Feet and Inches Calculator: Add & Subtract Lengths

This calculator adds or subtracts lengths given in feet and inches. You enter each length as feet and inches (for example, 5 ft 6 in), choose whether to add or subtract, and get the result in feet and inches, decimal feet, and total inches. The tool is designed for construction, woodworking, sewing, and any project where you need to combine or compare measurements without converting by hand.

Formula

The core formula converts each length to total inches, performs the addition or subtraction, then converts back to feet and inches. For a length given as F feet and I inches:

Total inches = F × 12 + I

For addition: Result inches = (F₁ × 12 + I₁) + (F₂ × 12 + I₂)

For subtraction: Result inches = (F₁ × 12 + I₁) − (F₂ × 12 + I₂)

To express the result in feet and inches, divide the total inches by 12. The whole number is feet, and the remainder is inches. For example, 74 inches ÷ 12 = 6 feet with 2 inches left over (because 6 × 12 = 72, and 74 − 72 = 2).

All variables are in inches or feet as labeled. The calculator uses integer inches for input (0–11) to avoid fractional inches in the entry fields, but the result displays inches with one decimal place so you can handle fractions like 6.5 inches.

Worked Example

Using the default values: first length = 5 ft 6 in, second length = 3 ft 4 in, operation = Add.

1. Convert first length to inches: 5 × 12 + 6 = 60 + 6 = 66 inches.
2. Convert second length to inches: 3 × 12 + 4 = 36 + 4 = 40 inches.
3. Add: 66 + 40 = 106 inches.
4. Convert back: 106 ÷ 12 = 8 feet with remainder 10 inches (8 × 12 = 96, 106 − 96 = 10).
5. Decimal feet: 106 ÷ 12 = 8.83 ft.

The calculator shows: Result (total inches) = 106.0 in, Result (ft & in) = 8 ft 10 in, Result (decimal feet) = 8.83 ft. The same process works for subtraction; for example, 66 − 40 = 26 inches = 2 ft 2 in.

Why Use a Feet and Inches Calculator?

Feet and inches are a mixed-unit system, which makes arithmetic error-prone. Unlike metric, where everything is a power of ten, you must constantly remember that 1 foot = 12 inches. A small mistake—like forgetting to carry the 12—can ruin a cut or a layout. This calculator removes that mental load by converting everything to a single unit (inches), doing the math, and converting back.

Common uses include:

• Adding up multiple board lengths for a framing project.
• Subtracting a gap from a total length to find the exact piece you need.
• Combining measurements taken in different units (one in feet, one in inches).
• Checking whether two lengths add up to fit a space.

It is especially handy when you work with fractional inches, because the calculator handles the decimal conversion for you.

How to Read the Results

The calculator gives three outputs, each useful in different situations:

• Result (total inches) – the raw sum or difference in inches. Use this when you need to work in inches only, such as when ordering material by the inch.
• Result (ft & in) – the conventional display, like 8 ft 10 in. This is what you’d write on a cutting list or read aloud on a jobsite.
• Result (decimal feet) – the same length expressed as a decimal, like 8.83 ft. This is useful when using a calculator that expects decimal feet, or when converting to metric (multiply by 0.3048 to get meters).

Note that the feet-and-inches output rounds the inches to one decimal place. If you need exact fractions (like 1/16 inch), you’ll need to convert the decimal remainder yourself. For instance, 0.5 in = 1/2 in, 0.25 in = 1/4 in, 0.125 in = 1/8 in.

Common Mistakes and How to Avoid Them

The most frequent error is mixing units without converting. For example, adding 5 ft 6 in to 3 ft 4 in by simply adding 5+3=8 and 6+4=10 gives you 8 ft 10 in, which happens to be correct here, but only because the inches sum to less than 12. If the inches sum to 12 or more, you must carry over. For instance, 5 ft 9 in + 3 ft 6 in: 9+6=15 inches, which is 1 ft 3 in, so the result is 8 ft + 1 ft + 3 in = 9 ft 3 in. A calculator prevents this slip.

Another common mistake is entering inches greater than 11. The calculator restricts inches to 0–11 because that’s the valid range for a foot-based system. If you have a measurement like 5 ft 14 in, convert it to 6 ft 2 in before entering. Similarly, when subtracting, the result can be negative if the second length is larger than the first. The calculator will show a negative total inches, which may be valid in some contexts (like a difference) but not for physical lengths. Check your inputs if you see a negative result.

Rounding is another pitfall. The decimal feet output rounds to two decimal places, which is about 1/8 inch (0.01 ft ≈ 0.12 in). For precision work, use the total inches output, which has one decimal place (0.1 in ≈ 3 mm). If you need 1/16-inch accuracy, you’ll have to do manual fraction conversion.

How to Choose Between Feet, Inches, and Decimal Feet

Your choice of output depends on what you’re doing next:

• Construction and carpentry – use feet and inches, because tape measures and lumber are marked that way.
• Engineering or CAD – use decimal feet, because most software expects a single unit.
• Metric conversion – use total inches and multiply by 2.54 to get centimeters, or use decimal feet and multiply by 0.3048 for meters.
• Sewing or crafts – total inches are often easier when working with fabric widths.

For example, if you’re building a shelf that must fit between two walls 6 ft 2 in apart, and you want a 1/2-inch gap on each side, subtract 1 inch total: 6 ft 2 in − 1 in = 6 ft 1 in. The calculator handles that directly.

Limitations and Alternatives

This calculator assumes standard 12-inch feet. It does not handle fractional inches directly (like 1/8 or 1/16), so you’ll need to convert fractions to decimals before entering. For example, 5 ft 6 1/2 in becomes 5 ft 6.5 in. It also doesn’t support negative lengths in the input, but subtraction results can be negative if you subtract a larger length from a smaller one.

For more complex operations—like multiplying lengths or working with square feet—you’ll need a different tool. This calculator is strictly for addition and subtraction of linear lengths. If you need to add multiple lengths (more than two), you can do it iteratively: add the first two, then add the result to the third, and so on.

For metric users, the same math works with centimeters and meters, but the conversion factor is 100 instead of 12. This calculator is specific to the imperial system, which is still common in the United States and for certain trades worldwide.

Frequently Asked Questions

How do I subtract feet and inches with borrowing?

Borrowing works like in regular subtraction, but you borrow 12 inches instead of 10. For example, to subtract 2 ft 8 in from 5 ft 3 in, you can’t subtract 8 from 3, so borrow 1 ft from the 5 ft, turning it into 4 ft and adding 12 inches to the 3, giving 15 inches. Then 15 − 8 = 7 in, and 4 − 2 = 2 ft, so the answer is 2 ft 7 in. The calculator does this automatically by converting to inches first.

Can I enter fractions of an inch like 1/4 or 1/16?

Not directly. The input fields accept decimal inches only (e.g., 0.25 for 1/4, 0.5 for 1/2, 0.0625 for 1/16). You’ll need to convert the fraction to a decimal before entering. For quick reference: 1/8 = 0.125, 1/4 = 0.25, 3/8 = 0.375, 1/2 = 0.5, 5/8 = 0.625, 3/4 = 0.75, 7/8 = 0.875.

Why does the result show 8 ft 10 in instead of 8.83 ft?

Both are correct; they’re just different representations. 8 ft 10 in means 8 feet plus 10/12 of a foot, which is 8.833… feet. The calculator shows both so you can use whichever is more convenient for your task. The feet-and-inches display is rounded to one decimal place, so 8 ft 10 in is exact here, but if the remainder were 10.4 inches, it would round to 10.4 in.

What if I need to add more than two lengths?

You can use the calculator repeatedly. Add the first two lengths, note the result, then add that result to the third length, and so on. For example, to add 2 ft 3 in + 1 ft 9 in + 4 ft 6 in, first add 2 ft 3 in + 1 ft 9 in = 4 ft 0 in (since 3+9=12 in = 1 ft), then add 4 ft 0 in + 4 ft 6 in = 8 ft 6 in. This works because addition is associative.

Is the result in decimal feet the same as feet and inches?

Yes, they represent the same length. Decimal feet is a single number where the fractional part is a decimal fraction of a foot. Feet and inches splits that into whole feet and remaining inches. For instance, 6.5 ft is the same as 6 ft 6 in because 0.5 ft = 6 in. To convert decimal feet to inches, multiply by 12: 0.5 × 12 = 6.

Can I use this for metric measurements?

No, the calculator is designed for feet and inches only. For metric, you’d use centimeters or meters, which are decimal-based and easier to add and subtract manually. If you have a metric measurement and need it in feet and inches, convert first: 1 inch = 2.54 cm, 1 foot = 30.48 cm.

Related

• Centimeter to Inches Calculator: Convert cm to in
• Dividing Remainder Calculator: Quotients and Remainders
• Double Recipe Calculator: Scale Ingredients Easily

in ft1 | First length feet | ft | 5 | 0 | 1000
in in1 | First length inches | in | 6 | 0 | 11
sel op | Operation | Add=1, Subtract=2
in ft2 | Second length feet | ft | 3 | 0 | 1000
in in2 | Second length inches | in | 4 | 0 | 11
out total_in | Result (total inches) | in | (ft1*12 + in1) + (op==1 ? (ft2*12 + in2) : -(ft2*12 + in2)) | 1
out ft_in | Result (ft & in) | - | floor(total_in/12) + " ft " + round(total_in - floor(total_in/12)*12, 1) + " in" | 0
out dec_ft | Result (decimal feet) | ft | total_in / 12 | 2