What it means
The idea is that a mining company does not really own a mine, it owns a finite quantity of ore that it will gradually convert into cash. Charging the full purchase cost in year one would understate the first year's profit and overstate every later year's, so the cost is matched to production instead.
Extract a tenth of the reserve this year and a tenth of the cost becomes an expense. The starting number is the depletion base, which is broader than the purchase price.
It includes the acquisition cost of the property or lease, exploration and development costs that have been capitalised, and capitalised restoration obligations, less any expected residual value of the land once extraction finishes. Getting this base right matters more than the arithmetic that follows.
The denominator is an estimate and it moves. Recoverable reserves are engineering judgements that get revised as drilling data improves or as prices change what is economic to extract, and when they are revised the depletion rate is recalculated for future production only.
Prior years are not restated, which mirrors the treatment of a change in the useful life of a machine. Cost depletion has a sibling called percentage depletion, a tax provision in some jurisdictions that allows a deduction based on a fixed percentage of gross revenue from the property.
Because that deduction can eventually exceed the amount actually invested, tax and accounting figures can diverge sharply, so companies compute both and claim the larger where the law permits. Financial statements prepared under standard accounting rules use the cost method.
One practical trap is the difference between units extracted and units sold. Depletion is calculated on units extracted, but the portion relating to unsold inventory sits in the balance sheet as part of stock cost rather than in the income statement.
Only when the ore or timber is sold does that share of depletion reach cost of sales.
In practice
Real-world examples.
Example
A timber company buys 12,000 acres of standing forest and calculates a depletion rate per thousand board feet of merchantable timber. When a survey after a storm reduces the estimated recoverable volume, it recalculates the rate on the remaining unrecovered cost and applies the higher rate to future harvests without restating earlier years.
Example
An oil producer capitalises the cost of a producing lease and applies depletion on a units-of-production basis using proved reserves. A downward reserve revision after disappointing drilling results raises the charge per barrel, cutting reported profit even though production volumes and prices are unchanged.
Example
A sand and gravel operator running three pits keeps a separate depletion calculation for each, because the purchase cost per recoverable tonne differs widely. Management uses the per-tonne rates alongside haulage costs to decide which pit to work when demand is soft.
Formula
Calculation
Formula: Depletion rate per unit = (Depletion base - Residual land value) / Estimated recoverable units, then Depletion expense = Depletion rate x Units extracted in the period
Worked example. Kestrel Aggregates buys a limestone quarry for $10,000,000 and capitalises a further $2,400,000 of access roads, stripping and site development, giving a depletion base of $12,400,000. The land is expected to be worth $400,000 as farmland once quarrying ends.
Net depletable amount: $12,400,000 - $400,000 = $12,000,000.
Engineers estimate recoverable stone of 4,000,000 tonnes.
Depletion rate: $12,000,000 / 4,000,000 = $3.00 per tonne.
In its first year the quarry extracts 250,000 tonnes, so depletion for the year is 250,000 x $3.00 = $750,000.
Of that production, 200,000 tonnes are sold and 50,000 tonnes remain in stockpiles. So 200,000 x $3.00 = $600,000 is charged to cost of sales, and 50,000 x $3.00 = $150,000 stays in inventory on the balance sheet until the stone is sold.Case study
Seen in the real world.
This is a fictional and illustrative example. Brackenhill Minerals, an invented company, acquired a slate quarry for $8,000,000, spent $1,600,000 on access and processing infrastructure, and expected the land to be worth $600,000 for amenity use once extraction ceased.
The depletion base in this invented case was $8,000,000 + $1,600,000 = $9,600,000, and the net depletable amount was $9,600,000 - $600,000 = $9,000,000. With estimated recoverable slate of 1,500,000 tonnes, the rate was $9,000,000 / 1,500,000 = $6.00 per tonne. Production of 120,000 tonnes in the first full year gave a depletion charge of $720,000.
Three years later, in this illustrative scenario, a fresh geological survey cut remaining recoverable reserves from 1,140,000 tonnes to 800,000 tonnes because a fault line made part of the deposit unworkable. The unrecovered cost at that point was $9,000,000 minus three years of charges totalling $2,160,000, which left $6,840,000. Dividing by the revised 800,000 tonnes produced a new rate of $8.55 per tonne, raising the annual charge by more than 40% and turning a thin operating margin into a reported loss, without a single tonne of production being lost.
Watch out
Common mistakes.
- Using depletion and depreciation interchangeably. Depreciation spreads the cost of equipment and buildings over time, whereas depletion spreads the cost of the resource itself over units extracted.
- Charging depletion on units sold rather than units extracted. Extraction drives the charge, and the portion relating to unsold stock belongs in inventory until the material is sold.
- Treating the reserve estimate as fixed. Reserves are revised regularly, and each revision changes the rate applied to all future production.
Questions
People also ask.
Is cost depletion the same as percentage depletion?
No; cost depletion allocates the actual amount invested, while percentage depletion is a tax rule based on a share of gross revenue and can exceed the amount invested.
Does depletion affect cash flow?
Not directly, since it is a non-cash charge added back in the cash flow statement, though it reduces taxable profit and therefore the tax paid.
Can the depletion base include future restoration costs?
Yes, where an obligation to restore the site exists, the discounted cost is generally capitalised into the asset and recovered through the depletion charge.
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