The SEC-prescribed computation is designed to avoid an obvious way the number could be distorted. Form N-1A instructs a fund to calculate portfolio turnover by taking the lesser of the fund's total purchases or total sales of portfolio securities during the period, and dividing that figure by the average monthly value of the securities the fund held over the same period. Securities with a maturity of one year or less at the time they were acquired are excluded from the calculation entirely. Taking the lesser of purchases and sales, rather than either figure alone or their sum, matters because a fund that received a large wave of new investor money and simply bought more of what it already held would otherwise show an inflated turnover figure driven by growth rather than by trading decisions. The lesser-of convention strips that effect out, so the number reflects actual reshuffling of the portfolio rather than its growth.
What drives turnover higher is trading activity, and the reasons vary by strategy. An actively managed fund that frequently changes its view on which companies to hold will show meaningfully higher turnover than one that buys and holds for years. An index fund's turnover comes almost entirely from changes to the index it tracks, such as a company being added to or removed from the index, or from managing shareholder purchases and redemptions, so a broad, stable index typically produces low turnover nearly by definition. A sector fund concentrated in a fast-moving industry, or a fund following a strategy that rebalances frequently, can show high turnover even without any change to its overall stated approach.
The cost consequence of turnover is straightforward: every trade has a cost. Buying and selling securities inside a fund incurs transaction costs, such as brokerage commissions and the bid-ask spread, and those costs come directly out of the fund's assets rather than appearing as a separate line item most investors ever see. A fund with high turnover pays these costs more often, which is a drag on return that exists independently of the fund's stated expense ratio.
The tax consequence is the one that matters most in a taxable account, and it is called tax drag when discussed as a general phenomenon. When a fund sells a security for more than its cost, it realizes a capital gain, and current tax law generally requires the fund to distribute realized net gains to shareholders at least annually. Those distributions are taxable to a holder in a taxable account in the year they are received, whether or not that holder sold any shares or has held the fund at a loss overall. A high-turnover fund therefore tends to generate larger and more frequent taxable distributions than a low-turnover one, all else equal, which is a meaningfully different consideration in a taxable brokerage account than in a tax-advantaged retirement account, where those distributions are not currently taxed.
A high turnover ratio is sometimes cited as one piece of evidence in disputes over excessive trading in a brokered account, but it is evidence, not a rule with a bright-line threshold, and using it that way is a separate subject covered on the pages for churning and wrap fee programs. On its own, for an ordinary fund, turnover is a cost and tax signal to weigh against the fund's strategy and stated objective, not a verdict about whether the fund is well or poorly run.
