Tax Basis: Definition, How It's Calculated, and Why It Matters

Tax basis (also called cost basis or adjusted basis) is the benchmark figure the IRS uses to determine how much of an asset's sale proceeds represent taxable gain and how much is simply a tax-free return of the owner's investment. Under IRC §1011, a taxpayer's adjusted basis in property is the property's original basis (usually its cost under IRC §1012) increased by capital improvements and decreased by allowed or allowable depreciation, amortization, depletion, and other basis-reducing events. When an asset is sold or otherwise disposed of, gain or loss equals the amount realized minus the adjusted basis — making basis one of the most consequential numbers in federal income taxation.

How Original Basis Is Established

For most purchased property, original basis equals the purchase price plus capitalized acquisition costs:

• Purchase price (cash paid plus any debt assumed)
• Closing costs, commissions, and settlement fees paid by the buyer
• Sales tax on the purchase
• Installation, testing, or start-up costs required to place the asset in service
• Legal fees to defend or establish title

Under IRC §1012, "cost" includes amounts paid with borrowed funds — a taxpayer who finances 80% of a $500,000 building has a $500,000 original basis, not a $100,000 basis.

Special cases require different starting points:

How Basis Is Adjusted Over Time

Once original basis is established, a series of events adjust it upward or downward. The result — adjusted basis — is what the taxpayer uses when computing gain or loss.

Upward Adjustments (Increase Basis)

• Capital improvements: expenditures that extend the useful life, add a new structural component, or adapt the property to a new use — distinguished from ordinary repairs (which are deducted currently and do not affect basis)
• Amounts included in income: if a shareholder recognizes taxable income from S corporation or partnership operations, their basis in the entity interest increases accordingly (under IRC §§705 and 1367)
• Reinvested dividends: in mutual fund accounts, dividends and capital gains distributions that are reinvested increase the owner's cost basis in the fund shares

Downward Adjustments (Decrease Basis)

• Depreciation and amortization: each year's depreciation deduction reduces adjusted basis by the amount deducted — or the amount allowable, even if not actually claimed (IRC §1016(a)(2))
• Casualty losses: the amount of a deductible casualty loss reduces basis
• Partial disposition or sale: selling a portion of an asset (e.g., a parcel of land) reduces the remaining basis by the basis allocated to the sold portion
• Tax-free distributions: return-of-capital distributions from a corporation or fund reduce the shareholder's basis before they become taxable gain
• Depletion: for natural resources, each depletion deduction reduces the property's basis

The "Allowable" Depreciation Rule

One of the most frequently misunderstood aspects of basis is the allowable vs. allowed rule in IRC §1016(a)(2). Basis must be reduced by the greater of depreciation actually claimed ("allowed") or depreciation the taxpayer was entitled to claim ("allowable"). A taxpayer who forgets to take depreciation for three years still must reduce basis by those three years' worth of deductions when computing gain on sale. This rule prevents taxpayers from avoiding depreciation recapture by simply not claiming the deduction.

Basis in Business Entity Interests

For owners of pass-through entities, basis tracks not just the original investment but ongoing tax events:

S corporation shareholder basis under IRC §1367:

• Increases: allocated ordinary income, separately stated income items, and non-deductible expenses that increase AAA
• Decreases: distributions, allocated losses and deductions, and non-deductible expenses

Shareholders cannot deduct losses allocated to them beyond their stock and loan basis. Suspended losses carry forward until the basis is restored (see the at-risk rules for an additional limitation layer).

Partnership basis under IRC §705 follows a similar structure, with partners also including their share of partnership liabilities in outside basis (IRC §752) — a significant difference from S corporation treatment.

Basis and Gain Calculation

The gain or loss on a sale is:

Gain (Loss) = Amount Realized − Adjusted Basis

Where amount realized includes cash received, the fair market value of other property received, and liabilities assumed by the buyer (or relieved from the seller). For example:

A taxpayer buys commercial real estate for $600,000 and takes $150,000 of MACRS depreciation over five years. Adjusted basis = $600,000 − $150,000 = $450,000. The property sells for $750,000. Gain = $750,000 − $450,000 = $300,000. Of that, up to $150,000 is subject to depreciation recapture at up to 25% (unrecaptured §1250 gain); the remaining $150,000 is long-term capital gain.

Special Basis Situations CPAs Encounter Most

Gifted property with built-in loss: If a donor's adjusted basis exceeds FMV at the time of the gift, the donee uses a dual basis — the donor's basis for computing gain, the FMV at the gift date for computing loss, and neither (no gain or loss) if the sale price falls between them (IRC §1015(a)).

Like-kind exchanges: A taxpayer who completes a §1031 exchange takes a substituted basis in the replacement property equal to the relinquished property's adjusted basis, adjusted for boot paid or received. This defers — but does not eliminate — the embedded gain. The low basis carries forward into the new property.

Installment sales: In an installment sale under IRC §453, the seller allocates their adjusted basis pro-rata across all payments using the contract price. Each payment received consists of a return-of-basis portion and a taxable gain portion determined by the gross profit ratio.

MACRS and bonus depreciation: Because MACRS and bonus depreciation reduce basis quickly — sometimes to zero in year one — a taxpayer who sells a piece of equipment after claiming 100% bonus depreciation has a zero basis, and the entire sale price is gain subject to depreciation recapture.

How CPAs Use Tax Basis in Practice

• Asset sale structuring: In a business sale, the allocation of purchase price among asset classes (under IRC §1060) directly determines each asset's starting basis for the buyer and the character of gain for the seller — CPAs negotiate these allocations intensely
• Inherited asset planning: Advising clients to hold appreciated assets until death to obtain a stepped-up basis, permanently eliminating embedded gain
• Loss harvesting: Tracking adjusted basis in securities portfolios to identify positions with unrealized losses that can be sold to offset capital gains
• S corporation planning: Monitoring shareholder basis to ensure losses pass through currently rather than being suspended, and advising clients on strategies to increase basis (direct loans, electing QSSS status, etc.)
• Partnership transactions: Computing outside basis for each partner to determine the deductibility of allocated losses and the tax consequences of distributions
• Like-kind exchange analysis: Calculating the low carryover basis in replacement property to quantify the deferred gain and assess the long-run tax cost of the strategy

Related Terms

• Depreciation — systematic basis reduction that generates current deductions
• Depreciation Recapture — the tax consequence of reducing basis below original cost
• MACRS — the IRS depreciation system that sets the pace of basis reduction for tangible property
• Step-Up in Basis — the inherited-property rule that resets basis to FMV, eliminating embedded gain
• Capital Gains — the tax on the excess of amount realized over adjusted basis
• At-Risk Rules — a loss limitation that interacts with basis in pass-through entities
• Realized vs. Recognized Gain — the distinction between the total economic gain from a disposition and the portion that is actually included in taxable income