How to Convert Feet and Inches to Decimal Feet
Convert feet-and-inches measurements to decimal feet for construction area, volume and estimating calculations, with examples and common mistakes.
Construction dimensions are often written as feet and inches, but area and volume formulas are easier to run when each measurement is a single decimal number. The conversion is simple: divide the inches by 12 and add the result to the feet.
The formula
decimal feet = feet + (inches ÷ 12)
For 8 ft 6 in:
8 + (6 ÷ 12) = 8.5 ft
For 12 ft 3 in:
12 + (3 ÷ 12) = 12.25 ft
For 5 ft 7 1/2 in, convert the fraction first: 7 1/2 inches is 7.5 inches.
5 + (7.5 ÷ 12) = 5.625 ft
The US construction unit converter handles common US customary and metric units when the measurements come from different drawings or product sheets.
Common inch fractions in decimal feet
| Inches | Decimal feet | | --- | ---: | | 1 in | 0.0833 ft | | 2 in | 0.1667 ft | | 3 in | 0.25 ft | | 4 in | 0.3333 ft | | 6 in | 0.5 ft | | 8 in | 0.6667 ft | | 9 in | 0.75 ft | | 10 in | 0.8333 ft | | 11 in | 0.9167 ft |
Keep more precision during the calculation than you intend to show in the final answer. Rounding every intermediate dimension can create a visible error when several areas or volumes are added together.
Do not treat inches as hundredths of a foot
The notation 8 ft 6 in does not mean 8.6 ft. A foot contains 12 inches, not 10. The decimal 0.6 ft equals 7.2 inches. Entering 8.6 instead of 8.5 into an area or volume calculation quietly overstates the result.
This matters most when:
- multiplying length × width for square footage;
- multiplying length × width × depth for cubic footage;
- adding many repeated pieces;
- applying labor or material rates to the final quantity.
Converting back to feet and inches
Take the whole number as feet. Multiply the decimal remainder by 12 to get inches.
For 9.375 ft:
- Whole feet = 9.
- Remainder = 0.375.
- Inches = 0.375 × 12 = 4.5.
- Result = 9 ft 4 1/2 in.
NIST’s current general tables of units define 12 inches as 1 foot. For ordinary construction measurements, that is the relationship to use. Legacy survey data can require special handling, so do not convert old coordinate or boundary records as if they were ordinary jobsite lengths.