Rules: (1) All non-zero digits are significant. (2) Zeros between non-zero digits are significant. (3) Leading zeros are not significant. (4) Trailing zeros after decimal are significant. (5) Trailing zeros without decimal are ambiguous (use scientific notation). Addition/subtraction: result has same number of decimal places as the least precise measurement. Multiplication/division: result has same number of significant figures as the least precise factor. In JEE calculations: typically 3-4 significant figures are expected. Rounding: if the digit after the last significant figure is >5, round up; if <5, round down; if exactly 5, round to even.