Editorial disclosure: this article is educational content explaining how stock market indexes generally work. It is not investment advice, and it does not describe any specific current event, company, or security.
Ask an investor what the Toronto Stock Exchange’s main index is “made of,” and most will answer with one word: resources. Yet on any given day, the two sectors most associated with Canada’s stock market, energy and materials (which includes mining), together typically account for well under half of the benchmark’s total value. So how does an index so strongly identified with oil wells and gold mines end up dominated, in practice, by banks and railways? The answer lies in how index weighting actually works, and it explains a lot about why commodity swings do not always move the broader market the way outsiders assume they should.
Market-cap weighting, not sector quotas
The S&P/TSX Composite Index, like most major benchmarks worldwide, is constructed using free-float market-capitalization weighting. That means each constituent’s influence on the index is determined by the total market value of its publicly tradable shares, not by which sector it belongs to, not by production volume, and not by revenue. A company is included and weighted purely because of how much investor capital is parked in its shares relative to every other eligible company.
This distinction matters because commodity-linked businesses, even large ones, often carry more debt, reinvest more heavily in physical assets, and trade at lower valuation multiples than sectors such as financials or technology. A bank with steady fee income and a large loan book can command a higher market capitalization than a mining company with comparable or even larger revenue, simply because investors are willing to pay more per dollar of earnings for a business they perceive as more stable or more scalable. Index providers such as S&P Dow Jones Indices, which administers the TSX benchmark, apply the same mechanical rules to every eligible stock. There is no target allocation for energy or materials built into the methodology; the weight emerges from where the market decides to put its money.
Why Canada’s index still feels resource-heavy
Even so, the perception that the TSX is a commodities index is not baseless. Canada’s economy has a long history tied to natural resource extraction, and the exchange has historically listed a large number of mining and energy companies, including many smaller exploration firms that would struggle to get listed on other major exchanges. That breadth means energy and materials often represent a meaningful share, commonly in the range of a fifth to a third of the index depending on commodity price cycles, even though financials alone frequently carry a larger combined weight.
Commodity price cycles also amplify the visible effect of these sectors. When oil, copper, or gold prices move sharply, the earnings and share prices of resource companies tend to move with more volatility than a diversified bank or utility. This means that even a modest index weighting can translate into outsized day-to-day swings attributable to the resource sector, creating the impression that commodities are driving the whole market even when their formal weight is more modest. Index-tracking funds and exchange-traded products that mirror the TSX Composite inherit this same sensitivity, since they hold each constituent in proportion to its float-adjusted market value.
What weighting mechanics mean for how the index behaves
Understanding this mechanism helps explain a few recurring patterns. First, a rally or selloff in a single dominant sector, whether financials or resources, can move the headline index number substantially, since capitalization-weighted benchmarks are inherently concentrated toward their largest constituents. Second, comparing the TSX to other national indexes purely by sector label can be misleading, because the underlying weighting methodology, share structure, and free-float rules differ across exchanges and index providers, affecting how comparable the composition figures really are.
Finally, periodic index rebalancing, typically conducted quarterly by the index provider, adjusts weights as share prices and float availability change, meaning the sector composition of the TSX Composite is not fixed but shifts gradually over time as capital flows and corporate actions play out. Readers evaluating claims about “how resource-heavy” a benchmark is should look at the actual published sector weights from the index provider rather than relying on the exchange’s general reputation, since the two can diverge in meaningful ways.
