The T5008 Statement of Securities Transactions is issued by your broker each tax year. Box 21 reports the proceeds of disposition — what you received when you sold.
Box 20 reports the broker’s cost or book value — what the broker believes you paid. Box 21 is almost always correct. Box 20 frequently is not.
Brokers calculate ACB based solely on purchases made through their own platform. If you transferred shares in from another brokerage, received DRIP reinvestment shares at a different cost basis, held an ETF that distributed return-of-capital amounts in prior years, or pooled positions across multiple accounts, your broker has none of that history. The Box 20 figure reflects an incomplete picture.
The CRA does not require T5008 Box 20 to be accurate. It is advisory. You are responsible for calculating and reporting the correct ACB on your tax return. If you use the broker’s number when your real ACB is higher, you will report a larger capital gain and pay more tax than you legally owe.
The checker above takes values from your T5008 slip and your own records. It computes the difference, shows the resulting capital gains from each perspective, and lists the most common reasons for a mismatch so you can identify which applies to your situation.
What this tool covers: A single security, single-brokerage disposition. For cross-brokerage pooling, multi-year DRIP and return-of-capital tracking, and a reviewable audit trail linked to each adjustment, use myCostBase. For a complete pre-filing workflow covering T5008 reconciliation, multi-broker pooling, ETF ROC, DRIPs, and USD trades, see the Canadian Adjusted Cost Base Checklist.