If a room measures 144 in by 120 in, multiplying gives 17,280 square inches. The result in square feet is 120, because each foot contributes a factor of 12 on one axis. Dividing an area by 12 would produce 1,440, which is still the wrong unit.
You can check both paths when building a calculator:
const lengthIn = 144; const widthIn = 120; const areaFromSides = (lengthIn / 12) * (widthIn / 12); const areaFromSquareInches = (lengthIn * widthIn) / (12 ** 2); console.assert(areaFromSides === 120); console.assert(areaFromSquareInches === areaFromSides); The same trap appears in decimal feet. Twelve feet six inches is 12.5 feet, not 12.6. At a width of 10 feet, the correct answer is 125 sq ft. For an L shape, add non-overlapping sections after converting both dimensions of each section. Keep the measurements precise until the final display. If you estimate flooring boxes, round the number of boxes up separately from the area calculation.
Disclosure: I work on Square Footage Calculator. The site has a mixed feet-and-inches example and links to a separate flooring calculator for box coverage and waste.
