Author: Robert K Valentine Jr.
The depreciation you report on your financial statements doesn’t have to match the depreciation you claim on your tax return — and for most companies, it doesn’t. Tax depreciation follows MACRS, the recovery periods and conventions written into the Internal Revenue Code. Book depreciation — the depreciation shown on your balance sheet and income statement under GAAP — is a management estimate: you choose the useful life, the method, and the convention, as long as the choice reasonably matches how the asset is used up over time. This article covers the two methods most companies use for book purposes — straight-line and declining balance — and a simple convention for handling assets placed in service mid-month.
Book Depreciation vs. Tax Depreciation
Under ASC 360, a company depreciates a fixed asset over its estimated useful life — not a life prescribed by the IRS — using whatever systematic and rational method best reflects the pattern in which the asset’s benefits are consumed. That’s why a machine might be depreciated over 10 years for the financial statements but 7 years under MACRS for tax purposes, and it’s why the resulting book-tax difference is exactly what deferred tax accounting is built to reconcile. None of the conventions in this article — the methods, the useful lives, or the mid-month rule described below — come from the tax code. They’re common financial-statement policy choices, and your company should apply whichever ones it adopts consistently, asset to asset and year to year.
The Straight-Line Method
Straight-line is the simplest and most common book depreciation method. It spreads the depreciable cost evenly over the asset’s useful life:
Annual depreciation = (Cost − Salvage value) ÷ Useful life
The examples in this article assume a salvage value of $0 for simplicity, but the same formula applies with any salvage value — just subtract it from the cost first.
Declining Balance Methods
Declining balance methods front-load depreciation, expensing more in the early years of an asset’s life and less in later years — a reasonable policy for equipment that loses usefulness or value faster when it’s newer. Each variant applies a multiple of the straight-line rate to the asset’s beginning-of-year book value (not its original cost):
- 200% declining balance (double-declining balance): Rate = 2 ÷ Useful life
- 175% declining balance: Rate = 1.75 ÷ Useful life
- 150% declining balance: Rate = 1.5 ÷ Useful life
- 125% declining balance: Rate = 1.25 ÷ Useful life
200% DB is the most aggressive and most commonly used of the four; 175%, 150%, and 125% DB simply dial back how quickly the asset depreciates, which some companies prefer for assets that lose value quickly but not that quickly. Because a declining balance calculation only ever depreciates a fraction of what’s left, it never reaches zero on its own. Standard practice — and the approach used in the example below — is to switch to straight-line partway through the asset’s life, in whichever year the straight-line amount on the remaining book value and remaining life first equals or exceeds the declining balance amount. That switch is what allows the asset to fully depreciate by the end of its useful life instead of leaving a small, permanent, undepreciated remainder.
The Modified Full-Month Convention
Instead of prorating depreciation to the exact day an asset is placed in service, many companies adopt a simpler monthly convention for book purposes. One common version — used throughout this article — is the modified full-month convention:
- If the asset is placed in service from the 1st through the 15th of the month, that entire month counts as a full month of depreciation.
- If the asset is placed in service from the 16th through the end of the month, that month doesn’t count at all — depreciation begins the following month.
This is a company accounting policy, not an IRS rule — it has no relationship to the mid-month or half-year conventions used in MACRS. Like any book depreciation policy, it should simply be applied the same way to every asset, every year.
Example 1: Straight-Line Depreciation of a Machine
- Asset: Machine
- Cost: $10,000.00
- Salvage value: $0
- Useful life: 10 years
- Placed in service: March 11, 2025
- Method: Straight-line, modified full-month convention
March 11 falls between the 1st and the 15th, so March counts as a full month. That gives the machine 10 months of depreciation in 2025 (March through December) instead of 12.
$10,000.00 ÷ 10 years ÷ 12 months = $83.33 per month
2025: $83.33 × 10 months = $833.33. Every full calendar year after that depreciates at $83.33 × 12 = $1,000.00, until the 10-year, 120-month life is used up:
| Year | Months | Depreciation Expense | Accumulated Depreciation | Ending Book Value |
|---|---|---|---|---|
| 2025 | 10 | $833.33 | $833.33 | $9,166.67 |
| 2026 | 12 | $1,000.00 | $1,833.33 | $8,166.67 |
| 2027 | 12 | $1,000.00 | $2,833.33 | $7,166.67 |
| 2028 | 12 | $1,000.00 | $3,833.33 | $6,166.67 |
| 2029 | 12 | $1,000.00 | $4,833.33 | $5,166.67 |
| 2030 | 12 | $1,000.00 | $5,833.33 | $4,166.67 |
| 2031 | 12 | $1,000.00 | $6,833.33 | $3,166.67 |
| 2032 | 12 | $1,000.00 | $7,833.33 | $2,166.67 |
| 2033 | 12 | $1,000.00 | $8,833.33 | $1,166.67 |
| 2034 | 12 | $1,000.00 | $9,833.33 | $166.67 |
| 2035 | 2 | $166.67 | $10,000.00 | $0.00 |
Notice that the schedule runs across 11 calendar years even though the machine has a 10-year life. That’s a direct result of the convention: 10 months of depreciation in the placed-in-service year leaves 2 months still owed at the end of Year 10, which get picked up as a short final year in 2035.
Example 2: 200% Declining Balance Depreciation of a Laptop Computer
- Asset: Laptop Computer
- Cost: $2,500.00
- Salvage value: $0
- Useful life: 5 years (assumed for this example)
- Placed in service: March 18, 2025
- Method: 200% declining balance, switching to straight-line, modified full-month convention
March 18 falls after the 15th, so March doesn’t count at all under the modified full-month convention — depreciation starts in April, giving 9 months of depreciation in 2025 instead of 12.
With a 5-year useful life, the 200% declining balance rate is 2 ÷ 5 = 40% per year, applied to the beginning-of-year book value:
2025: $2,500.00 × 40% × 9/12 = $750.00. Book value drops to $1,750.00.
2026: $1,750.00 × 40% = $700.00. Book value drops to $1,050.00.
2027: $1,050.00 × 40% = $420.00. Book value drops to $630.00.
At the start of 2028, 2.25 years of life remain (5 years minus the 9 months used in 2025, minus 3 full years in 2026–2027). This is where the declining balance and straight-line amounts need to be compared:
- 200% DB amount: $630.00 × 40% = $252.00
- Straight-line amount on the remainder: $630.00 ÷ 2.25 years = $280.00
Since the straight-line amount ($280.00) is now larger than the declining balance amount ($252.00), the laptop switches to straight-line for 2028 and every year after, which is what fully depreciates it by the end of its 5-year life instead of leaving a small remainder forever:
| Year | Months | Method | Calculation | Depreciation Expense | Ending Book Value |
|---|---|---|---|---|---|
| 2025 | 9 | 200% DB | $2,500.00 × 40% × 9/12 | $750.00 | $1,750.00 |
| 2026 | 12 | 200% DB | $1,750.00 × 40% | $700.00 | $1,050.00 |
| 2027 | 12 | 200% DB | $1,050.00 × 40% | $420.00 | $630.00 |
| 2028 | 12 | Straight-line (switched) | $630.00 ÷ 2.25 years | $280.00 | $350.00 |
| 2029 | 12 | Straight-line | $350.00 ÷ 1.25 years | $280.00 | $70.00 |
| 2030 | 3 | Straight-line | $70.00 ÷ 0.25 years | $70.00 | $0.00 |
As with the straight-line example, the modified full-month convention stretches this 5-year asset across 6 calendar years (2025–2030), with the final 3 months recovered in a short stub year.
Key Takeaways
- Book depreciation is a management estimate, not a tax calculation. The method, useful life, and convention are company accounting policy choices under GAAP, not rules imposed by the IRS.
- Straight-line divides evenly; declining balance methods (200%, 175%, 150%, and 125% DB) front-load the expense by applying a fixed rate to a shrinking book value.
- Declining balance almost always needs to switch to straight-line partway through an asset’s life — whenever the straight-line amount on the remaining book value and remaining life exceeds the declining balance amount — so the asset fully depreciates by the end of its useful life.
- The modified full-month convention counts the placed-in-service month as a full month if the asset went into service on or before the 15th, and skips that month entirely if it went into service on the 16th or later.
- A mid-month placed-in-service date always creates a short final year, stretching the depreciation schedule across one more calendar year than the asset’s stated useful life.
This article is for general informational purposes only and does not constitute accounting or tax advice. Book depreciation methods, useful lives, salvage values, and conventions are estimates and policy elections made under GAAP (ASC 360); they should be applied consistently and reviewed with your accountant or auditor. They are separate from — and are not required to match — the depreciation method used for tax reporting.
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