Why FX handling causes so many ACB errors
For foreign-currency securities, both proceeds and adjusted cost base must be expressed in CAD. The problem is not just converting one annual total. The problem is converting each transaction on its own date so the ledger remains correct over time.
That is where many spreadsheet workflows fail: the rate source is inconsistent, the date is wrong, or the conversion logic gets reconstructed later from memory.
myCostBase launch approach
myCostBase uses Bank of Canada daily rates for the transaction date selected in the ledger and keeps the rate visible on the transaction row. When that date has no published observation, the fallback is the previous business-day rate and the rate date remains visible.
For exchange-traded shares, keep both trade and settlement dates; CRA’s T5008 guide uses settlement date as the transaction-completion date in Box 14. Bank of Canada is a practical default, but CRA also accepts qualifying alternative rate sources and averages in certain circumstances. A documented override lets the ledger reflect an applicable alternate method without losing the audit trail.
A worked example
Say you buy 100 shares of a US-listed stock at USD $42.00 and the documented rate applicable to the transaction is 1.3620. The CAD cost is 100 × $42.00 × 1.3620 = $5,720.40—the amount added to the ACB, not the USD figure alone.
You later sell those shares at USD $55.00 when the documented rate is 1.3910. Proceeds are 100 × $55.00 × 1.3910 = $7,650.50. The capital gain is $7,650.50 − $5,720.40 = $1,930.10 CAD—a figure that reflects both the price change and currency movement between the two dates.
Overrides and auditability
If you need to use a different documented FX rate, the override should be recorded with a note. The override is part of the ledger, not a side calculation in a separate spreadsheet.
That makes later review possible when you need to answer why a given CAD amount differed from a broker display or an old worksheet.
Missing FX rate warnings
If an FX rate is missing for an imported USD trade, that should be treated as a warning condition. The product should not silently continue with an incomplete conversion trail because the downstream gain or loss depends on that rate.
Related workflows
If the security also has ETF adjustments, read ETF Adjustments and Return of Capital. If your primary question is why the final broker number still differs after FX is applied correctly, read T5008 Reconciliation and Reports.