Depletion

Depletion is the systematic allocation of the recorded cost of a natural resource — an oil or gas reserve, a mineral deposit, or a stand of timber, for example — from the balance sheet to expense as that resource is actually extracted and sold. It plays the same conceptual role for natural resources that depreciation plays for buildings and equipment and that amortization plays for intangible assets: all three spread an asset's cost over the period it generates economic benefit, rather than expensing the full cost immediately.

Depletion is most commonly calculated using the units-of-production method, which divides an asset's depletable cost by the total estimated units of the resource expected to be extracted over its life, producing a depletion rate per unit. That rate is then multiplied by the actual number of units extracted and sold during a given period to determine the depletion expense for that period, meaning depletion expense naturally rises and falls with production volume rather than following a fixed schedule based purely on time, the way straight-line depreciation does for a building.

A mining company pays $10 million for a mineral deposit estimated to contain 2 million tons of ore, giving a depletion rate of $5 per ton. If the company extracts and sells 300,000 tons during the year, depletion expense for that year is $1.5 million, found by multiplying the 300,000 tons by the $5 per-ton rate, leaving $8.5 million of remaining depletable cost on the balance sheet to be allocated to expense as extraction continues in future years.

Because the depletion rate depends entirely on the total estimated units recoverable from the deposit, a revision to that estimate — discovering the deposit contains more or less recoverable material than originally thought — changes the depletion rate used going forward for all remaining units, a treatment similar to how a change in an asset's estimated useful life affects future depreciation. Natural resource companies routinely revise these reserve estimates as new geological data becomes available, which is why depletion rates on a given property can change meaningfully from one year to the next even without any change in extraction activity itself.

All three concepts spread an asset's cost over its useful economic life, but they apply to different categories of assets and are calculated somewhat differently: depreciation typically follows a method based on time or usage for tangible fixed assets, amortization follows a similar pattern for intangible assets, and depletion is almost always tied directly to the physical quantity of resource actually extracted, since a natural resource's “useful life” is best measured by how much of it physically remains rather than by the passage of time alone.

Natural resource companies in the United States can sometimes use percentage depletion for income tax purposes instead of the cost-based method described above, deducting a fixed percentage of gross income from the resource rather than tracking cost against estimated reserves, a method that in some cases can allow total tax deductions to exceed the resource’s original cost over its life, subject to specific statutory limits that vary by the type of resource involved.