Nonzeros digits are always significant, for example, 238 mm has three significant digits (or figures), 1.2 kPa has two significant digits and 3 atm has one significant digit.

Zeros between nonzero digits are always significant, for example, 208 mm has three significant digits, 1.024 kPa has four significant digits and 30301 atm has five significant digits.

Leading zeros do not count as significant digits, for example, 0.000205 mm has three significant digits, 0.01 Pa has one significant digit and 0.00000035 atm has two significant digits.

Trailing zeros are significant only if there is a decimal point in the number, for example, 1010000 mm has three significant digits, 1000. mm has four significant digits, 0.010 Pa has two significant digits and 10.0 atm has three significant digits.

Exact numbers have infinite number of significant digits, for example, 10100 monkeys, 4 apples and 55 orca whales has infinite numbers of significant digits.