Tape measures speak in feet and inches, but arithmetic refuses to: adding 5 ft 7 in to 2 ft 9 in in your head means carrying twelves, not tens, and the errors multiply on real projects. This Feet and Inches Calculator adds or subtracts two tape-measure dimensions properly and reports the result every way you might need it — feet-and-inches, decimal feet, total inches, centimetres and metres.
Worked Example: 5 ft 7 in + 2 ft 9 in
The calculator first converts everything to the common currency of total inches: 5 ft 7 in = 67 inches, and 2 ft 9 in = 33 inches. The sum is 100 inches, which re-expresses as 8 ft 4 in — the carrying done by division (100 ÷ 12 = 8 remainder 4) rather than by hope. The same total in the other dialects: 8.33 decimal feet, 254 cm, 2.54 m. Notice what the head-calculation does wrong, because it is the mistake this page exists to retire: adding the columns separately gives 7 ft 16 in, and the untrained instinct ships “7 ft 16 in” or fumbles the carry into 7 ft 6 in or 8 ft 6 in — three wrong answers from one sum, each plausible enough to cut lumber by. Subtraction borrows in twelves the same way: 8 ft 4 in minus 2 ft 9 in borrows a foot (7 ft 16 in) before subtracting to reach 5 ft 7 in, and the borrow is the step mental arithmetic skips under load. Fractions ride the same rails when they appear: enter 7.5 inches and the machinery is unchanged, because decimal inches are the honest intermediate — the companion Inches to Fraction calculator handles the return trip, turning a decimal result back into the sixteenths your tape actually displays.
Why Twelve-Based Arithmetic Trips Everyone
Feet-and-inches is a mixed-radix system — twelves stacked on a decimal world — and human brains are built for uniform bases. Every other measurement you add (money, metric, decimal feet on a blueprint) carries at ten; the tape measure alone carries at twelve, and it does so while your hands are full and the saw is waiting. The professional coping strategies are all represented in this calculator’s outputs. Total-inches thinking (the method above) converts the problem to ordinary arithmetic and back at the end — the method finish carpenters use for running totals across a wall of cabinets. Decimal-feet thinking is the surveyor’s and engineer’s dialect: 8.33 ft enters formulas, spreadsheets and slope calculations without translation, and construction calculators on site flip between the two all day. Metric thinking sidesteps the radix entirely — 2.54 m carries at ten everywhere — which is why so much of the world’s building happens in millimetres and why the cm/m outputs sit on this page: enter the imperial measurements you were given, read off the metric the drawing wants. Whichever dialect your next step speaks, converting once, exactly, at the boundary is the whole discipline — the errors happen in a dozen small mental conversions, never in one deliberate one.
Where This Arithmetic Runs the Job Site
Framing and layout: stud walls accumulate dimensions — a 16-inch on-centre layout across a 12 ft wall is nine spaces of sixteen inches summed without drift, and a single carried-twelve error early in the run telegraphs to the last stud. Cabinetry and trim: filler strips, crown runs and countertop overhangs are sums and differences of tape readings taken in awkward corners, the borrow-and-carry heartland. Height and clearance: a person’s 5 ft 7 in is 67 inches and 170.2 cm (the form the rest of the world wants it in); doorway, ceiling and appliance clearances are the same conversion with consequences. Fencing, decking and flooring: board counts and cut lists are repeated addition of identical feet-inches chunks, where the total-inches method doubles as the waste check. Sewing and crafts run the identical arithmetic at smaller scale (hems, seam allowances, quilt borders in inches and eighths). In every trade the pattern repeats: measurements arrive in feet and inches because tapes are printed that way, decisions need totals, and the conversion between the two is a known, mechanical, error-prone step — precisely the kind of step a calculator should own so the craftsperson can own the craft.
Heights, Doors and Rims: The Conversions Everyone Actually Needs
Strip away the job site and the most common feet-and-inches sums are body-and-building benchmarks, each one a small application of this calculator. Human height: the average Dutch man, the world’s tallest population on average, stands about 183 cm — 72.05 inches, which this calculator renders as 6 ft 0 in; the average American man at about 175.3 cm converts to 69.02 inches, or 5 ft 9 in. The two-centimetre honesty of those decimals is the point: a person described as “about five-nine” spans 68–70 inches of reality, and any conversion quoting more precision than the input owns is decoration. The standard door is 6 ft 8 in — 80 inches, 203.2 cm — a dimension chosen so that nearly everyone passes under it, and the reason “six-eight” is a unit of its own in builders’ speech. The standard ceiling is 8 ft (243.8 cm), and the modern 9 ft ceiling is a 12.5% raise you can verify against any room with this arithmetic. The basketball rim stands exactly 10 ft — 3.048 m — above the floor, a height set in 1891 and never since improved upon, which makes every playground argument about lowered rims a feet-and-inches calculation with a known right answer. Run each of these through the calculator once — height as a single measurement against zero — and the pattern of the page becomes a habit: measurements live in the system they were spoken in, and conversion is the toll paid exactly once, at the border, in the open.
Decimal Feet in the Wild: Surveyors and Property Lines
One profession never left decimal feet at all. Surveyors and civil engineers measure in feet and tenths of feet — a grade stake or a site plan reading 8.33 ft means 8 ft 4 in by this page’s arithmetic (0.33 × 12 ≈ 4), and their tapes are even graduated in tenths so the dialect is native at both ends. Property documents speak it too: a lot line recorded as 125.75 ft converts to 125 ft 9 in (0.75 × 12 = 9 exactly), and a depth of 98.5 ft is 98 ft 6 in — conversions a buyer walks with a tape when the fence and the deed disagree. The trap is symmetric and worth stating both ways: a carpenter reading a surveyor’s 8.33 as “eight foot three” loses an inch; a surveyor reading a carpenter’s 8’3″ as 8.3 ft loses the same inch from the other direction. The two dialects share digits and differ in meaning, which is the most dangerous kind of difference there is. This calculator sits deliberately on the border — feet-and-inches in, decimal feet and total inches printed beside the answer — so that a figure crossing from one trade’s paper to another’s hands gets translated exactly once, in the open, with both forms visible for the sanity check.
One Room, Measured: A Baseboard Perimeter
A last everyday run-through: a bedroom 12 ft 6 in by 10 ft 3 in needs baseboard around its perimeter. In total inches the walls are 150 and 123; the perimeter is 2 × (150 + 123) = 546 inches = 45 ft 6 in of trim before door openings are subtracted and waste is added — computed here as two additions (12 ft 6 in + 10 ft 3 in = 22 ft 9 in) doubled, or as one total-inches chain. Either route lands on the same 546, which is the quiet promise of the method: convert first, and every path through the arithmetic agrees.
Common Mistakes
Adding feet to feet and inches to inches and reporting “7 ft 16 in” as a finished answer. Borrowing tens in subtraction out of decimal habit. Reading 8.33 ft as 8 ft 3 in (decimal feet are tenths, not inches: 0.33 ft = 4 inches — the single commonest conversion error on site). Entering a fraction like 7½ as 7.2 or 7.12 instead of 7.5. And rounding intermediate sums during a multi-piece layout instead of carrying exact total inches to the end — layout errors compound, and they compound in the direction of the gap you discover last.
Related Tools
Continue with the Inches to Fraction Calculator, the Scale Calculator, the Length Converter, or browse the full Conversion Calculators hub.
Frequently Asked Questions
How do you add feet and inches?
Convert both measurements to total inches, add, then divide by 12: 5 ft 7 in (67 in) + 2 ft 9 in (33 in) = 100 in = 8 ft 4 in. The calculator performs the whole chain and shows decimal feet and metric too.
What is 5 ft 7 in plus 2 ft 9 in?
8 ft 4 in — 100 inches total, which is also 8.33 decimal feet, 254 cm or 2.54 m.
How do I subtract feet and inches?
The calculator borrows in twelves automatically: 8 ft 4 in − 2 ft 9 in = 5 ft 7 in. By hand, convert to total inches (100 − 33 = 67) and convert back.
Is 8.33 feet the same as 8 feet 3 inches?
No — this is the classic trap. Decimal feet are tenths: 0.33 ft = 0.33 × 12 ≈ 4 inches, so 8.33 ft = 8 ft 4 in. Never read the decimal part as inches.
Can I enter fractional inches like 7.5?
Yes — decimal inches (7.5, 7.25, 7.75) are accepted in the inches fields. To express a decimal result back as a tape fraction (11/16 and friends), use the Inches to Fraction Calculator.
How tall is 5 ft 7 in in centimetres?
67 inches × 2.54 = 170.18 cm. Enter the height as the first measurement with the second set to zero and the calculator shows the metric line.