The leading digit in a decimal is the digit to the immediate left of the decimal point. If the decimal is less than one, the leading digit is the first non-zero digit to the right of a decimal point.
The leading digit estimation is useful in figuring out a mathematical result when precision is unnecessary. For example, leading digit estimation with decimals allows for cost estimation without worrying about cents. The rule for rounding is adding one to the digit when the digit to the right is five or greater and leaving the digit unchanged if the digit to the right is less than five.