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http://en.wikipedia.org/wiki/Depreciation

Methods of depreciation

There are several methods for calculating depreciation, generally based on either the passage of time or the level of activity (or use) of the asset.

Straight-line depreciation

Straight-line depreciation is the simplest and most often used technique, in which the company estimates the "salvage value" of the asset after the length of time over which it is depreciated, and assumes the drop in the asset's value is in equal, constant yearly increments over that amount of time. The salvage value is an estimate of the value of the asset at the time it will be sold or disposed of; it may be zero. For example, a vehicle that depreciates over 5 years, is purchased at a cost of US$17,000, and will have a "salvage value" of US$2000 will depreciate at US$3,000 per year. ($17,000 − (5 x $3000)) = $2000

If the vehicle were to be sold and the sales price exceeded the depreciated value (net book value) then the excess depreciation would be considered as income by the tax office (cap l gains). If the sales price is less than the book value, the resulting cap l loss is tax deductible.

If a company chooses to depreciate an asset at a different rate from that used by the tax office then this generates a timing difference in the income statement due to the difference (at a point in time) between the taxation department's and company's view of the profit.

Declining-balance depreciation

The declining-balance method is a type of accelerated depreciation, because it recognizes a higher depreciation cost earlier in an asset's lifetime. This may be a more realistic reflection of an asset's actual resale value, as well as the expected benefit from the use of the asset: many assets are most useful when they are new. In the U.S., a form of double declining-balance depreciation, MACRS, is used for tax purposes and is based on time.

In declining-balance depreciation, each period's depreciation is based on the previous year's net book value, the estimated useful life, and a factor. The factor is commonly two; this is known as double declining-balance. Each period we calculate depreciation:

For the double-declining balance method, using the vehicle example from above, we compute the depreciation after the first year:

We subtract $6800 from our previous year's net book value to obtain our new net book value: . For the second year, we use this new value to calculate depreciation. Notice that it is significantly lower than the first year:

This process continues until we reach the salvage value or the end of the asset's useful life. Since declining-balance depreciation doesn't always depreciate an asset fully by its end of life, some methods also compute a straight-line depreciation each year, and apply the greater of the two. This has the effect of converting from declining-balance depreciation to straight-line depreciation at a midpoint in the asset's life.

Activity depreciation

Activity depreciation methods are not based on time, but on a level of activity. This could be miles driven for a vehicle, or a cycle count for a machine. When the asset is acquired, we estimate its life in terms of this level of activity. Assume the vehicle above is estimated to go 50,000 miles in its lifetime. We calculate a per-mile depreciation rate: ($17,000 cost - $2,000 salvage) / 50,000 miles = $0.30 per mile. Each year, we then calculate the depreciation expense by multiplying the rate by the actual activity level.

Sum of years digits depreciation

Sum of Years Digits is a historical depreciation method that results in a more accelerated write off than straight line, but less than declining balance or later methods. Salvage value is counted in the method. There are no property classes of later methods.

• Given;
  • N = Depreciable life of asset
  • B = Cost basis
  • S = Salvage value
  • D(t) = Depreciation charge for year t
  • 
    
  • 
    

Example; If an asset costs $1000, has a depreciable life of 5 years and a salvage value of $90 compute its depreciation schedule.

Year	D(t)	Sum of D(t)	Remaining Book Value
1	$303	$303	$697
2	$242	$546	$454
3	$182	$728	$272
4	$121	$849	$151
5	$61	$910	$90

The equation for year 1 would look like this:

Note: Most depreciation schedules round to the nearest dollar.

Units of Production depreciation

Units of Production depreciation is used in the U.S. in cases where MACRS is inappropriate, and the value to depreciate is based in the asset, such as a mine or natural resources. The method calculates the depreciation based on the units of the asset place n service as compared to the total units of the asset.

Units of time depreciation

Units of Time Depreciation is similar to units of production, and is used for depreciation equipment used in mine or natural resource exploration, or cases where the amount the asset is used is not linear year to year.

Taxes

When a company spends money for a service or anything else that isn't a tangible asset, this expenditure is usually immediately tax deductible, and the company enjoys an immediate tax benefit.

However, when a company buys some physical asset that will last longer than one year, like a computer, car, or building, the company cannot immediately deduct the cost and enjoy an immediate tax benefit. Instead, the company must depreciate the cost over the useful life of the asset, taking a tax deduction for a part of the cost each year. Eventually the company does get to deduct the full cost of the asset, but this happens over several years; the number of years depends on an estimate of how long it typically takes that type of asset to become effectively useless, and require a replacement. A computer may depreciate completely over five years; a factory building, over 30 years. The maximum allowable useful life estimate under U.S. income tax regulations is 40 years. Other countries have other systems, many of which remove the choice of depreciation rate and method from the company altogether. In these jurisdictions accounting depreciation and tax depreciation are almost always significantly different numbers, as in many instances a form of "accelerated depreciation" can be used for tax purposes to lower net income (or, in some instances, a fixed asset may be allowed to be expensed for tax purposes; Section 179 of the Internal Revenue Code allows for this treatment in some circumstances).