Market concentration calculator
How much of the US share market sits in its biggest companies, and does it matter? This calculator shows how concentrated the market has been every month since 1926, and what happened next: for the biggest companies, for the rest of the market, and for the market as a whole compared with cash. The figures update monthly.
What the calculator shows
How concentrated the market is now: the share of the whole US share market held by its biggest 10% of companies, compared with every month since 1926.
The biggest companies against the rest: what the biggest companies and the rest of the market went on to return over 5 to 20 years, starting from low, average and high concentration.
Shares against cash: whether investing in the whole market beat leaving the money in cash, even when concentration was high.
Past episodes: each time concentration was high, when the biggest companies’ lead turned and when the market next fell.
Any period you choose: pick a start and end date to see how the biggest companies, the rest and the whole market did.
Things to keep in mind
History has only had a handful of times when the market was this concentrated: the early 1930s, the late 1950s and early 1960s, around 2000, and 2017 to today (2026). That isn’t much to go on, so treat the results as a guide rather than a prediction.
High concentration has rarely come on its own. It has often arrived alongside high share prices or a market crash, so it’s hard to say how much of what followed was down to concentration itself.
The biggest companies have usually given back their lead in the end, but the timing has varied from months to many years. In several past episodes the market kept rising strongly for years after concentration first got high.
What it means for investors
Concentration has been a poor guide to when to buy or sell. Getting out of shares when concentration was high often meant missing years of gains, and over 10 years or more shares beat cash in most periods, even when concentration was high.
A broad, low cost, diversified fund means you own both the biggest companies and the rest of the market, without needing to guess which will lead next.
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Market timing calculator: what waiting for a better moment has cost investors
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This calculator uses data from Eugene F. Fama and Kenneth R. French (Kenneth R. French Data Library) and Robert J. Shiller. It is general information about past market behaviour, not financial advice. Past returns don’t tell you future returns.
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