What are the actual formulas for SL, DB, and DDB? I've seen it a bunch of different ways in various blogs and financial magazine sites. Nothing but SL ever seems to make sense.

Closed-form formulas aren't as important as understanding the process for calculating the depreciation expense for each year. (This is why we have calculators and spreadsheets.)

Straight-line is obvious. The depreciable amount is cost less salvage value. If an asset's useful life is n years, depreciation expense is 1/n * (cost less salvage value) per year for n years. Easy peasy. Simple linear function; same amount of depreciation each year.

Double declining balance: Start with the same depreciable amount. If the useful life is again n years, you depreciate 2/n of the remaining depreciable amount each year, with a floor at zero. So the depreciable amount left after k years will be initial depreciable amount * (1 - 2/n)^k, where k = 1, 2,..., n - 1. Year n's depreciation expense is whatever depreciable value is left at the end of year n -1/start of year n, since that's the end of the asset's useful life. Not as easy, and a bit less peasy. The

*rate* of depreciation is constant (up till the final year), but the

*amount* of depreciation declines each year as the depreciable amount amortizes. The link below has a really clear example that walks you through the calculation, if that helps.

And 100k% declining balance is a generalization of double declining balance. The depreciation rate is k/n instead of 2/n, but the algorithm is otherwise the same.

The double declining balance depreciation method is a form of accelerated depreciation that doubles the regular depreciation approach. It is

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