1. Convert each measurement, then multiply
One foot contains 12 inches. Divide the inches by 12 and add that amount to the whole feet.
Then multiply by the width: 12.5 × 10 = 125 sq ft.
12 ft 6 in is not 12.6 ft. Six inches is half a foot, so the correct decimal length is 12.5 ft.
What if both measurements have inches?
Convert each one before multiplying. For a 12 ft 6 in length and a 10 ft 3 in width:
- Length: 12 + 6 ÷ 12 = 12.5 ft.
- Width: 10 + 3 ÷ 12 = 10.25 ft.
- Area: 12.5 × 10.25 = 128.125 sq ft, displayed as 128.13 sq ft.
The same conversion works for wall width and height, or a section’s dimensions. The shape still determines which measurements and area formula you need.
2. Match your measurement to the input boxes
Use the Units menu for the whole calculation. In ft + in mode, one part of a measurement may be blank: the calculator treats that blank part as zero.
| You have | Choose units | Enter |
|---|---|---|
| 12 ft 6 in | ft + in | ft: 12 in: 6 |
| 10 ft | ft + in | ft: 10 in: blank or 0 |
| 150 in | ft + in | ft: blank or 0 in: 150 |
| 12.5 ft | Feet (ft) | 12.5 in the single ft input |
| 12 ft 6 1/2 in | ft + in | ft: 12 in: 6 1/2 or 6.5 |
Decimal feet also work in ft + in mode: enter 12.5 in ft and leave in blank or 0. Both boxes blank means the measurement is still missing.
3. Enter fractional inches
If your tape or note says 6½ inches, type 6 1/2 in the in box: use a space between the whole number and the fraction, and a slash between its numbers. You can also type 6.5.
- 1/4 in
- 0.25 in
- 1/2 in
- 0.5 in
- 3/4 in
- 0.75 in
For 12 ft 6 1/2 in, keep 12 in the ft box. Only the 6 1/2 goes in the in box. For a 10 ft width, this gives approximately 125.42 sq ft.
These decimals are inches. Half an inch is 0.5 in; half a foot is 6 in. Keep the value in the box for its actual unit.
4. If both measurements are in inches
Leave ft blank and enter each dimension in its in box. For example, enter 150 in for length and 120 in for width. The calculator returns 125 sq ft.
150 × 120 is 18,000 square inches. Divide that area by 144 to get square feet.
Why divide by 144 instead of 12?
A square foot is 12 in × 12 in, so it contains 144 square inches. Dividing a length by 12 converts inches to feet; dividing an area by 144 converts square inches to square feet.
Compare square feet with other area units →
One length, such as 150 in, does not tell you an area. A rectangle also needs its width. If you already have an area in square inches, divide that area by 144 directly.
5. Check common entry mistakes
- Using decimal feet and extra inches together
- In this tool, use whole feet + inches, or decimal feet with in blank or 0. For 12 ft 6 in, enter 12 and 6, or 12.5 and 0. Entering 12.5 and 6 triggers a correction prompt. This is an input rule to help prevent double counting; those two lengths can still be added mathematically.
- Pasting the full measurement into one box
- 12′6″ means 12 feet 6 inches. Separate it into ft: 12 and in: 6. Do not paste the symbols or the whole measurement into ft. For fractions, use 6 1/2 or 6.5 rather than 6½ or 6-1/2.
- Rounding before the calculation is finished
- Keep the original dimensions and unrounded conversions while calculating. Round the final area for display. A rounded area is not a whole-box purchase quantity.
Continue with your measurements
Return to the calculator to enter or correct your dimensions. Your measured area comes before any material waste allowance.
Sources and method
NIST’s unit tables give 12 inches per foot and 144 square inches per square foot. The examples use those relationships and match our calculator. Input-format instructions describe this tool’s supported formats.
Content checked October 17, 2026.