Feet and Inches to Decimal Calculator

decimal feet = feet + (inches ÷ 12). The inches portion is converted to a fraction of a foot first, then added to the whole-feet count. 5 ft 9 in becomes 5 + (9 ÷ 12) = 5 + 0.75 = 5.75 ft. The division by 12 works because a foot has always been defined as exactly 12 inches, so the fraction of a foot that any number of inches represents never needs rounding beyond the decimal itself.

5 ft 9 in: 5 + 9/12 = 5.75 ft, or 69 total inches, which is 1752.6 mm (69 × 25.4). 6 ft 3 in: 6 + 3/12 = 6.25 ft, or 75 total inches, 1905 mm. 10 ft 6 in: 10 + 6/12 = 10.5 ft, or 126 total inches, 3200.4 mm. 8 ft 11 in: 8 + 11/12 = 8.9167 ft, or 107 total inches, 2717.8 mm. Each of these can be checked two ways at once, since decimal feet × 304.8 and total inches × 25.4 always land on the same millimeter figure when the arithmetic is done correctly.

Surveyors record property lines, elevations, and station distances in decimal feet because it lets them sum hundreds of measurements without converting fractions at every line. A benchmark elevation of 542.67 ft or a boundary run of 128.42 ft adds directly into a running total the way any decimal number would. CAD platforms and GIS software store coordinates the same way internally, and civil engineering drawings for roads, pipes, and grading plans use decimal feet for the same reason: it's faster to add, subtract, and average a column of decimals than a column of mixed fractions.

Some common feet-and-inches lengths and their exact decimal-feet equivalents:

The inches field on this calculator expects a decimal number, so a tape-measure reading like 9 1/2 inches needs to be converted to 9.5 before it's typed in. Use the inches to decimal calculator to turn any fractional reading, down to 1/64 inch, into the decimal form this page needs. 5 ft 9 1/2 in, entered as 5 feet and 9.5 inches, works out to 5 + 9.5 ÷ 12 = 5.7917 ft.

Both the feet and inches fields accept negative numbers, but they are added independently rather than combined as one signed length. Typing -5 into feet and 9 into inches gives -5 + 9/12 = -4.25 ft, not the -5.75 ft you might expect from "negative 5 feet 9 inches." To represent a true negative length of 5 ft 9 in, enter -5 for feet and -9 for inches, which correctly returns -5 + (-9/12) = -5.75 ft.

If the starting value is already a single decimal number in inches rather than separate feet and inches fields, use the inches to decimal feet converter instead. To go the other way, from a decimal-feet number back into feet and inches, use the decimal feet to inches calculator or the decimal to feet and inches converter if the starting number is decimal inches. Every one of these tools shares the same underlying 12-inches-per-foot relationship, so a result on one always cross-checks against a result on another.

The most frequent error is entering a fractional inch value directly, such as typing 9.5/8 or "9 1/2" into the inches field, when the field only accepts a plain decimal like 9.5. Another is mixing up which field is feet and which is inches, so double-check that the larger whole number goes into the feet box. A third is forgetting that the inches value should stay below 12: entering 5 ft 15 in works arithmetically (5 + 15/12 = 6.25 ft) but it's clearer to first convert to 6 ft 3 in before running the conversion, since 15 inches is really 1 ft 3 in.