How to Use Significant Figures in Addition and Subtraction

When adding or subtracting measurements in chemistry, we quite often cannot get exact measurements. Either that, or the measurements we do get have so many figures we would not be able to effectively write them all out. This is when we round using significant figures.

Difficulty: Easy

Instructions

Things You'll Need:

• Paper
• Pencil with an eraser

How to Use Significant Figures in Addition and Subtraction

1. Count the amount of significant figures to the right of the decimal point of each number you are adding or subtracting. If you had the numbers 40.3516432 and 22.4315 the first number would have seven significant figures to the right of the decimal point, and the second number would have 4 significant figures to the right of the decimal point.

2. Add or subtract your numbers as you would any other numbers. If we were subtracting 22.4315 from 40.3516432 we would get an answer of 17.9201432.

3. Round your answer to the same amount of significant figures as the least accurate number, or in other words the number with the least amount of significant figures to the right of the decimal point. In this case it was 4 significant figures. Our final answer would then be 17.92