Count the significant figures in any number, or round a number to a chosen number of significant figures.
Significant figures are the digits in a number that carry meaningful precision - all non-zero digits, zeros between them, and trailing zeros after a decimal point, but not leading zeros used just to place the decimal.
Leading zeros are never significant; zeros between non-zero digits are always significant; trailing zeros are significant only if a decimal point is present.
0.04500 has 4 significant figures (the leading zeros don't count, but the trailing zeros after the decimal do).
The digits carrying meaningful precision in a measurement, used to communicate measurement precision.
All non-zero digits count; zeros between them count; leading zeros don't; trailing zeros count with a decimal point.
They prevent calculated results from falsely implying more precision than the original measurements support.