Introduction
Capital gains are a fact of life for corporations that hold investments, sell assets, or transfer property. The size of those capital gains depends on one number that is frequently miscalculated or poorly tracked: the adjusted cost base (ACB).
For incorporated business owners — particularly those whose corporations hold investment portfolios, real estate, or business assets — understanding ACB is foundational to understanding the tax consequences of any disposition.
What Is the Adjusted Cost Base?
The adjusted cost base of a property is its cost for tax purposes — adjusted upward or downward by certain events that have occurred since the property was acquired. When a property is sold, the capital gain is the proceeds of disposition minus the ACB minus the selling costs.
For shares purchased on a stock exchange, the ACB starts as the purchase price. For business assets, the ACB is the original purchase price adjusted for certain additions and reductions. For real estate, the ACB includes the original purchase price plus certain capitalised acquisition costs.
The lower the ACB, the larger the capital gain — and the larger the tax.
How ACB Is Adjusted Over Time
For most investment assets, the ACB changes in three main ways:
Reinvested distributions: When a mutual fund, ETF, or similar investment distributes income that is automatically reinvested (rather than paid out in cash), those reinvested amounts are included in the investor's income in the year of distribution — and the ACB of the investment is increased by the same amount. Failing to track these adjustments results in double taxation when the investment is ultimately sold.
Return of capital distributions: Certain distributions are classified as return of capital rather than income. These reduce the ACB of the investment. Over time, a significant accumulation of return of capital distributions can reduce the ACB below the original purchase price — potentially creating a larger capital gain on disposition than expected.
Corporate reorganisations and stock splits: Share splits, consolidations, and corporate reorganisations affect the number of shares held and the ACB per share. These adjustments must be tracked to maintain an accurate ACB.
ACB for Identical Properties
When a corporation holds multiple lots of the same security — purchased at different times and at different prices — the ACB is calculated as an average across all identical shares. The total cost of all shares held is divided by the total number of shares to produce the average ACB per share.
This "identical property" rule means that a corporation cannot selectively sell its highest-ACB shares to minimise capital gains. All shares of the same class of the same issuer are treated as a single pool.
The Superficial Loss Rule
Where a corporation sells an investment at a loss and repurchases the same (or identical) investment within 30 days before or after the sale, the loss is denied as a superficial loss under section 54 of the Income Tax Act. The denied loss is added to the ACB of the repurchased shares — deferring it rather than eliminating it.
For corporate investment portfolios that are actively managed or periodically rebalanced, the superficial loss rule should be monitored. Accidental triggering of superficial losses — through automatic dividend reinvestment programs that purchase new units within the 30-day window — is a common source of ACB complications.
Why Poor ACB Tracking Is Expensive
A corporation that has held an investment portfolio for many years without carefully tracking ACB adjustments may have a significant discrepancy between its records and the actual ACB at the time of sale. If the ACB is understated, the capital gain is overstated — meaning the corporation pays more tax than it legally owes.
The CRA does not automatically correct these errors in the corporation's favour. The taxpayer is responsible for maintaining accurate ACB records.
ACB on Business Assets
For business assets sold by a corporation — equipment, vehicles, intellectual property — the relevant tax figure is the undepreciated capital cost (UCC), not the ACB. When a capital asset is sold for more than its UCC, the excess is recaptured CCA — fully included in income, not treated as a capital gain. Amounts above the original cost (a rare event) are capital gains.
When to Speak With a CPA
For corporations with investment portfolios or significant capital assets, reviewing ACB records as part of the annual tax engagement ensures that capital gains calculations are accurate and that errors do not accumulate over time. Retroactive ACB corrections are possible but become more complex with each passing year.