Depreciation questions show up in almost every introductory and intermediate accounting unit, and they're graded harder than most students expect. The calculations aren't difficult once you know which method applies, but assignments rarely tell you outright which one to use. You're expected to read the scenario, pick the correct method, justify the choice, and apply it correctly across the asset's useful life. This guide walks through the three methods you'll actually be tested on, how to choose between them, and where students consistently lose marks.

Why Depreciation Methods Matter Beyond the Calculation

Depreciation isn't just an accounting mechanic. It reflects an economic assumption about how an asset loses value over time. A delivery van loses more value in its early years than its later ones. A factory machine might wear down at a steady rate regardless of age. A mining excavator's value might depend entirely on how many hours it's operated, not how many years have passed.

AASB 116 Property, Plant and Equipment requires the depreciation method to reflect the pattern in which the asset's future economic benefits are expected to be consumed. That single sentence is why assignments ask you to justify your method choice rather than just compute a number. Markers want to see that you understand the asset's actual usage pattern, not just that you can run a formula.

Straight-Line Depreciation

Straight-line is the most common method and the one most students learn first.

Formula: (Cost − Residual Value) ÷ Useful Life

Say a business buys office equipment for $50,000, expects it to have a residual value of $5,000 after 5 years, and plans to use it evenly across that period.

($50,000 − $5,000) ÷ 5 = $9,000 depreciation expense per year

The expense stays flat every year, which makes straight-line the right choice when an asset delivers roughly equal value across its useful life, such as office furniture, buildings, or fixtures.

Where students lose marks: forgetting to subtract residual value before dividing, or applying a full year's depreciation to an asset purchased partway through the financial year. If the equipment above was bought on 1 October in a business with a 30 June year-end, only nine months of depreciation should be recognised in that first year, not a full twelve.

Diminishing Balance (Reducing Balance) Method

Diminishing balance charges more depreciation in the early years of an asset's life and less as time goes on. It suits assets that lose value quickly at first, such as vehicles, computers, and most technology equipment.

Formula: Depreciation Rate × Opening Carrying Amount

Take a $40,000 vehicle depreciated at 30% per year using this method.

Year 1: $40,000 × 30% = $12,000. Carrying amount falls to $28,000. Year 2: $28,000 × 30% = $8,400. Carrying amount falls to $19,600. Year 3: $19,600 × 30% = $5,880. Carrying amount falls to $13,720.

Notice that the rate applies to the carrying amount, not the original cost. This is the single most common error in assignments using this method. Students frequently keep applying the rate to the original $40,000 every year instead of the reduced balance, which produces flat depreciation figures identical to straight-line and defeats the entire purpose of the method.

Unlike straight-line, diminishing balance rarely reduces the carrying amount exactly to residual value by the end of the useful life. Some assignments expect you to adjust the final year's depreciation to bring the carrying amount down to the residual value precisely. Check the assignment brief for this requirement before submitting, because it's easy to miss.

Units of Production Method

Units of production ties depreciation directly to actual usage rather than time. It's used for assets where wear and tear correlates with output or activity, such as manufacturing equipment, mining machinery, or vehicles measured by kilometres travelled.

Formula: (Cost − Residual Value) ÷ Total Estimated Units × Units Produced in the Period

A printing press costing $120,000 with a residual value of $20,000 is expected to print 500,000 pages over its life. If it prints 80,000 pages in the current year:

($120,000 − $20,000) ÷ 500,000 = $0.20 per page $0.20 × 80,000 = $16,000 depreciation for the year

This method produces different depreciation figures every period depending on actual output, which is exactly what makes it appropriate for equipment where usage varies year to year. It also means you need accurate activity data for the period, something assignment scenarios usually provide explicitly, so read the figures carefully rather than assuming a flat rate applies.

How to Choose the Right Method in an Assignment

Most assignments give you clues rather than telling you the method directly. Look for these signals in the scenario:

• Steady, even use over the asset's life points to straight-line.
• Heavy early use, obsolescence risk, or technology that loses value fast points to diminishing balance.
• Output figures, machine hours, or kilometres travelled given in the question point to units of production.
• Explicit instructions naming the method, which should always override your own judgement.

If the scenario doesn't specify a method and gives no clear usage pattern, straight-line is the reasonable default assumption, but you should state that assumption explicitly in your answer rather than silently picking one.

Common Mistakes That Cost Marks Beyond the Calculation

A correct number doesn't guarantee full marks. Markers are checking for a few things beyond arithmetic:

No journal entries. Many assignments expect the depreciation expense to be recorded through a journal entry (debit Depreciation Expense, credit Accumulated Depreciation), not just the calculated figure on its own.

Confusing carrying amount with fair value. Carrying amount is cost minus accumulated depreciation. It's an accounting figure, not a market valuation, and treating the two as interchangeable is a recurring error in written answers.

Ignoring impairment. If an assignment scenario mentions damage, obsolescence, or a drop in expected future benefits, it may be testing whether you recognise that AASB 136 impairment applies on top of ordinary depreciation, not instead of it.

Not justifying the method choice. Even when the calculation is right, skipping the explanation of why that method fits the asset's usage pattern is one of the most common reasons marks are lost on depreciation questions specifically, since most rubrics allocate marks for reasoning separately from computation.

If you're working through a depreciation-heavy assignment and want a second opinion on your method choice or your journal entries before submitting, structured accounting assignment help can be useful for checking your reasoning against the standard rather than just the numbers.

A Quick Reference for Revision

Frequently Asked Questions

Can a business change depreciation methods partway through an asset's life? Yes, but only if the change better reflects the pattern of economic benefit consumption, and it's treated as a change in accounting estimate under AASB 108, applied prospectively rather than restating prior periods.

Does land get depreciated? No. Land generally has an unlimited useful life and isn't depreciated, unlike the buildings or improvements sitting on it, which are depreciated separately.

What happens to depreciation if an asset is sold partway through the year? Depreciation is usually calculated up to the date of sale, then a gain or loss on disposal is recognised based on the difference between sale proceeds and the carrying amount at that date.