Accumulated depreciation is the running total of a long-term asset's cost that has been allocated to depreciation expense since the asset was first acquired and placed into service. It is a long-term contra asset account — meaning it carries a credit balance, opposite the normal debit balance of an asset account — reported on the balance sheet under Property, Plant, and Equipment.
How the balance builds each period
Every time a company records depreciation expense on the income statement, the offsetting credit increases Accumulated Depreciation rather than reducing the asset account itself directly. This means the original cost of an asset stays fixed on the books for as long as the company owns it, while Accumulated Depreciation grows period after period until it eventually equals the asset's full depreciable cost, at which point the asset is considered fully depreciated.
Why depreciation is tracked this way rather than reducing the asset directly
Keeping the original cost and the accumulated depreciation in two separate accounts preserves useful information that a single net figure would destroy: anyone reading the balance sheet can see both what the company originally paid for its productive assets and how much of that investment has already been expensed, rather than seeing only a blended net number with no way to tell whether the assets are mostly new or mostly worn down. Netting the two together produces the asset's book value, also called carrying value, which is the figure actually included in total assets.
A simple worked example
A machine purchased for $200,000 with no expected salvage value and a ten-year useful life depreciates $20,000 a year under straight-line depreciation. After three years, Accumulated Depreciation: Machinery carries a credit balance of $60,000, and the balance sheet shows the machine at its $200,000 original cost, less $60,000 of accumulated depreciation, for a net book value of $140,000. The $200,000 figure never changes while the company owns the asset; only the accumulated depreciation balance grows each year.
Limits of accumulated depreciation as an economic signal
A large accumulated depreciation balance relative to an asset's original cost indicates the asset is relatively old within its useful life, which analysts sometimes use as a rough proxy for how soon a company may need to reinvest in new equipment. However, accumulated depreciation reflects an accounting allocation schedule, not the asset's actual physical condition or current market value, so a heavily depreciated machine can still be in excellent working order, just as a lightly depreciated one can already be functionally obsolete.
Removing an asset from the books
When a fully or partially depreciated asset is finally sold, scrapped or retired, both its original cost and its associated accumulated depreciation are removed from the books together, and any difference between sale proceeds received and the asset's remaining net book value is recognized as a gain or loss on disposal. Failing to remove both accounts together when an asset leaves service is a common bookkeeping error, since it leaves the general ledger carrying cost and depreciation figures for equipment the company no longer actually owns.