All accurately known digits plus the first uncertain digit of a measurement is called significant digits or significant figures. e.g. if in a measurement, mass of a body lies between 5.6 kg & 5.7 kg & we estimate it as 5.62 kg then the first two digits 5 & 6 are certain while the third digit 2 is approximately known we can say there are 3 significant figures.
Following are the rules for counting significant figures
Rule 1: All non zero digits are significant.
Rule 2: All zeros occuring between two non zero digits are significant.
Rule 3: The digit zero (underlined) conventionally put on the left of a decimal for a number less than one is never significant. All zeros to the right the decimal point are not significant.
Rule 4: The terminal or trailing zero(s) in a number without a decimal point are not significant.
Rule 5: The trailing zero(s) in a number with a decimal point are significant.
Rule 6: The powers of ten are not significant.
Rule 7: No. of significant figures remains unchanged on changing the units of measurement, zeroes appearing in the base number in the scientific notation are significant.
|4.5 x 10 – 6||Two||6|
|5.300 x 10 6 mm||Four||7|