FIELD CALCULATIONS / WORKED EXAMPLE
Free feet and inches calculator: add and subtract
By Eutopix Labs · 9 September 2026
To add feet, inches and fractions, keep the units separate, use a common denominator for the fractions, and carry each 12 inches into one foot. A decimal point is not a separator between feet and inches.
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Calculate your measurement
Enter whole feet and inches as a decimal or fraction, such as 7 1/2. All calculations stay in your browser.
Need chained calculations, metric units and saved jobs? Get Onsite Math for iPhone and iPad. The core calculator is free.
Example: 8 ft 7½ in + 3 ft 9¾ in
- Add the feet: 8 + 3 = 11 ft.
- Add the inches: 7½ + 9¾ = 17¼ in.
- Carry 12 inches: 17¼ in = 1 ft 5¼ in.
- Combine the result: 12 ft 5¼ in.
Check the answer in inches
8 ft 7½ in is 103½ inches. 3 ft 9¾ in is 45¾ inches. Together they are 149¼ inches. Dividing by 12 gives 12 feet with 5¼ inches left over, so the two methods agree.
Why 8.75 feet is not 8 feet 75 inches
The decimal part of a foot must be multiplied by 12. Therefore 0.75 ft is 9 in, and 8.75 ft is 8 ft 9 in. Likewise, 8 ft 7½ in is 8.625 decimal feet.
When to round
Keep the full fraction while calculating. Round only the final result to the precision appropriate for the job. Repeated rounding can change a cut list or a total across several pieces.
Use the app for a longer calculation
Onsite Math handles feet, inches, fractions and metric conversions offline on iPhone and iPad. Enter measurements with their units and compare the result with this worked example before using it for a project.