Use a Rounding Numbers Calculator to round precisely to your desired decimal places. For 3.14159265: to nearest integer = 3, to 1 decimal = 3.1, to 2 decimals = 3.14, to 3 decimals = 3.142 (note: rounds up from 3.141), to 4 decimals = 3.1416. The standard rounding rule: if the next digit is 5 or greater, round up; if less than 5, round down. However, there are actually several rounding methods: Round Half Up (standard, 2.5→3, -2.5→-3), Round Half Down (2.5→2), Round Half to Even/Banker’s Rounding (2.5→2, 3.5→4 – reduces cumulative rounding error), Round toward zero (truncation), Round away from zero. For most purposes, standard “half up” is fine. When to round: displaying results to users (show appropriate precision), before comparisons (avoid false inequality from floating-point error), in intermediate calculations for specific algorithms, for reporting requirements (e.g., financial reports to 2 decimals). When NOT to round: during intermediate calculations (compounds errors), when exact values are needed, before further mathematical operations. Best practice: perform all calculations with full precision, round only the final result for display. Significant figures vs decimal places: 3.14159 to 3 sig figs = 3.14, to 4 sig figs = 3.142; but 0.00314159 to 3 sig figs = 0.00314 (leading zeros don’t count). The calculator helps you verify your rounding is correct, especially important for financial calculations where errors matter!