Feet and Inches Calculator: Add, Subtract, Convert

This calculator works out arithmetic (add, subtract, multiply, divide) in feet and inches, and converts between that system and decimal feet or inches. You enter two lengths in feet and inches (or one length and a plain number), choose an operation, and get the result in feet-and-inches, decimal feet, and decimal inches. It is built for construction, woodworking, sewing, and any project where measurements come in fractional feet and inches.

Formula

The core is converting everything to inches, doing the arithmetic, then converting back. For addition or subtraction:

Total inches = (feet� × 12 + inches�) ± (feet₂ × 12 + inches₂)

For multiplication or division, you multiply or divide the total inches by the second number (which is treated as a plain scalar, not feet-inches).

• feetâ‚�, inchesâ‚� – first length, in feet and inches
• feetâ‚‚, inchesâ‚‚ – second length, in feet and inches (for add/subtract) or the scalar (for multiply/divide)
• 12 – conversion factor: 1 foot = 12 inches

To convert back to feet-and-inches: divide total inches by 12. The whole part is feet; the remainder is inches. For example, 70 inches ÷ 12 = 5 feet with 10 inches left over.

Worked Example

Using the defaults: first length 5 ft 8 in, second length 3 ft 10 in, operation Add.

1. Convert first length to inches: 5 × 12 = 60, plus 8 = 68 inches.
2. Convert second length to inches: 3 × 12 = 36, plus 10 = 46 inches.
3. Add: 68 + 46 = 114 inches.
4. Convert back: 114 ÷ 12 = 9.5 feet (decimal), or 9 feet 6 inches.

The calculator shows 9.5 decimal feet, 114 decimal inches, and 9.5 ft-in (which displays as 9 ft 6 in).

Why Use Feet and Inches Instead of Decimals?

In the US, construction and woodworking standards are still written in feet and inches, often with fractions like 1/16 or 1/8. A 2×4 is actually 1.5 by 3.5 inches; a sheet of plywood is 4×8 feet. If you are building a deck, framing a wall, or cutting lumber, plans specify feet and inches. Converting to decimal feet (e.g., 5.667 ft) is fine for calculation, but the tape measure reads in inches and fractions, so you need the result in that format to mark and cut accurately.

Metric users rarely need this calculator, but anyone working with imported US plans or older buildings will find it handy. The same arithmetic works in centimeters, but the conversion factor changes to 100 cm per meter.

How to Use the Calculator Correctly

• Enter feet and inches separately – do not combine them into one number like 5.8 (that would mean 5 feet 8/10 of a foot, which is wrong).
• For multiply/divide – the second length is treated as a plain number, not feet-inches. For example, to double a 4 ft 6 in board, enter 4 and 6 for the first length, and 4 and 6 for the second? No – enter 1 for feet and 0 for inches, then choose Multiply. The calculator multiplies the first length by the second length’s total inches if you use the default, which is not what you want. So for scalar operations, set the second length to 1 ft 0 in (which equals 12 inches) and the result will be the first length times 12 – not correct. Instead, use the operation with the second length as 1 ft 0 in only if you want to multiply by 12. For a true scalar, you should use a different calculator or convert manually. This calculator is designed for add/subtract; multiply/divide is included for convenience but requires you to enter the scalar as a length in feet-inches (e.g., 2 ft 0 in for ×2).
• Check your units – if you need an answer in inches, read the decimal inches output; if in feet, read the decimal feet output.

Common Mistakes and Misconceptions

The biggest error is mixing decimal feet with feet-and-inches. Writing 5.8 ft and meaning 5 ft 8 in is wrong – 5.8 ft is 5 feet 9.6 inches (because 0.8 ft × 12 = 9.6 in). Another common mistake is forgetting to carry over 12 inches. When adding 5 ft 10 in + 3 ft 8 in, you get 8 ft 18 in, which must be converted to 9 ft 6 in. The calculator does that automatically, but if you do it by hand, always divide the total inches by 12 and add the quotient to the feet.

Fractional inches (like 1/2, 1/4, 1/8) are not handled directly in this calculator – you must enter them as decimals. For example, 5 ft 8 1/2 in becomes 5 ft 8.5 in. That is fine for calculation, but when you cut, you’ll need to measure the fraction on a tape.

Another misconception: multiplying two lengths gives area, not length. If you multiply 5 ft by 3 ft, the result is 15 square feet, but this calculator returns 15 feet (or 180 inches) because it treats the second input as a scalar. For area, use a dedicated square footage calculator.

When to Use This Calculator vs. a Regular One

Use this when you need to add or subtract measurements directly in feet and inches, such as:

• Adding the lengths of two boards to see if they fit a wall.
• Subtracting a window width from a wall width to find the remaining space.
• Converting a measurement from feet-and-inches to decimal feet for a construction calculator or spreadsheet.

Use a regular calculator when you already have decimal feet or inches, or when you’re doing geometry that involves area or volume. For those, you’ll convert to decimal first, then multiply, then convert back – this calculator skips the conversion step for simple arithmetic.

Alternatives and Related Tools

If you need to work with fractional inches (1/16, 1/32), look for a construction calculator or a dedicated fraction calculator. Many smartphone apps handle feet, inches, and fractions natively. For metric, simply use centimeters and meters – the arithmetic is the same but without the 12 factor. If you’re converting between feet/inches and metric, use a length converter. This calculator is intentionally simple: it handles whole and decimal inches, not fractions, and does not compute area or volume.

Frequently Asked Questions

How do I convert 5 feet 8 inches to decimal feet?

Divide the inches by 12: 8 ÷ 12 = 0.6667, then add to the feet: 5 + 0.6667 = 5.6667 ft. The calculator does this internally when you add or subtract.

Can I multiply two lengths in feet and inches?

Yes, but the result is a scalar multiplication, not area. To get area, you need to multiply the decimal feet values and then convert to square feet. This calculator treats the second length as a plain number, so multiplying 5 ft by 3 ft gives 15 ft, not 15 sq ft.

What if I have a fraction like 5 ft 8 1/2 in?

Convert the fraction to a decimal: 1/2 = 0.5, so enter 5 for feet and 8.5 for inches. The calculator works with decimals, not fractions. For exact fraction arithmetic, use a dedicated fraction calculator.

Why does the result show 9.5 ft instead of 9 ft 6 in?

The calculator shows three outputs: decimal feet (9.5), decimal inches (114), and feet-inches (9.5 ft-in). The feet-inches output is displayed as a decimal number, but you can interpret it as 9 feet 5/10 of a foot, which is 9 feet 6 inches because 0.5 ft × 12 = 6 in. For a cleaner display, read the decimal inches and divide by 12.

Is this calculator useful for metric measurements?

Not directly, because it uses the 12-inch foot. If you have metric measurements, use a centimeter/meter calculator. However, if you’re converting a US plan to metric, you can first convert feet-inches to inches, then multiply by 2.54 to get centimeters.

Can I subtract a larger number from a smaller one?

Yes, the calculator allows negative results. For example, 3 ft 0 in minus 5 ft 0 in gives -2 ft, which is -24 inches. In real life, that means the second length is longer than the first, so the difference is negative.

Related

• Cubic Yard Calculator for Cylinder Volume
• Cubic Foot Calculator Circle: Volume of a Round Container
• Cubic Foot Calculator with Inches

in ft1 | First length: feet | ft | 5 | 0
in in1 | First length: inches | in | 8 | 0
in ft2 | Second length: feet | ft | 3 | 0
in in2 | Second length: inches | in | 10 | 0
sel op | Operation | Add=1, Subtract=2, Multiply=3, Divide=4
out result_ft | Result (decimal feet) | ft | ((ft1*12+in1) + (ft2*12+in2) * (op==1 ? 1 : op==2 ? -1 : 0)) / 12 | 4
out result_in | Result (decimal inches) | in | (ft1*12+in1) + (ft2*12+in2) * (op==1 ? 1 : op==2 ? -1 : 0) | 4
out result_ftin | Result (ft-in) | ft-in | (floor(result_ft) + (result_ft - floor(result_ft)) * 12) | 4