ROI (Return on Investment) in tournament poker is calculated as (Total Prizes – Total Buy-ins) / Total Buy-ins × 100%. An ROI of 15% in regular MTTs indicates strong performance, while 30%+ ROI is considered excellent but requires large volume to be statistically significant. ROI varies significantly by format: small-field SNGs allow higher ROIs (20-40%) with lower variance, while large-field MTTs have lower average ROIs but larger top prizes. Comparing ROI without adjusting for field size, stake level, and guarantees can be misleading for evaluating actual performance.