myCostBase pools identical property across a taxpayer’s taxable brokerages using weighted-average cost. The ledger records documented FX conversions, ETF cost-base adjustments, and option grants and resolution events so each calculation can be reviewed across accounts. This page explains the formulas and tax-treatment assumptions behind those records.
The core ACB formula
After each new purchase or DRIP reinvestment, the per-share ACB is recalculated:
New ACB per share = (Previous total ACB + New shares × Price per share + Commission) ÷ Total shares held
When you sell, the capital gain or loss is:
Capital gain = Proceeds of disposition − (ACB per share × Shares sold) − Selling commissions
Return-of-capital adjustments reduce the total ACB without changing the share count; DRIP reinvestments increase both. For a full walkthrough with worked examples across stocks, ETFs, and partial sales, see How to Calculate Adjusted Cost Base in Canada.
Pooling across brokerages
Identical securities held in taxable accounts by the same taxpayer share one weighted-average ACB pool, even when they sit at different brokerages — no single broker can see the other side of the pool. This is the identical-property averaging rule in ITA s. 47(1), not a myCostBase convention. Registered accounts (RRSP, TFSA, FHSA, RESP) are excluded entirely. See Pooled ACB Across Brokerages for how transfers and account scope affect the pool.
Superficial loss rule
Selling a security at a loss and buying the same or an identical security within 30 days before or 30 days after the sale — by the taxpayer or a person affiliated with them — denies the loss if the replacement is still held at the end of that window. The definition is in ITA s. 54; the loss is deemed nil under s. 40(2)(g)(i). myCostBase flags affected dispositions and adds the denied loss to the replacement position’s ACB instead of letting it offset the current year’s capital gains. See The Superficial Loss Rule in Canada for the affiliated-persons definition and registered-account interactions.
FX conversion for USD trades
USD-denominated transactions are converted to CAD using a documented rate applicable to each event, with the source and observation date kept visible on the transaction row. myCostBase defaults to a Bank of Canada daily rate and supports documented overrides. CRA generally accepts Bank of Canada rates, qualifying alternative sources, and averages in certain circumstances described in Income Tax Folio S5-F4-C1. For exchange-traded shares, retain both trade and settlement dates; the CRA T5008 guide uses settlement date in Box 14. See USD and FX Conversion for fallbacks, overrides, and audit detail.
ETF adjustments
Return of capital reduces the ACB per unit without changing share count — the deduction required by ITA s. 53(2)(h)(i.1) for amounts a trust pays as a return of capital — while reinvested capital gain distributions and DRIP purchases increase both cost and share count. Each event is recorded as its own row in the same ledger as buys and sells, in date order. See ETF Return of Capital and ACB Adjustments for a worked example.
Options
For options reported on capital account, ITA section 49 generally treats a written option’s net premium as a capital gain when granted. Buying an offsetting option can create a capital loss at that later date, while exercise can move the original premium into the cost or proceeds of the underlying shares and require a grant-year return amendment. CRA’s archived IT-479R, Transactions in Securities addresses capital-versus-income classification and exchange-traded option outcomes. See Options Transactions and ACB for the full mechanics.
Where this methodology shows up
This is the methodology T5008 Reconciliation and Reports compares your broker’s figures against, and what myCostBase for Accountants is built to make traceable for a year-end handoff. For what’s outside this scope entirely, see Product Limits and Scope.