Significant figures and dimensional analysis

Significant figures are the digits known with certainty plus one last estimated digit.

All non-zero digits count; zeros between non-zero digits count; zeros to the left of the first non-zero digit never count (0.0032 has two).

Zeros at the right end count only when there is a decimal point in the number: 0.200 has three, while 100 written without a decimal point has one.

Exact numbers, such as a counted 2 balls or a defined conversion factor, have an unlimited number of significant figures.

In addition and subtraction the answer keeps as many decimal places as the entry with the fewest decimal places.

In multiplication and division the answer keeps as many significant figures as the entry with the fewest significant figures.

Rounding: drop the digit if it is less than 5, raise the previous digit by one if it is more than 5; if it is exactly 5, keep the previous digit if it is even and raise it by one if it is odd.

Dimensional analysis (the unit factor method) converts units by multiplying by a ratio equal to 1, such as (1 m / 100 cm), so the unwanted units cancel.