How to Add, Subtract and Multiply Feet and Inches (With Fractions)
To add or subtract lengths in feet and inches, convert both to inches, do the sum, and convert back. 5 ft 7½ in plus 2 ft 8¾ in is 67.5 + 32.75 = 100.25 inches, which is 8 ft 4 1/4 in. The feet and inches calculator does it for you, with fractions down to 1/16 inch.
Worked examples
| Sum | Answer |
|---|---|
| 5 ft 7½ in + 2 ft 8¾ in | 8 ft 4 1/4 in |
| 5 ft 7½ in − 2 ft 8¾ in | 2 ft 10 3/4 in |
| 5 ft 7½ in × 3 | 16 ft 10 1/2 in |
| 5 ft 7½ in ÷ 4 | 1 ft 4 7/8 in |
| How many times does 2 ft 8¾ in go into 5 ft 7½ in? | 2.06 times (2 whole, 2 in left over) |
The method, step by step
- Write each length as feet, inches and a fraction.
- Convert to inches: feet × 12, plus the inches, plus the fraction.
- Do the sum in inches (add, subtract, multiply or divide).
- Convert back: divide by 12 for feet, and the remainder is the inches. Turn the decimal part into sixteenths by multiplying by 16.
Fractions of an inch
A tape measure is marked in sixteenths. The decimal and millimeter value of each:
| Fraction | Decimal inch | Millimeters |
|---|---|---|
| 1/16 | 0.0625 | 1.59 |
| 1/8 | 0.1250 | 3.18 |
| 3/16 | 0.1875 | 4.76 |
| 1/4 | 0.2500 | 6.35 |
| 5/16 | 0.3125 | 7.94 |
| 3/8 | 0.3750 | 9.52 |
| 7/16 | 0.4375 | 11.11 |
| 1/2 | 0.5000 | 12.70 |
| 9/16 | 0.5625 | 14.29 |
| 5/8 | 0.6250 | 15.88 |
| 11/16 | 0.6875 | 17.46 |
| 3/4 | 0.7500 | 19.05 |
| 13/16 | 0.8125 | 20.64 |
| 7/8 | 0.8750 | 22.22 |
| 15/16 | 0.9375 | 23.81 |
Spaces along a length
To find how many studs, balusters or boards fit along a wall, divide the length by the spacing. For a 16 inch spacing:
| Wall | Spaces of 16 in | Whole spaces |
|---|---|---|
| 10 ft wall | 7.50 | 7 |
| 12 ft wall | 9.00 | 9 |
| 16 ft wall | 12.00 | 12 |
| 20 ft wall | 15.00 | 15 |
| 24 ft wall | 18.00 | 18 |
Add one for the stud at the end, and add extras for corners and openings, or use the stud calculator.
Tips
- Measure twice and add up once. Write each measurement down with its fraction.
- Use the actual size of lumber: a 2×4 is 1½ × 3½ inches, and a 1×6 is ¾ × 5½ inches.
- For metric, use the millimeter values and a metric tape. One inch is 25.4 mm.
- Round to the nearest 1/16 inch, which is as precise as you can mark with a pencil.
- Do not mix up decimal feet with feet and inches: 8.5 feet is 8 ft 6 in, not 8 ft 5 in.
Frequently asked questions
How do you add feet and inches?
Convert both lengths to inches, add them, and convert back. 5 ft 7½ in plus 2 ft 8¾ in is 67.5 + 32.75 = 100.25 inches, or 8 ft 4 1/4 in.
How do you subtract feet and inches?
Convert to inches, subtract, and convert back: 5 ft 7½ in minus 2 ft 8¾ in is 2 ft 10 3/4 in.
How do you multiply feet and inches?
Convert to inches, multiply by the number, and convert back. 5 ft 7½ in × 3 is 16 ft 10 1/2 in.
How do you divide feet and inches?
Convert to inches, divide, and convert back. 5 ft 7½ in ÷ 4 is 1 ft 4 7/8 in.
How many inches are in a foot?
12. A tape measure marks each inch in sixteenths, so the smallest mark is 1/16 inch, which is 0.0625 inch or about 1.59 millimeters.
More guides
Tools & materials for this project
As an Amazon Associate we earn from qualifying purchases. These links do not change what you pay.
Estimates only. Quantities depend on your materials, site and supplier. Confirm important numbers before you order, and follow local building codes for anything structural.