Math Checkup: Adding and Subtracting Feet and Inches
Knowing how to add and subtract feet and inches directly by hand can be a time saver. Otherwise, you would need to convert feet and inches to inches, add inches, and convert back to feet and inches. Adding feet and inches can not be done directly on your calculator.
To add feet and inches, you will need to remember how to:
- Add and subtract fractions
- Changing improper fractions to proper fractions.
Example Adding Feet and Inches
5’ – 9 7/8” + 2’ – 8 1/2″
| What I am calculating… | What I am thinking… |
| I line up the measurements on top of one another. The first column on the left is whole feet, followed by whole inches, followed by fractional inches. I will add each column separately. | |
| Add the fractions on the far right column. I need to make the bottom numbers the same. | |
| 11/8 is not a proper fraction. It is one full inch plus another 3/8. I will carry the full inch to the inch column, and leave the 3/8 in the fraction column. | |
| After adding the inches, I have 18. One foot is 12 inches. I will carry one foot over to the foot column, and there will be 6 inches (18 – 12 = 6) left over. I can add all the feet now. | |
| The final answer. |
Example Subtract Feet and Inches
7’ – 2 1/8” – 2’ – 9 13/16”
| What I am calculating… | What I am thinking… |
| Line up the numbers in columns. Feet on the left, whole inches in the center, and fractional inches on the right. Subtract each column separately. | |
| When subtracting fractions, make the bottom numbers the same. Notice that the top number (2/16) is smaller than the bottom (13/16). Just like in normal subtraction we need to borrow one from the whole inches. | |
| When borrowing 1 from the whole inches, it turns into a fraction in the fraction column. Make a fraction with the same bottom number. 1 is the same as 16/16 in this case. Add 16/16 to the 2/16 already existing. Now we have 18/16 on top. This is an improper fraction, but that is the point. Subtract and I am left with 5/16. | |
| Subtract the whole inches, but here again there are not enough inches on top. We need to borrow from the feet column. One foot turns into 12 inches in the inch column. Add the 12 inches to the one existing. Now there are 13 inches on top. Subtract 13 – 9 = 4. | |
| Subtract whole feet. 6 – 2 = 4 | |
| The final answer. |
Adding and Subtracting Feet and Inches Exercise Set 1