Pooling cost base across two brokerages
The same ETF at two institutions is one adjusted cost base, not two. Here is the calculation neither broker can do for you, worked in both directions.
September 10, 2026 · 7 min read
If you hold the same ETF at two institutions and each one shows you a cost figure, you may reasonably assume you have two cost bases to keep track of. You have one, and neither institution is computing it.
This is the narrow case where a broker's number is not merely a different presentation of yours. It is an answer to a different question. The general version is covered in adjusted cost base, explained; this page is only about the two-institution problem.
The rule, in one line
Identical property held by one taxpayer is a single pool, regardless of which institution holds it.
Not one pool per account. Not one per brokerage. One per taxpayer, per security. Units of the same ETF are identical property whether they sit in an account at one broker or another, so a sale from either account draws on the same weighted-average cost.
Two things follow immediately, and both catch people:
- Neither broker can compute your cost base, because neither can see the other side. What each one reports is a book value for the account it holds, accurate about its own records, and not the figure the rule asks for.
- Which account you sell from does not change the cost you deduct. The pooled average applies. Selling the "cheaper" units is not a thing that exists for identical property.
Registered accounts sit outside this entirely. Growth and realised gains inside a TFSA or RRSP are not taxable events, so pooling is a question about your taxable accounts only. If your holdings are split across a TFSA and a taxable account, the TFSA side is simply not part of the calculation.
Worked, in both directions
Suppose you hold units of one ETF in taxable accounts at two brokers.
At broker A, bought in two lots:
| Event | Units | Price per unit | Cost |
|---|---|---|---|
| Buy | 100 | 30.00 | 3,000.00 |
| Buy | 50 | 36.00 | 1,800.00 |
| A | 150 | 4,800.00 |
At broker B, one lot:
| Event | Units | Price per unit | Cost |
|---|---|---|---|
| Buy | 50 | 42.00 | 2,100.00 |
| B | 50 | 2,100.00 |
Each broker, looking only at its own records, reports a per-unit book value of 32.00 at A and 42.00 at B. Both are correct about the account and neither is your adjusted cost base.
Pool them:
- Total units: 150 + 50 = 200
- Total cost: 4,800.00 + 2,100.00 = 6,900.00
- Pooled cost per unit: 6,900.00 ÷ 200 = 34.50
Now sell 50 units from broker B, at 50.00, for proceeds of 2,500.00.
- Cost of the units sold: 50 × 34.50 = 1,725.00
- Capital gain: 2,500.00 − 1,725.00 = 775.00
Broker B, using its own book value of 42.00, would compute a cost of 2,100.00 and a gain of 400.00, understating the gain by 375.00. Not because B did anything wrong, but because B was never in a position to see A's units.
And in the other direction, which is the check worth doing. After the sale:
- Units remaining: 200 − 50 = 150
- Cost remaining: 6,900.00 − 1,725.00 = 5,175.00
- Cost per unit: 5,175.00 ÷ 150 = 34.50, unchanged, as it must be
A sale of identical property does not move the pooled average; it removes units at that average. If your remaining per-unit cost changed after a plain sale, the arithmetic is wrong somewhere. That reconciliation is the cheapest error check available and almost nobody runs it.
Where the pooled number and the broker's number come apart
Splitting a holding across institutions is one cause. Three more compound it, and they interact:
- A transfer in kind. Moving units between your own taxable accounts is not a disposition, so it does not change your cost base. What it frequently changes is the receiving broker's book value, because that broker may have no record of what you originally paid. Keep the statement from the sending institution. It is the only evidence of the original cost.
- Return of capital. A distribution characterised as return of capital lowers your cost base without being taxed when received. Split across two brokers, each adjusts only its own side. How return of capital affects your cost base.
- What your slip says. Box 20 on a T5008 is the broker's cost or book value, which is exactly the account-local figure this page is about. Box 20 is not your adjusted cost base.
One rule is deliberately out of scope here and you should not infer it is handled: if you are selling at a loss and repurchasing, the superficial loss rule can deny the loss outright, and it reaches your registered accounts and affiliated persons. Tax-loss selling and the superficial loss rule covers it. Luum does not apply superficial-loss adjustments to its cost base record.
What to actually do
The practical version is short:
- Keep one record per security, not one per account. The account is where units sit; the pool is what you report.
- Record every buy, sale, distribution and transfer with its date, units, amount and which institution, the institution matters for reconciling against slips later, not for the calculation.
- Reconcile in both directions after every sale: cost removed plus cost remaining should equal cost before, and the per-unit average should be unchanged.
- Expect your slips to disagree with you, and keep the working that explains why. A figure you can support is worth more than a figure that matches a slip.
Luum keeps this record pooled across accounts on the average-cost method, which is the calculation described above, how Luum tracks adjusted cost base sets out what it does and does not apply. It is educational, it is not tax advice, and it is not a substitute for your own records or for a qualified advisor on anything consequential.
Common questions
Do I really have to combine accounts at different brokers?
For identical property in taxable accounts, yes. The pool is a property of the taxpayer rather than of the account, so holdings of the same security at different institutions form one weighted average. Neither institution can do it for you, which makes it your record to keep.
Does it matter which account I sell from?
Not for the cost you deduct. The pooled average applies whichever account the units leave from, so choosing to sell the units you paid least for is not available for identical property. It can matter for other reasons, such as which account holds cash afterwards.
Do my TFSA and RRSP holdings go into the pool?
No. Registered accounts are outside the calculation because gains inside them are not taxable events. Only your taxable accounts are pooled, which is why a tracker that lumps registered and non-registered holdings together will produce a cost base that is wrong for tax purposes.
What if the two brokers disagree about my cost?
They usually will, and neither is necessarily wrong. Each is reporting a book value for its own account. Your job is the third number: the pooled average across both. Keep the working that reconciles it against each slip.
This article is educational and general in nature. It is not investment, tax, or legal advice, and it does not take your own circumstances into account. Verify tax treatment with the CRA or a qualified tax professional.