Sortino Ratio = (Portfolio Return - Minimum Acceptable Return) / Downside Deviation. The minimum acceptable return (MAR) is commonly set to 0% (any loss is unacceptable) or to the risk-free rate. Downside deviation = the standard deviation of returns that fall below the MAR, calculated by squaring only negative deviations (ignoring positive deviations), averaging them, and taking the square root. Unlike the Sharpe Ratio, which divides by total standard deviation (penalizing both upside and downside variance), the Sortino Ratio denominates only by downside variance.

A strategy that earns consistent small gains punctuated by occasional large gains will have a higher Sortino Ratio than Sharpe Ratio because its return distribution is positively skewed — the high upside volatility (good) inflates total standard deviation (penalizing the Sharpe) but does not inflate downside deviation (leaving the Sortino unaffected). Conversely, an options-selling strategy that generates consistent small income but has occasional catastrophic drawdowns will have a much lower Sortino than Sharpe — the tail risk is precisely what the Sortino penalizes most harshly.

The Sortino Ratio is most valuable when evaluating asymmetric return strategies: long options positions (unlimited upside, defined loss), trend-following (large wins when trends are strong, small losses when wrong), covered call selling (limited upside, full downside exposure). For these strategies, the Sharpe Ratio misclassifies upside volatility as risk — the Sortino Ratio correctly focuses only on the downside.

For symmetric strategies — long-only equity portfolios, balanced funds, diversified multi-asset portfolios — Sharpe and Sortino ratios track closely because returns are approximately normally distributed and upside/downside volatility are roughly equal. In these cases, either metric is appropriate; the Sharpe is more widely quoted and provides comparability across more sources. The choice between Sharpe and Sortino matters most when comparing strategies with meaningfully different distributional shapes.

Sortino Ratio benchmarks by strategy type: diversified equity funds typically achieve Sortino ratios of 1.0-1.5 over full market cycles. Trend-following CTAs (commodity trading advisors) often show Sortino ratios of 0.5-1.2 — lower than their Sharpe might suggest because occasional large drawdowns occur during trend reversal periods. Best-in-class equity managers with consistent strategies and controlled drawdowns can sustain Sortino ratios of 1.5-2.5 over long horizons. Any strategy claiming a Sortino above 3.0 over a short period should be scrutinized for survivorship bias, overfitting, or unmeasured tail risk.

The Sortino Ratio is particularly revealing when evaluating alternative investment strategies that appear attractive on Sharpe Ratio but hide significant drawdown risk. A merger arbitrage fund, a short volatility strategy, or a credit strategy that collects steady premiums will show high Sharpe ratios in calm markets — because the denominator (total standard deviation) is low when volatility is suppressed. But the Sortino Ratio, by measuring how often and how severely returns go negative, will reveal the asymmetric downside exposure that the Sharpe hides until a crisis event materializes.