Calculating ACB for Identical Shares: The Averaging Method Ontario Investors Need to Know

If you've bought shares of the same company at different times and different prices, you might assume you can choose which purchase to "sell" for tax purposes — selling your highest-cost shares first, for example, to minimize the gain. Canadian tax law doesn't work that way for identical shares. Instead, every purchase gets blended into a single running average, and that average is what determines your capital gain or loss when you sell.

Getting this adjusted cost base (ACB) calculation wrong is one of the more common ways Ontario investors misreport capital gains — usually without realizing it, until the numbers don't reconcile with a brokerage statement or a CRA review.

Why the CRA Requires Averaging, Not Lot Selection

Some other countries let investors choose which specific purchase "lot" they're selling (first-in-first-out, or a specific identification method). Canada does not offer that flexibility for identical properties. Once you own more than one block of the same class of shares in the same corporation, they're combined into a single averaged cost base — there is no way to selectively sell your "cheap" shares or your "expensive" shares for tax purposes. You're always selling a proportional slice of the average.

This matters most when share prices have moved a lot between purchases. An investor who bought early at a low price and later added more at a much higher price can't isolate the recent, high-cost shares to reduce a taxable gain — the average blends everything together.

The Averaging Formula

The mechanics are straightforward, even if investors rarely calculate it by hand:

New average ACB per share = (Total ACB before the purchase + Cost of the new purchase, including commissions) ÷ Total shares held after the purchase

Every time you buy more shares of the same class of the same company, you recalculate the average. Every time you sell, you use the current average to determine the ACB of the shares sold — you don't get to pick which "batch" left your account.

Worked Example (Illustrative Only — Use Your Actual Numbers)

When the investor sells 100 shares at $25, the capital gain is calculated against the current average ACB of $21.32 per share — not against the price paid for any specific batch. The gain on that sale is roughly $368 (100 × ($25.00 − $21.32)), and only the applicable inclusion rate of that gain is taxable — currently 50% for all taxpayers, as of mid-2026 (verify the current rate before relying on it).

What Counts as "Identical" Property

The averaging rule applies to identical properties — the same class of shares in the same corporation, held in the same account type (for example, a non-registered account). It generally does not blend together:

• Shares held in a registered account (like an RRSP or TFSA) with shares of the same company held in a non-registered account — gains and losses inside registered accounts aren't calculated the same way.
• Different classes of shares in the same company (common vs. preferred), which are not identical properties to each other.
• Options, warrants, or convertible securities, which have their own cost-base rules separate from the underlying shares.

Mixing these up — averaging shares that shouldn't be pooled together, or failing to pool ones that should be — is one of the more common reasons an investor's self-reported ACB doesn't match what a broker or the CRA calculates.

What Happens If You Sell at a Loss

If your averaged ACB is higher than your sale proceeds, the result is a capital loss. Capital losses can only be used to offset capital gains, not other income, but they aren't wasted if you don't have a gain to apply them against that year — they can generally be carried back three years or carried forward indefinitely to offset gains in other years.

Frequently asked questions