# Feet and Inches Calculator — Add, Subtract, Divide > Add, subtract, multiply and divide feet and inches with exact tape fractions to 1/64. Mix in metric, chain up to 20 measurements, see every subtotal. - **Category:** Construction & Home - **Calculator:** https://youcalc.com/en/construction-home/feet-and-inches-calculator/ - **Price:** Free · no sign-up ## About this calculator A tape measure is marked in feet, inches and binary fractions, and none of those add up in a way a phone calculator understands. This one does the arithmetic in the units the work is actually in: enter 8 ft 4 1/2 in, add 2400 mm to it, divide the total into five equal spacings, and read the answer back as a tape fraction. Each step is just two boxes, feet and inches, and the inches box is deliberately permissive — 8, 8.5, 5/16, 8 5/16 and 300 mm are all understood, so leaving the feet box empty and typing 102 means 102 inches, while typing 8.5 in the feet box alone means decimal feet. Choosing × or ÷ turns that step into a plain count instead. You can chain up to twenty steps, mixing feet-and-inches, decimal feet, total inches and metric in the same run. Every figure is held as an exact fraction rather than a decimal, so the answer does not drift as the chain gets longer. ## How to read your results The headline is your total in feet and inches, snapped to the tape fraction you chose — 1/16 in by default, which is how most tapes are marked. Directly beneath it the same total appears as decimal feet, total inches, yards and metric. Those describe the EXACT value, while the headline is rounded to the nearest 1/16, so the two can legitimately disagree; the badge always states which case you are in and, when the answer is rounded, exactly how many millimetres the displayed fraction sits from the true value. If a result is finer than the display denominator it says "smaller than 1/64 in" rather than silently showing zero. Below the result, the running subtotal lists what the total was after each row, because the chain is evaluated strictly left to right and that order should never be a surprise. ## How it's calculated Every measurement is converted on entry to an exact rational number of inches — a numerator and a denominator held as arbitrary-precision integers, reduced by their greatest common divisor. Feet become twelfths, tape fractions keep their own denominator, and metric input uses the exact international definitions rather than decimal approximations: 1 in = 25.4 mm exactly, so a millimetre is exactly 5/127 in, a centimetre 50/127 in and a metre 5000/127 in. The foot is the international foot, 0.3048 m exactly, not the US survey foot of 1200/3937 m that NIST deprecated at the end of 2022 — over 1000 ft the two differ by about 0.6 mm. The chain is evaluated strictly left to right with NO operator precedence, which is tape-calculator convention and is what the trade expects: 2 ft + 3 ft × 2 is (2 + 3) × 2 = 10 ft. Multiplication and division take a dimensionless count rather than a length, because a length times a length is an area — a different quantity — and that is rejected with a message rather than silently coerced. Floating-point arithmetic is used exactly once per displayed figure, at the very end, derived straight from the exact fraction and never chained from an already-rounded number. Rounding to the tape fraction is half away from zero, so −1/32 in rounds to −1/16 in exactly as +1/32 rounds to +1/16; and the total is rounded BEFORE being split into feet and inches, which is what stops an answer of 95.99 in printing as the impossible "7 ft 12 in" instead of 8 ft. ## Worked example - **Your inputs:** A stair stringer run measures 12 ft 6 in and has to be divided into five equal treads. Then 2400 mm of decking is added to the total. - **Results:** 12 ft 6 in is 150 in exactly. Divided by 5 that is 30 in, or 2 ft 6 in per tread — exact at 1/16, with no rounding badge. Adding 2400 mm converts to 2400 × 5/127 = 12000/127 in = 94.4882 in, giving 124.4882 in in total, which displays as 10 ft 4 1/2 in with a badge noting the shown fraction is 0.3 mm above the true value. ## Frequently asked questions ### How do I add feet and inches together? Add the inches first, then carry every 12 inches into the feet column. 8 ft 4 in + 3 ft 7 in is 11 ft 11 in, because 4 + 7 = 11 inches, which is under twelve so nothing carries. If the inches had come to 14, you would write 2 in and carry 1 ft. Fractions add the same way but need a common denominator first: 1/2 + 3/8 is 4/8 + 3/8 = 7/8. This calculator does the carrying and the common denominators for you, and shows the subtotal after each row so you can check the working. ### Why does the calculator not follow BODMAS? Because it is a tape calculator, and that is the convention every physical construction calculator uses. Rows are applied strictly left to right, so 2 ft + 3 ft × 2 is (2 + 3) × 2 = 10 ft, not 2 + (3 × 2) = 8 ft. In practice this is what people mean: you total up a run of measurements and then double it, rather than writing an algebraic expression. To make it impossible to get caught out, the running subtotal after every row is shown with the result, so you can see the order the chain was evaluated in. ### Why can I not multiply one length by another? Because a length times a length is an area, not a length — 3 ft × 4 ft is 12 square feet, which this calculator has no way to display and no business guessing at. Multiply and divide therefore take a plain count: "8 ft 4 in × 3" means three of that measurement, and "12 ft 6 in ÷ 5" means five equal spacings. The interface makes that impossible to get wrong: the moment you choose × or ÷, that step's feet and inches boxes are replaced by a single count box, so there is nowhere to put a second length. For areas and volumes, use the area or concrete calculators instead. ### What does the "rounded" badge actually mean? It means the exact answer does not land on one of the marks at your chosen tape fraction, so the feet-and-inches line has been snapped to the nearest one. The badge tells you the direction and size of that gap in millimetres — positive if the displayed fraction is longer than the true value, negative if it is shorter. The decimal, yard and metric figures underneath are NOT snapped; they describe the exact value. So if the headline says 7 ft 10 1/2 in and the millimetre row says 2400.0, both are right: one is the tape reading and the other is the true length. ### How precise is it, and why cap decimals at six places? Internally there is no precision limit at all — every value is an exact fraction, so dividing by 3 and multiplying back by 3 returns exactly what you started with, which is not true of ordinary decimal arithmetic. The six-decimal cap applies only to what you can type. It exists because 1/64 in is 0.015625 in, so six decimal places is already several times finer than any tape or machinist rule, and because an unbounded pasted decimal is a way to make a calculator show NaN. You can enter fractions as fine as 1/128, while the display caps at 1/64 because no tape is marked finer. ### Does it use the US survey foot? No, it uses the international foot: 0.3048 metre exactly, which makes an inch exactly 25.4 mm. The US survey foot was a slightly different unit, 1200/3937 m, used in American geodetic and land-survey work; NIST and NOAA deprecated it as of 31 December 2022 in favour of the international foot. The difference is about two parts per million — imperceptible on a door frame, but roughly 0.6 mm over 1000 ft, and a genuine 3.2 m over the width of the continental United States, which is why survey data was the one place it mattered. ### Can I mix metric and imperial in the same calculation? Yes, and that is one of the main reasons to use it. Leave the feet box empty and type the metric measurement into the inches box — 2400 mm, 12 cm or 1.2 m — and it is read as metric, with no unit to select. The conversion is exact rather than a rounded factor, so a metric measurement added to an imperial one loses nothing. This is the common case on a job where the drawings are metric and the timber is sold in feet, or the other way round. The result is then shown in both systems at once, so you never have to convert it back by hand. ## Related calculators - [Unit Converter](https://youcalc.com/en/conversions-units/unit-converter/) - [Stair Calculator](https://youcalc.com/en/construction-home/stair-calculator/) - [Deck Calculator](https://youcalc.com/en/construction-home/deck-calculator/) - [Flooring Calculator](https://youcalc.com/en/construction-home/flooring-calculator/) ## Sources - https://www.nist.gov/pml/owm/si-units-length — National Institute of Standards and Technology - https://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors/nist-guide-si-appendix-b9 — National Institute of Standards and Technology - https://www.nist.gov/pml/us-surveyfoot — National Institute of Standards and Technology - https://www.legislation.gov.uk/ukpga/1985/72/schedule/1 — The National Archives --- Open full calculator: https://youcalc.com/en/construction-home/feet-and-inches-calculator/ · YouCalc — https://youcalc.com