Feet and Inches Calculator

Add, subtract, multiply, or divide lengths in feet and inches.

Supports fractions and works in feet/inches, square feet, and square inches.

Updated August 2, 2026

CreatorFrank Zhao

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Introduction / overview

The Feet and Inches Calculator is a small workbench for anyone who measures in feet and inches but gets tired of converting everything to decimals first. You enter two lengths — each as feet plus inches, fractions welcome — pick an operator, and the result appears in a format you can actually read: $3\ \mathrm{ft}\ 9\ \mathrm{in}$ instead of $45$ inches, or an area in square feet instead of a raw number.

The neat trick: type a fraction like $5 + 3/8$ straight into the inches box — no need to work out $5.375$ yourself. The calculator understands common fraction notation and does the conversion for you.

Who is this for?

DIYers and woodworkers adding or cutting board lengths measured in feet and inches.
People sizing rooms, furniture, or materials who need a quick area in square feet.
Anyone working with fractional-inch measurements — fine woodworking, sewing, or metalwork — who wants results at a sensible precision.

The result updates the moment you finish a value, and the calculator is fully bidirectional — edit the result and one of the inputs recalculates to match, which is handy for working backwards from a target size. If you only need to switch a single measurement between units, our Feet to Inches Calculator is the quicker one-stop for plain conversions.

How to use / quick start

1Enter the first length: type the whole Feet into the feet box and theInches into the inches box. Leave a part blank if it’s zero — $0\ \mathrm{ft}\ 9\ \mathrm{in}$ just needs the 9 in inches.
2Pick the Operator: $+$ , $-$ , $\times$ , or $\div$ .
3Enter the second length the same way. Want fractional inches? Write them like $1/4$ , $5+3/8$ , or $3*4$ — the inches box accepts simple fraction expressions.
4Read the result. For add and subtract it appears as $\mathrm{X\ ft\ Y\ in}$ , with a “Fraction inches” readout beneath it; for multiply you get an area; for divide a plain number.

Worked example — add two shelf boards: 2 ft 3 in plus 1 ft 6 in

2\ \mathrm{ft}\ 3\ \mathrm{in}

=

2 \times 12 + 3

=

27\ \mathrm{in}

1\ \mathrm{ft}\ 6\ \mathrm{in}

=

18\ \mathrm{in}

27 + 18

=

45\ \mathrm{in}

=

3\ \mathrm{ft}\ 9\ \mathrm{in}

In the calculator, set the operator to add, type $2$ and $3$ for the first value and $1$ and $6$ for the second — the result reads $3\ \mathrm{ft}\ 9\ \mathrm{in}$ .

Worked example — subtract with a fraction: 5 ft 3/8 in minus 2 ft

5\ \mathrm{ft}\ 3/8\ \mathrm{in}

=

5 \times 12 + 0.375

=

60.375\ \mathrm{in}

60.375 - 24

=

36.375\ \mathrm{in}

=

3\ \mathrm{ft}\ 0.375\ \mathrm{in}

Type $3/8$ straight into the inches box — the calculator handles the fraction for you.

How to read the results

The values you typed stay black; the auto-calculated result appears in blue so you always know which one you entered.
For add/subtract, the “Fraction inches” line shows the same remainder three ways — as a decimal, as an improper fraction, and as a mixed fraction — rounded to the precision you chose.
For multiply, the answer is an area; for divide, a plain (unit-less) number.

Real-world examples

Adding cabinet pieces for a kitchen install

You’re fitting a run of cabinets. One cabinet face is $2\ \mathrm{ft}\ 3\ \mathrm{in}$ wide and the next is $1\ \mathrm{ft}\ 6\ \mathrm{in}$ . The total run is:

(2 \times 12 + 3) + (1 \times 12 + 6)

=

45\ \mathrm{in}

=

3\ \mathrm{ft}\ 9\ \mathrm{in}

Now you know the combined width before ordering countertop or scribing the filler strip. Want the same total in centimeters for a metric supplier? Switch the result unit to cm and read it directly.

Cutting a length of trim into equal pieces

You have a $6\ \mathrm{ft}$ length of trim and want to know how many $4\ \mathrm{in}$ pieces it can make. Use division:

\frac{6\ \mathrm{ft}}{4\ \mathrm{in}}

=

\frac{72\ \mathrm{in}}{4\ \mathrm{in}}

=

18

Eighteen full pieces — with the blade kerf in mind you’d order slightly longer, but it’s the right starting number. For a decimal-inches cut list instead, our Inches to Fraction Calculator can turn awkward decimals into clean sixteenths for the tape.

Floor area of a small room in square feet

A room is $12\ \mathrm{ft}\ 6\ \mathrm{in}$ by $10\ \mathrm{ft}$ . Choose multiplication:

(12 \times 12 + 6) \times (10 \times 12)

=

150 \times 120

=

18\,000\ \mathrm{in}^2

=

125\ \mathrm{ft}^2

A handy 125 sq ft for flooring estimates. The result can be shown in square feet or square inches; for converting the final area to square meters or yards, pair it with our Area Converter.

Working backwards from a target size

You need a bookshelf section to total exactly $4\ \mathrm{ft}\ 6\ \mathrm{in}$ , and one shelf already measures $1\ \mathrm{ft}\ 8\ \mathrm{in}$ . Set subtraction, enter $4\ \mathrm{ft}\ 6\ \mathrm{in}$ as the first value and $1\ \mathrm{ft}\ 8\ \mathrm{in}$ as the second:

(4 \times 12 + 6) - (1 \times 12 + 8)

=

54 - 20

=

34\ \mathrm{in}

=

2\ \mathrm{ft}\ 10\ \mathrm{in}

That’s the gap to fill. Because the result field is editable, you can also type a target result and let the calculator reveal the matching input — the smart two-way solving does the algebra.

Common scenarios / when to use

DIY & home improvement

Adding board lengths, finding the difference between a cut and an opening, or sizing a piece of stock. Fractional-inch inputs make trim and molding math painless.

Flooring & area planning

Multiply length by width in feet and inches to get an area in square feet — ideal before buying flooring, tiles, or underlay.

Cutting & dividing materials

Dividing a long length into equal pieces, or working out how many fixed-size pieces fit. Division returns a plain count, so 6 ft ÷ 4 in = 18 pieces, instantly.

Furniture & clearance checks

Seeing whether a sofa fits a wall, or how much room is left after a piece is placed. Subtraction gives the gap directly, including negative answers when it won’t fit.

When it’s less useful

This tool works in feet and inches, so it’s awkward for pure metric jobs — switch the result to cm or m, or reach for the Length Converter for a wide range of units. And for structural or load-bearing engineering, always have a professional verify the numbers — this is a handy everyday helper, not an engineering tool.

Tips & best practices

Type fractions, not just decimals

The inches boxes accept expressions like $1/4$ , $5+3/8$ , or $3*4$ . This saves a conversion step and keeps mistakes out of fine measurements.

Remember the 1 ft = 12 in rule: if your inches part reaches 12 or more, it simply rolls over into the feet — the calculator normalizes it for you.

Choose a precision that matches the job

Coarse framing usually only needs $1/2$ inch; fine woodworking often calls for $1/16$ or $1/32$ . Pick the precision so the result reads cleanly on your tape instead of showing long decimals.

The “Fraction inches” line shows the same remainder as a decimal, an improper fraction, and a reduced mixed fraction — perfect for marking a fractional cut.

Watch out for common gotchas

A negative subtraction answer just means the second value is bigger — that’s a real “doesn’t fit by X” signal.
Division needs a non-zero divisor; if both feet and inches of the second value are zero, the calculator shows a “non-zero divisor” message instead of an answer.
For area, remember the result is square units — $\mathrm{ft}^2$ not $\mathrm{ft}$ — so don’t compare it to a length.

For the most accurate results

Measure carefully, enter the exact fraction rather than a rounded decimal, and double-check that feet and inches are in the right boxes. Want the same measurement in plain inches for a quick comparison? Our Height in Inches Calculator converts a feet-and-inches height straight to total inches.

Calculation method

Every operation starts the same way: each length is converted to a single number of inches, because $1\ \mathrm{ft} = 12\ \mathrm{in}$ . Let $f_1,\ i_1$ be the feet and inches of the first value and $f_2,\ i_2$ those of the second.

Total inches of each length

T = f \times 12 + i

Addition & subtraction

R_{\mathrm{in}} = (f_1 \times 12 + i_1) + (f_2 \times 12 + i_2)

R_{\mathrm{in}} = (f_1 \times 12 + i_1) - (f_2 \times 12 + i_2)

The answer in inches is then split back into whole feet and remaining inches:

\mathrm{feet} = \left\lfloor \frac{|R_{\mathrm{in}}|}{12} \right\rfloor

\mathrm{inches} = |R_{\mathrm{in}}| - \mathrm{feet} \times 12

Multiplication (area)

A_{\mathrm{in}^2} = (f_1 \times 12 + i_1) \times (f_2 \times 12 + i_2)

Because each side is in inches, the product is in square inches. Since $1\ \mathrm{ft}^2 = 144\ \mathrm{in}^2$ :

A_{\mathrm{ft}^2} = \frac{A_{\mathrm{in}^2}}{144}

Division (unit-less ratio)

R = \frac{f_1 \times 12 + i_1}{f_2 \times 12 + i_2}

Dividing a length by a length cancels the units, so the answer is a plain number — for example, how many times one length fits into another. Division by zero is undefined, so the calculator asks for a non-zero divisor instead.

Fractional-inch rounding

For add and subtract, the fractional remainder is rounded to the nearest selected step. With a precision of $1/16$ , for example, the remainder is rounded to the nearest sixteenth:

\mathrm{rounded} = \frac{\mathrm{round}(R_{\mathrm{in}} \times 16)}{16}

Related concepts / background info

The 12-inches-to-a-foot rule

Feet and inches are a compound imperial unit: one foot always equals twelve inches. That’s the single conversion behind every formula here. Writing $30\ \mathrm{in}$ as $2\ \mathrm{ft}\ 6\ \mathrm{in}$ is just splitting it by twelves — the quotient is the feet and the remainder the inches.

Square feet vs square inches

Multiplying two lengths gives an area. Because a square foot is $12 \times 12$ square inches, the conversion factor is $144$ . Flooring, tiling, and painting estimates are usually quoted in square feet, so that’s the default — but the result can be switched to square inches, yards, meters, or centimeters.

Why division has no unit

When you divide two lengths with the same unit, the unit cancels out — $\mathrm{ft} / \mathrm{ft} = 1$ . That’s why the divide result is a plain ratio or count, like “this board is 1.5 times that one,” rather than a new measurement.

Why this calculation matters

Most real-world measurements are taken directly in feet and inches, but most math is easier in a single unit. Converting once, calculating, and converting back is exactly what this calculator does for you — removing the most error-prone step in carpentry and layout work. For single-value unit switches, the Feet to Inches Calculator and Length Converter are great companions.

Frequently asked questions

How do I enter a fraction like 1/4 of an inch?

Type $1/4$ straight into the inches box. You can also use a whole number plus a fraction, like $5+3/8$ , which means $5.375$ inches. Simple expressions with $+$ , $-$ , $*$ , or $/$ are accepted.

How many inches are in a foot?

Exactly $12$ . To convert inches to feet, divide by 12 — the quotient is the feet and the remainder is the leftover inches. For example, $62\ \mathrm{in} = 5\ \mathrm{ft}\ 2\ \mathrm{in}$ .

Why is my subtraction result negative?

A negative answer simply means the second length is bigger than the first. It’s a useful “doesn’t fit by this much” number — for example, $2\ \mathrm{ft} - 5\ \mathrm{ft} = -3\ \mathrm{ft}$ .

What does the Precision choice do?

For add and subtract, it rounds the fractional inch to the nearest step — from a coarse $1/2$ inch up to a fine $1/64$ . Pick the step that matches how precisely you can read your tape measure; it only affects the displayed fraction, not the underlying calculation.

What’s the difference between square feet and square inches?

Multiplying two lengths in inches gives square inches. Because a foot is 12 inches, a square foot is $12 \times 12 = 144$ square inches, so $1\ \mathrm{ft}^2 = 144\ \mathrm{in}^2$ . The multiply result can be shown in either, or in square yards, meters, or centimeters.

Why does the division result have no unit?

Dividing a length by a length cancels the unit out — think of it as “how many times does one length fit into the other.” So $6\ \mathrm{ft} \div 4\ \mathrm{in} = 18$ , a plain count of 4-inch pieces.

Why do I see “Division by zero isn’t allowed” when dividing?

Division by zero has no mathematical answer. If the second value is $0\ \mathrm{ft}\ 0\ \mathrm{in}$ , the calculator can’t compute a ratio, so it shows that message and waits for a non-zero divisor instead of guessing.

Can I use this to calculate a room’s area?

Yes — choose multiplication and enter the room’s length and width in feet and inches. The area appears in square feet by default, which is the unit most flooring and painting estimates use. For converting that area to other units, our Area Converter can finish the job.

Limitations / disclaimers

A few practical boundaries

The result is only as accurate as your inputs — a tape measure read to the nearest 1/8 in will always carry that small uncertainty into the answer.
Fraction inputs support simple expressions; they aren’t a full algebra engine, so keep expressions to plain numbers with +, -, *, /.
Division by zero is undefined and returns no answer; the multiply result is an area, so don’t compare it to a length.
This is a general-purpose helper for everyday measurements, not a substitute for professional engineering or structural review on safety-critical work.