The single-pool rule
Canada taxes capital property using a pooled average cost. All identical property you own — the same class of shares in the same company — sits in one pool, and your ACB per unit is the total cost divided by the total units. Nothing in the rule is scoped to a brokerage account. Two hundred shares of a company held 100 at Broker A and 100 at Broker B is one pool of 200.
Why each broker’s gain is wrong
Every broker computes cost base only from the activity it can see inside its own walls. If you bought the same stock at two firms at different prices, neither firm has your true ACB — each has a partial one. Report a sale using a single broker’s figure and your gain can be materially off.
Transferring shares between brokers
Moving shares "in kind" from one broker to another is not a disposition — you haven’t sold anything, so there’s no gain or loss and your ACB carries over unchanged. The catch: the receiving broker often records the transfer at the market value on the transfer date, not your real cost. That figure will be wrong on their statements, and you have to override it with your actual pooled ACB when you file. See our full guide on in-kind transfers and ACB.
What does — and doesn’t — pool
Your pool includes only property you own. Your spouse’s shares of the same company are their own pool, not yours. But watch the superficial loss rule: a loss you trigger can still be denied if your spouse (or a corporation you control) buys the identical security within the 30-day window — pooling and affiliation are separate tests that can both bite.