DIY Cost Tools
Feet and Inches Calculator
Measurement Calculator
Feet and Inches Calculator
Add or subtract two measurements in feet, inches, and fractions. The calculator returns a normalized feet-and-inches result plus decimal feet, decimal inches, centimeters, and millimeters.
Add or subtract measurements
How to use the calculator
Enter the whole feet, whole or decimal inches, and optional fractional inches for both measurements. Choose addition or subtraction. You can leave a numerator at zero when no fraction is needed. The result updates as you type, so it is useful for trim layouts, framing checks, room dimensions, and material cut lists.
Feet and inches formula
Each measurement is converted to total inches before the operation is performed:
Total inches = (feet x 12) + inches + numerator / denominator
For addition, the two inch totals are added. For subtraction, the second total is removed from the first. Decimal feet equal total inches divided by 12. Centimeters equal inches multiplied by 2.54, and millimeters equal inches multiplied by 25.4.
Worked example
Suppose a project uses one piece measuring 6 ft 8 5/16 in and another measuring 1 ft 3 3/4 in. Their inch totals are 80.3125 and 15.75. Added together, they equal 96.0625 inches, or 8 ft 0 1/16 in. The same length is 8.005208 decimal feet, 243.999 centimeters, or 2,439.988 millimeters.
Accuracy and assumptions
- The mixed-number answer is rounded to the nearest 1/16 inch.
- Metric and decimal answers are calculated from the unrounded total.
- The fraction denominator may be 2, 4, 8, 16, 32, or 64.
- Negative results are supported when the second measurement is larger during subtraction.
- Field measurements can still vary because of tape condition, viewing angle, saw kerf, and material movement.
Frequently asked questions
How many inches are in one foot?
There are exactly 12 inches in one foot. Converting both inputs to inches makes mixed-unit addition and subtraction reliable.
Why can the fraction and decimal appear slightly different?
The feet-and-inches display rounds to a practical 1/16 inch. Decimal and metric values keep more precision, so their displayed equivalent may be a little more exact.
Can I enter more than 12 inches?
Yes. The result is normalized automatically. For example, 2 feet 15 inches is reported as 3 feet 3 inches.
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Sources and review
Conversion constants follow the SI reference information published by the National Institute of Standards and Technology. Calculator logic and examples were reviewed by the DIY Cost Tools editorial team on September 22, 2026.
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