The digits which tell us the number of units we are reasonably sure of having counted in making a measurement are called significant figures. Or in other words, the number of meaningful digits in a number is called the number of significant figures. A choice of change of different units does not change the number of significant digits or figures in a measurement.
For example, 2.868 cm has four significant figures. But in different units, the same can be written as 0.02868 m or 28.68 mm or 28680 mm. All these numbers have the same four significant figures.
From the above example, we have the following rules.
1. All the nonā'zero digits in a number are significant.
2. All the zeroes between two nonā'zeroes digits are significant, irrespective of the decimal point.
3. If the number is less than 1, the zeroes on the right of decimal point but to the left of the first nonā'zero digit are not significant. (In0.02868 the underlined zeroes are not significant).
4. The zeroes at the end without a decimal point are not significant. (In 23080 mm, the trailing zero is not significant).
5. The trailing zeroes in a number with a decimal point are significant. (The number 0.07100 has four significant digits).
1. 30700 has three significant figures.
2. 132.73 has five significant figures.
3. 0.00345 has three and
4. 40.00 has four significant figures.