T5008 box 20 is not your adjusted cost base
Your broker reports a cost or book value in box 20. CRA says plainly it may not be your ACB, here is what it is, why it diverges, and what to do.
September 8, 2026 · 5 min read
Every year your brokerage sends you a T5008, and box 20 on it carries a number labelled cost or book value. It looks like the number you need. Very often it is not.
You do not have to take that from us. CRA tells the brokers who prepare the slip, in its instructions for completing box 20:
"This amount may or may not reflect the investor's ACB for the purpose of determining their gain or loss from the disposition of the security."
That is CRA, in the guidance it gives the people filling in the slip. It is worth knowing before you copy the number onto your return.
What box 20 actually contains
CRA defines it as "the total amount paid to purchase a security, including any transaction charges related to the purchase, adjusted for reinvested distributions, returns of capital and corporate reorganizations."
So it is not a naive purchase price. A brokerage that does this well is already adjusting for the two things people most often forget. Box 20 is a serious attempt at your cost base. It is simply an attempt made from one institution's records.
The four places it comes apart
It sees one account. This is the big one, and it is arithmetic rather than carelessness. Canada pools identical property per taxpayer, not per account. If you hold the same ETF at two brokerages, neither one's box 20 can be correct, because neither can see the other's units. Adjusted cost base, explained works through why the pool has that shape.
Transferred-in positions arrive with whatever came with them. When you move a holding between institutions, its cost base travels as a number supplied by the previous one, or gets estimated, or reset to the transfer-date price, or lost. The receiving brokerage reports what it was given.
Return of capital may not have been applied. ROC reduces your cost base rather than being taxed as income, and it arrives on a T3 slip in the spring, often after the statements it should have adjusted.
Corporate actions get recorded inconsistently. Splits, mergers and especially spin-offs require cost base to be allocated between securities under specific rules, and a spin-off allocation is a judgement call based on the issuer's own guidance.
What CRA expects of the two parties
CRA asks the slip preparer to "take reasonable measures in order to ensure that the amount reported in box 20 is correct", and separately tells the investor they "may have to make adjustments to the amount indicated in box 20 at the time of determining and reporting their gain or loss."
That is the whole relationship in two sentences. The brokerage does its best from what it holds. The figure you report is yours, and so is the liability if it is wrong.
What to do about it
Keep the trade-level record, every buy, sell, distribution and corporate action, for as long as you hold the security plus six years after you dispose of it. Monthly statements are not that record.
Check your T3 slips each year for return of capital and reinvested distributions, and apply the adjustment in the year it happens rather than reconstructing it a decade later.
If the same security sits at more than one institution, maintain the pool yourself, because no single brokerage is positioned to. To run one security's pool through the arithmetic, the adjusted cost base calculator does it in your browser, and how Luum tracks adjusted cost base sets out which adjustments Luum applies automatically and which it leaves to you.
Once you have your own figure, the T5008 and cost base checker puts it beside box 20 on each slip and shows what the difference does to the totals you would report.
And if the disposition in question was a loss, the superficial loss rule may change the answer again.
This is educational information about how the slip works, not tax advice. Verify with the CRA or a qualified tax professional before you file.
Box 20 diverges most reliably when you hold the same security at more than one institution, since no broker can see the other side, pooling cost base across two brokerages works that case through.
Common questions
Is box 20 on my T5008 my adjusted cost base?
No, and the CRA says so directly. Box 20 carries the cost or book value your broker holds, which is an account-local number. Your adjusted cost base is a taxpayer-level figure. Box 20 is a useful starting point for reconciliation and it is not the number you are obliged to report.
Why would box 20 differ from my own records?
The common causes are a transfer in from another institution where the original cost did not travel with the shares, a return-of-capital distribution that lowered your cost base without the broker adjusting for it, and holding the same security at more than one institution so that no single broker sees the whole pool.
Which figure do I report on Schedule 3?
Your own adjusted cost base, computed across all of your accounts. If it differs from box 20, keep the working that reconciles the two so you can support the number you filed. The slip is information the CRA also receives, not a determination of your cost base.
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.