Straight-Line vs. Double-Declining-Balance Depreciation

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SL = \frac{\text{Cost} - \text{Salvage}}{\text{Life}} \qquad DDB_t = \text{Book value}_{t-1} \times \frac{2}{\text{Life}}

$\text{Cost}$: the asset's original purchase cost
$\text{Salvage}$: the asset's estimated residual value at the end of its useful life
$\text{Life}$: the useful life, in years, over which the asset is depreciated
$SL$: straight-line depreciation: the same dollar expense every year
$DDB_t$: double-declining-balance depreciation expense in year t, applied to the PRIOR year's book value

Why it must be true

Depreciation spreads an asset's cost over its useful life — but "spreads" can mean two very different shapes. Straight-line is the simplest honest guess: assume the asset wears out evenly, so charge the same expense every year. Double-declining balance assumes the opposite — an asset (a delivery truck, a laptop) loses most of its usefulness early, so front-load the expense: charge a fixed RATE against whatever book value remains, which shrinks every year, so the dollar expense shrinks with it.

Same asset, same total depreciation over its life, completely different year-by-year earnings impact: straight-line is flat and predictable; DDB front-loads big expenses (and big tax shields) early, then tapers off.

The derivation

Straight-line divides the total depreciable amount — cost minus what it'll be worth at the end — evenly across the years:

SL = \frac{\text{Cost} - \text{Salvage}}{\text{Life}}

Double-declining balance instead applies a FIXED rate — twice the straight-line rate, hence "double" — to whatever book value is left, which itself shrinks each year as depreciation is taken:

\text{rate} = \frac{2}{\text{Life}}, \qquad DDB_t = \text{Book value}_{t-1} \times \text{rate}

Because the base keeps shrinking, DDB depreciation shrinks geometrically year over year — until it would cut below the salvage value, at which point the expense is capped so book value never falls below salvage.

When to reach for it

Comparing how a reported depreciation expense (and therefore earnings) would differ under an even, straight-line assumption versus a front-loaded, accelerated assumption for the same asset.

Listen for

“straight-line depreciation / double-declining balance”“accelerated depreciation”“book value declining each year”“front-loaded expense vs level expense”

Back-of-the-envelope

Estimate it in your head first — then the calculator only confirms.

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DDB's rate is always 2/Life, applied to book value, NOT to (Cost − Salvage) — salvage only matters at the end, as a floor.
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DDB expense strictly decreases each year (until the floor); straight-line never changes. If a 'DDB' answer choice is flat or rising, it's wrong.
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Total depreciation over the FULL life is identical under both methods (Cost − Salvage) — only the timing differs. A useful check when sanity-testing multi-year totals.

Traps in applying it

✗Applying the DDB rate to the original cost every year instead of the shrinking book value.
✗Forgetting the floor: DDB depreciation must stop reducing book value below salvage value, even if the formula's raw output would go lower.
✗Using (Cost − Salvage) in the DDB rate calculation — DDB's rate ignores salvage entirely; only straight-line's numerator subtracts it.

Limits & criticisms

Both methods are ESTIMATES of a real economic pattern (useful life and residual value) that won't be known with certainty until the asset is retired or sold — a large gain or loss on disposal is common evidence the original assumptions were wrong. Neither method necessarily tracks the asset's TRUE economic value at any point in time; they are accounting conventions chosen partly for tax and earnings-smoothing reasons, not because either curve is a physical law of how assets wear out.

Where it came from

Accelerated depreciation methods like double-declining balance became standard accounting and tax practice in the mid-20th century — the US Internal Revenue Code's post-WWII and 1954 reforms explicitly permitted accelerated methods to encourage capital reinvestment, since front-loaded depreciation defers tax payments (a real cash-flow benefit) even though total depreciation over the asset's life is identical either way. Analysts still compare the two methods directly whenever they need to judge how much of a reported earnings difference between two companies is a genuine operating difference versus simply a different depreciation policy.

One identity, 2 questions

The exam can hide any variable. Each face below is the same equation solved for a different unknown — drill them separately.

Even expense every year

SL = \frac{\text{Cost} - \text{Salvage}}{\text{Life}}

The simple face: spread the depreciable amount evenly — same expense every single year.

Drill this face →

Front-loaded, shrinking expense

DDB_t = \text{Book value}_{t-1} \times \frac{2}{\text{Life}}

The accelerated face: a fixed rate against a shrinking base — big expense early, tapering fast.

Drill this face →

On the BA II Plus

Worked example: An asset costs $100,000.00 with a salvage value of $3,500.00 and a 6 periods-year useful life. Using double-declining balance, what is the depreciation expense in year 2 periods?

1.2 [÷] 6 [=]double-declining rate
2.100000 [×] [RCL] [=]year 1 expense; repeat against remaining book value through year 2

→ $22,222.22

Where it leads

Master this and the following come almost for free:

Free Cash Flow to the Firm

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