Straight-Line Depreciation: Formula, Examples, and When to Use It

You just added a $12,000 server to the register. Useful life: 5 years. Residual value: $500. What's the monthly depreciation? If you can answer that in under five seconds, you already know straight-line depreciation. If you hesitated, this guide will make it second nature.

Straight-line is the default depreciation method for the majority of SME asset registers. It's simple, predictable, and the method auditors expect to see. This guide covers how to calculate straight-line depreciation from the formula through a complete lifecycle example, including what happens when start conventions, useful life changes, and mid-year acquisitions enter the picture.

The Straight-Line Formula

The formula has three inputs:

Monthly Depreciation = (Acquisition Cost - Residual Value) / Useful Life in Months

That's it. No rates, no remaining book value lookups, no year-over-year recalculations. The amount stays the same every month from start to finish.

Breaking it down:

• Acquisition Cost - what you paid for the asset (including delivery, installation, and any costs to get it ready for use)
• Residual Value (also called salvage value) - what you expect the asset to be worth at the end of its useful life. For many assets, this is zero.
• Useful Life - how long you expect to use the asset, expressed in months for monthly depreciation

The difference between cost and residual value is called the depreciable amount - the total value that gets allocated across the asset's useful life.

Monthly vs. Annual: Why Monthly Matters

Textbooks present straight-line depreciation as an annual calculation: ($12,000 - $500) / 5 years = $2,300 per year. That works for annual financial statements, but month-end close requires monthly precision.

Annual: ($12,000 - $500) / 5 = $2,300/year

Monthly: ($12,000 - $500) / 60 = $191.67/month

The monthly figure is what hits your general ledger each period. It's also what drives the journal entry: DR Depreciation Expense $191.67 / CR Accumulated Depreciation $191.67.

Why not just divide the annual amount by 12? For full-year calculations, the result is identical. The distinction matters when an asset is acquired mid-year. A server purchased in October with a full-month convention gets three months of depreciation that year, not a prorated fraction of the annual amount. Monthly calculations handle partial years naturally.

A Complete Worked Example

Let's follow that $12,000 server through its full lifecycle.

Year-by-year summary:

After 60 months, the net book value equals exactly the residual value ($500). The asset is fully depreciated. No more expense is recorded, but the asset stays on the register until it's disposed.

The journal entries each month:

These entries are generated automatically when you post a depreciation batch. (Minty Assets generates these journal entries for every asset in the batch, grouped by category with the correct GL accounts.)

How Start Conventions Affect the First Month

When you buy an asset on March 15th, how much depreciation do you record for March? The answer depends on your company's start convention.

Using our $12,000 server with $191.67 monthly depreciation:

The convention is set once at the company level and applies to every asset. Full month is the most common choice for SMEs because it's simple and slightly conservative (you recognize expense earlier).

The total depreciation over the asset's life is identical regardless of convention. The only difference is timing: when the first and last partial months occur. An asset starting with the next-month convention depreciates for one extra month at the end to reach the same residual value.

What Happens When Things Change

Real assets don't always follow their original plan. Two situations require recalculation.

Residual Value Revision

Your $12,000 server has been running for 24 months. Accumulated depreciation is $4,600. You now estimate the residual value is $0 instead of $500.

Revised calculation:

• Remaining depreciable amount: $12,000 - $4,600 - $0 = $7,400
• Remaining useful life: 36 months
• New monthly depreciation: $7,400 / 36 = $205.56

The change applies prospectively - you don't restate the prior 24 months. You simply spread the remaining depreciable amount over the remaining life.

Capital Improvement

You install a $3,000 RAM and SSD upgrade in month 24, extending the server's useful life by 12 months (to 72 months total).

Revised calculation:

• New cost basis: $12,000 + $3,000 = $15,000
• Accumulated depreciation to date: $4,600
• Remaining book value: $15,000 - $4,600 = $10,400
• Remaining depreciable amount: $10,400 - $500 = $9,900
• Remaining useful life: 48 months (72 total minus 24 elapsed)
• New monthly depreciation: $9,900 / 48 = $206.25

Again, prospective only. The prior entries stay as they are.

Common Mistakes to Avoid

Using tax lives instead of book lives. MACRS assigns a 5-year life to computers. But if you plan to use that server for 7 years, your book depreciation should use 84 months. Tax and book depreciation serve different purposes - one minimizes taxes, the other reflects economic reality. Experienced controllers maintain both but never confuse the two.

Forgetting residual value. Setting residual value to zero when the asset has genuine salvage value overstates depreciation expense. For vehicles, residual value can be 15-20% of cost. For specialized machinery, it might be the scrap metal value. When in doubt, err toward a small residual value rather than zero.

Rounding errors that compound. $191.67 per month for 60 months is $11,500.20 - not $11,500.00. That $0.20 discrepancy compounds across hundreds of assets. Good asset management systems handle this by adjusting the final month's depreciation to land exactly on the residual value.

Depreciating below residual value. Once net book value reaches the residual value, depreciation stops. This sounds obvious, but in spreadsheets with formula-driven calculations, it's easy to miss the cutoff and keep depreciating into negative territory.

When Straight-Line Is the Right Choice

Straight-line works when an asset delivers roughly equal value each period. Office furniture doesn't become less useful in year three than in year one. Buildings don't lose structural capacity on a curve. Software licenses provide the same access throughout their term.

It's also the right default when you have no strong reason to use an accelerated method. Auditors rarely question straight-line. It produces predictable, level expense that simplifies budgeting. And for assets below your materiality threshold, the difference between straight-line and accelerated methods is too small to justify the added complexity.

For assets that genuinely lose value faster in early years - vehicles, production equipment, technology hardware - consider declining balance or double declining balance instead. The right method matches how the asset actually wears out, not just what's easiest to calculate.