Bottom Line Up Front (BLUF): How to Add and Subtract Feet and Inches
To add or subtract feet, inches, and fractions on a job site, convert all measurements to a common denominator (typically 16ths, 32nds, or 64ths). Sum the fractions, convert 16/16ths into 1 inch, sum total inches, and convert every 12 inches into 1 foot.
The Complete Guide to Tape Measure Math & Fractions
1. Reading the Lines on a Standard Tape Measure
A standard imperial tape measure splits every inch into 16 increments. The longest mark is the 1-inch line, followed by the 1/2-inch mark, 1/4-inch marks, 1/8-inch marks, and smallest 1/16-inch ticks. Finer precision tapes include 1/32" and 1/64" lines.
2. Adding and Subtracting Compound Measurements
When calculating room spans or framing cuts, always align fractions first. Example: 8' 5 3/8" + 4' 9 3/4". Convert 3/4" to 6/8". Sum fractions: 3/8 + 6/8 = 9/8 = 1 1/8". Sum inches: 5" + 9" + 1" = 15" = 1' 3". Sum feet: 8' + 4' + 1' = 13' 3 1/8".
3. Imperial to Metric Quick Conversion Rules
1 inch = exactly 25.4 millimeters (2.54 cm). 1 foot = exactly 0.3048 meters (304.8 mm). Converting fractional inches to metric requires decimalizing the fraction (e.g. 5/8" = 0.625") before multiplying by 25.4.
Fractional Inch to Decimal & Millimeter Reference Table
| Fraction (in) | Decimal Inches (in) | Decimal Feet (ft) | Millimeters (mm) | Centimeters (cm) |
|---|---|---|---|---|
| 1/16" | 0.0625" | 0.0052' | 1.5875 mm | 0.1588 cm |
| 1/8" | 0.1250" | 0.0104' | 3.1750 mm | 0.3175 cm |
| 1/4" | 0.2500" | 0.0208' | 6.3500 mm | 0.6350 cm |
| 3/8" | 0.3750" | 0.0313' | 9.5250 mm | 0.9525 cm |
| 1/2" | 0.5000" | 0.0417' | 12.7000 mm | 1.2700 cm |
| 5/8" | 0.6250" | 0.0521' | 15.8750 mm | 1.5875 cm |
| 3/4" | 0.7500" | 0.0625' | 19.0500 mm | 1.9050 cm |
| 7/8" | 0.8750" | 0.0729' | 22.2250 mm | 2.2225 cm |
| 1" | 1.0000" | 0.0833' | 25.4000 mm | 2.5400 cm |
Frequently Asked Questions about Feet & Inches Calculations
How do I calculate square footage from feet and inches?
Convert both length and width to total decimal feet (e.g. 12' 6" = 12.5 ft, 10' 3" = 10.25 ft), then multiply them: 12.5 × 10.25 = 128.125 sq ft.
What is the smallest mark on a standard tape measure?
Most residential tape measures mark down to 1/16 of an inch. Precision carpentry and cabinetmaker tapes often mark down to 1/32 or 1/64 of an inch.
How do I convert decimal inches back to tape fractions?
Multiply the decimal part by 16 (or 32), and round to the nearest whole number. For example, 0.375 × 16 = 6, meaning 6/16" which reduces down to 3/8".