Construction math with feet, inches, and fractions
This construction calculator handles a complete expression, so you can add measured lengths, subtract an allowance, and divide the result without converting each fraction to a decimal. It is free to use without an account. Enter dimensions with their units, use parentheses to group steps, and press equals when the expression is ready. Results stay in place while you edit, with a label showing when they need recalculating.
For a quick two-measurement operation, the Feet & Inches Calculator remains available. This tool adds multi-step expressions, dimension-aware arithmetic, unit conversions, and reusable calculation history.
Add and subtract feet and inches
Suppose two measured lengths are 12 feet 6 3/8 inches and 3 feet 2 1/4 inches. Enter 12' 6 3/8" + 3' 2 1/4". In inches, the addition is 150 3/8 + 38 1/4 = 188 5/8. The result is exactly 15 feet 8 5/8 inches. Load worked example fills this expression deliberately; it does not calculate until you press equals.
For subtraction, 8' 1/2" − 2' 3 3/4" gives 5 feet 8 3/4 inches. The calculator borrows across feet and inches automatically. Negative values are supported for differences: 3" − 5" gives -2 inches. A negative measurement can represent a shortfall; it is not a physical material quantity to order.
A tight hyphen in 12'-6" joins feet and inches. A spaced minus in 12' - 6" subtracts six inches and gives 11 feet 6 inches. The keypad always inserts subtraction with spaces. Use explicit units for every measurement: 12 + 6 is the number 18, while 12' + 6" is 12 feet 6 inches.
Divide a length without accumulated rounding
To divide 11 feet 6 inches into three equal lengths, enter 11' 6" ÷ 3. The run is 138 inches, and 138 ÷ 3 = 46, so each length is exactly 3 feet 10 inches. The divisor has no unit because it is a count of equal parts.
Some answers fall between tape divisions. 1' ÷ 7 is exactly 1 5/7 inches. Its 1/16-inch tape reference is approximately 1 11/16 inches. Repeatedly adding that rounded reference seven times would total 11 13/16 inches instead of 12 inches. Use Result, history reuse, and memory retain the original exact fraction; multiplying the stored answer by seven returns exactly one foot.
For a list of cumulative marks measured from one datum, use the Jobsite Layout Calculator. It calculates each position independently and includes the actual endpoint. The calculator on this page solves expressions; it does not turn a rounded spacing into a field layout automatically.
Multiply feet and inches to calculate square feet
Length multiplied by length is area. An opening 8 feet 6 inches by 3 feet 4 inches is 102 in × 40 in = 4,080 in². Dividing by 144 square inches per square foot gives exactly 28 1/3 square feet, displayed as approximately 28.3333 ft². The result selector offers square feet and square meters because the answer is an area.
To deduct a rectangular opening from a wall area, group the products: (10' × 8') − (3' × 7') gives 59 ft². Multiplication and division run before addition and subtraction. Parentheses make the intended sequence explicit. Enter the multiplication operator between groups; adjacent parentheses do not imply multiplication.
Keep length, area, volume, and ratios distinct
The units follow the calculation. A 10-foot by 12-foot area with a 4-inch depth is 10' × 12' × 4", or exactly 40 cubic feet. That is 1 13/27 cubic yards, displayed as approximately 1.4815 yd³. This is geometric volume only. The Concrete Calculator handles its own material-estimating workflow.
- Length × unitless number gives length
6' × 3 = 18'. A count does not add a dimension.- Length ÷ length gives a ratio
36 in ÷ 12 in = 3, with no units.- Area ÷ length gives length
80 ft2 ÷ 8 ft = 10 ft.- Volume ÷ area gives length
40 ft3 ÷ 120 ft2 = 4 in.
Addition and subtraction require matching dimensions. 10 ft2 + 2 ft cannot produce a meaningful construction measurement, so the calculator rejects it. It also rejects division by zero and results outside length, area, volume, or unitless values, such as inverse length. It does not silently relabel square or cubic units as inches.
Construction measurement conversions
Calculate a value, then use Show as to convert it within its dimension. Length options include feet and inches, decimal inches, feet, yards, millimeters, centimeters, and meters. Area can display in square feet or square meters. Volume can display in cubic feet, cubic yards, or cubic meters. Typed units also accept forms such as sq ft, ft², cubic yards, and m3.
One foot equals 12 inches, one yard equals 3 feet, and one inch equals exactly 25.4 millimeters. Therefore 25.4 mm converts to exactly 1 inch, and 2.5 m converts to exactly 2,500 mm. One square foot equals exactly 0.09290304 m²; one cubic yard equals exactly 0.764554857984 m³. Area conversion squares the length factor, and volume conversion cubes it.
Decimal feet are not feet-and-inches notation: 8.5 ft means 8 feet 6 inches, not 8 feet 5 inches. Likewise, a decimal inch is a fraction of an inch. Enter 38.625 in and select Feet & inches to get exactly 3 feet 2 5/8 inches.
Fraction precision, history, and field limits
The default tape reference is the nearest 1/16 inch. You can select 1/4, 1/8, or 1/32 inch. An approximation symbol, ≈, identifies a rounded display. Half increments round away from zero, including negative results. Decimal units display up to four decimal places. Open Exact value & rounding to see the fraction behind the displayed answer. A rounded zero can still have a small nonzero exact value.
Common fractions and the custom fraction entry avoid confusing division with a tape fraction. Digits joined by a slash, such as 3/8, form a single fraction. For division, use the divide key or surround the slash with spaces. Thus 8 / 1/2 means eight divided by one half, which is 16. Mixed fractions require a numerator smaller than the denominator.
History keeps the most recent 20 successful calculations in this browser tab. Select a calculation to edit it, or reuse its exact result in another expression. Copy Result includes the expression, displayed result, exact value, and rounding precision. History and memory use temporary browser storage and survive a reload when that storage is available. They are not sent to analytics or stored in an account.
Expressions are limited to 512 characters, 128 tokens, and 16 nested parentheses. Decimal input allows six decimal places. Exact reduced fraction parts are limited to 30 digits, and intermediate values are limited to one trillion in base inches, square inches, cubic inches, or unitless values. Very large or complex expressions receive an error instead of an unreliable answer.
Check actual dimensions and allowances before cutting. Arithmetic cannot account for an unmeasured kerf, uneven surface, or nominal size that differs from the material on site. Keep your measurements in the Field Measurement Pad, or practice the underlying skills in the Construction Math Trainer.