Book Cost vs. Adjusted Cost Base: Why Your Broker's Number Is Not Your Tax Number
On a Canadian brokerage statement, book cost (some brokers label it book value) is what you paid for a holding, including purchase charges, adjusted for reinvested distributions, return of capital and corporate reorganizations to the extent your broker tracks them. The adjusted cost base CRA expects on Schedule 3 is defined by different law and may differ from it. This guide explains when, why, and what to do about it.
What is book cost?
Book cost is a securities-regulation term. It was added to National Instrument 31-103 by the CRM2 client-reporting amendments, defined 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." Its purpose is performance reporting. It puts a cost next to market value so you can see roughly how a position has done. It was never designed to be a tax figure.
Adjusted cost base is an Income Tax Act term, built from s. 47 (identical-property averaging), s. 53 (the list of additions and deductions), and s. 54 (definitions). It is the number Schedule 3 needs. The full definition, the formula and eleven worked examples are in our guide to adjusted cost base.
A brokerage-reported book value may not reflect your ACB and has to be verified against your own records. CRA says the same thing about the slip built from it. Its T5008 guidance says the amount in Box 20 "may or may not reflect your adjusted cost base" and that "you are required to make the adjustments, as needed." When the agency receiving the number disclaims it, treat it as a starting point, not an answer. The same warning applies to the year-end slip, covered in T5008 and Schedule 3.
On the slip itself, that difference lives in Box 20, and CRA states outright that the amount there may or may not reflect your adjusted cost base. Our guide to T5008 Box 20 and Box 21 covers what CRA instructs filers to report in each.
The structural problem is one pool per taxpayer, not per account
The biggest divergence is not an error at all. It is a design limit. Under s. 47, all of your holdings of an identical security are one pooled position with one averaged ACB, per taxpayer, across every non-registered account you own at every broker. Your broker can only see its own accounts. If you hold the same ETF at two institutions, neither one's book cost can be your ACB, even if each tracked its own side perfectly. The pool is yours, not the account's, and only you (or software that sees all your accounts) can compute it.
Eleven ways book cost drifts from ACB
| Event | What book cost typically shows | What the tax rules require |
|---|---|---|
| Same security at two brokers | Two separate per-account figures | One averaged pool across all non-registered accounts (s. 47) |
| Transfer in-kind from another broker | Often reset to market value at transfer, or blank/zero | A transfer changes nothing, and the original cost carries over |
| Return of capital (T3 Box 42) | Applied late, inconsistently, or not at all | Reduces ACB each year, and below zero becomes a deemed capital gain |
| Phantom reinvested distributions | Frequently missed | Increase ACB, and missing them means being taxed twice |
| Superficial loss repurchase | Never tracked (brokers say so outright) | Denied loss is added to the ACB of the repurchased shares |
| US-dollar securities | Book cost in USD, or converted at one rate | Each buy and sell converted at the Bank of Canada rate for its own date, with the pool kept in CAD |
| ESPP / RSU / option shares | Only the price you paid | Add the s. 7 employment benefit already taxed on your T4 |
| Mergers, spinoffs, corporate actions | Frequently mishandled or booked at odd values | Specific ACB allocation rules, including the s. 86.1 spinoff election |
| In-kind move into a TFSA/RRSP | Position just disappears from the account | Deemed disposition at market value, with gains taxable and losses permanently denied |
| Dual-listed journaling (e.g. DLR) | Currency lines confuse the book figure | Journaling is not a disposition and the CAD pool continues (see the Norbert's Gambit guide) |
| Late or amended T3 slips | Book cost finalized before the data exists | ACB adjustments follow the final (sometimes amended) slip |
The direction of the error matters. Book cost too low (a missed phantom distribution, an unadded employment benefit) means you overpay tax. Book cost too high (a transfer booked at market value, unapplied return of capital) means you underreport a gain, which is the direction CRA reassesses.
The three traps worth dollar examples
The transfer reset. You bought at $30, transferred the shares to a new broker when they traded at $45, and the receiving broker booked $45 as book cost (some book nothing at all). Sell at $50 and the statement implies a $5-per-share gain. Your actual gain is $20 per share, because a transfer is not a disposition and your ACB never changed. Filing the broker's figure here underreports the gain by 75 percent. If you ever need to rebuild a transferred position by hand, the starter template from our broker import guide is built for exactly that.
The phantom distribution double tax. Your ETF reports a $500 reinvested capital gains distribution on a T3. No cash arrives and no units are added, but you pay tax on the $500 this year, and it increases your ACB by $500. Skip that adjustment and you will pay tax on the same $500 a second time when you sell. Return of capital is the same problem mirrored. Box 42 quietly reduces ACB every year, and an ETF or REIT holder who never applies it carries an inflated cost base for a decade. See ETF return of capital and your ACB.
The employer-shares trap. Your ESPP sells you a $100 share for $85. The $15 discount is an employment benefit, taxed on your T4 in the year of purchase, and it is added to your ACB, making it $100. The plan administrator's statement shows $85. File from the statement and you pay income tax on the $15 and then capital gains tax on the same $15 again. RSUs work the same way, with an ACB equal to fair market value at vesting rather than zero.
When book cost is probably fine
Honestly, sometimes it is. Your broker's book cost is likely close to your ACB if every statement below is true.
One broker, one non-registered account. Canadian-dollar securities only. No ETFs, REITs, or funds paying return of capital or phantom distributions. Nothing transferred in from another institution. No sale at a loss followed by a repurchase within 30 days (by you, your spouse, or your registered accounts). No employer stock plans. No mergers, spinoffs, or elections.
Uncheck any one of those and the number needs independent verification. In practice the investors most likely to rely on book cost, ETF holders with a couple of accounts, are among the least able to.
What happens if you file the broker's number anyway
CRA receives every T5008 electronically and matches the proceeds against your Schedule 3, so the sale itself is never invisible. The cost side is on you. The burden of proving your ACB rests with the taxpayer, and in Tax Court cases where no cost base could be substantiated, CRA has assessed using full proceeds with gross-negligence penalties upheld. The normal reassessment window for individuals is three years, but where a misstatement resulted from neglect or carelessness there is no time limit, and the s. 163(2) penalty is the greater of $100 or 50 percent of the understated tax.
None of this means the broker's number is malicious. It means the responsibility boundary is drawn exactly where the disclaimers say it is. The broker reports what it can see, and you report your ACB.
Broker notes, as of August 2026
Questrade's help pages say Box 20 "may or may not represent the actual adjusted cost base" and explain that a zero usually means a transfer arrived without cost data. Wealthsimple's help centre says book cost "is never guaranteed to be accurate", that it is your responsibility to verify the reported ACB, and that it does not adjust for superficial losses. Interactive Brokers reports gains on a FIFO basis by default, which is a US convention that does not match CRA's required average-cost method, and its Box 20 cost reporting can still miss the full Canadian ACB picture. Wording changes without notice, but the pattern does not.
How ActiveACB computes the number CRA actually wants
ActiveACB rebuilds your ACB from your broker's raw transaction exports instead of trusting its book cost. Every buy, sell, and distribution event flows into one averaged pool per security, across every account you upload, the way s. 47 requires. US-dollar trades are converted at the Bank of Canada rate for each transaction's own date, superficial losses are detected and added back to the ACB of the repurchased shares automatically, ETF return of capital and phantom distributions are applied from the verified factor data, and the result is a per-sale gain/loss report with the cost base your T5008's Box 20 leaves blank. The full approach is documented in the methodology. Run your own broker export through the ACB calculator. Your first calculation is free.
Frequently asked questions
Can I just use the book value on my statement for my taxes?
Not reliably. CRA's T5008 guidance says Box 20 may or may not reflect your ACB, and brokers present book cost as a figure you must verify yourself. It is only safe in the narrow case of one account, CAD-only holdings, no return of capital, no transfers, no loss repurchases, and no employer shares.
Why is my book cost different at each broker?
Each broker only sees purchases inside its own accounts. The Income Tax Act pools all your holdings of an identical security into one averaged ACB across every non-registered account you own, so no single broker's figure can be the tax number when you hold the security in more than one place.
My transferred shares show a $0 or blank book cost. What do I do?
Your ACB did not change. The cost data just did not follow the shares. Recover the purchase history from your previous broker's statements or trade confirmations and compute the ACB from that. Do not file zero, and do not file the market value on the transfer date.
Do I average my ACB with my spouse's shares?
No. The ACB pool is per individual taxpayer. Your spouse matters for a different rule. Their purchase of the same security within the 61-day superficial loss window can deny your loss, because spouses are affiliated persons.
What are phantom distributions and why do they matter?
They are non-cash reinvested capital gains distributions reported on a T3. You pay tax on them in the year they occur, and they increase your ACB. If the increase is never applied, you pay tax on the same amount again when you sell. They appear on the fund provider's tax breakdown, not as cash in your account.
My ESPP or RSU shares show only what I paid. Is that my ACB?
No. The employment benefit that was already taxed on your T4 is added to your ACB. For an ESPP, ACB is the full market value at purchase, not the discounted price, and for RSUs it is the market value at vesting. Filing the plan statement's figure means paying tax on the benefit twice.
Is a blank Box 20 on my T5008 an error?
No. Issuers legitimately leave it blank when they do not know your cost base, most commonly after transfers or for long-held positions. The proceeds in Box 21 still reach CRA, so the disposition must be reported, with an ACB you compute yourself.
How far back can CRA reassess a wrong cost base?
Normally three years from your notice of assessment. Where a misstatement resulted from neglect, carelessness, or wilful default, there is no time limit, and the gross-negligence penalty is the greater of $100 or 50 percent of the understated tax.
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