciation schedule calculator covers that ground.
Depreciation policy also feeds directly into capital budgeting. The depreciation tax shield — the charge multiplied by your marginal tax rate — is a real cash flow, and its timing changes the answer a net present value calculation gives you. Faster depreciation moves that shield earlier and raises NPV, which is precisely why accelerated tax methods exist as a policy instrument.
Key terms
- Depreciable base
- Cost less salvage value. The total amount that will be charged to expense across the asset's life, regardless of which method you use.
- Accumulated depreciation
- The running total of all depreciation charged on an asset since it was placed in service. A contra-asset account shown as a deduction from gross property, plant and equipment.
- Book value (carrying amount)
- Cost minus accumulated depreciation. Under straight-line it declines in equal steps from cost to salvage value.
- Half-year convention
- A simplification that charges six months of depreciation in the first year and six in the year after the life ends, regardless of the actual in-service date. Required by US tax rules under MACRS in most cases, and used by some companies for book purposes as well.
- Composite life
- Gross property, plant and equipment divided by annual depreciation expense — the implied average useful life of an entire asset base. A useful screen for whether depreciation policy has been quietly relaxed.