Feet, Inches, and Fraction Math on the Job

Formula

Total inches = feet×12 + inches + fraction. Operate in inches (or rational inches), then convert back: feet = floor(total/12), rem inches = total mod 12.

Units

Feet, inches, and binary fractions (1/2, 1/4, 1/8, 1/16, 1/32, 1/64).

Step-by-step

1. Write each length as ft + in + fraction.
2. Convert fractions to a common denominator (often 16ths).
3. Add/subtract inch totals; carry or borrow 12 inches across feet.
4. Simplify the fraction; decide a practical rounding only at the saw.

Example — add 5′ 6½″ + 2′ 3¼″

Inches: 6.5 + 3.25 = 9.75″ → 9¾″. Feet: 5+2 = 7. Sum = 7′ 9¾″.

Example — subtract 12′ 3⅛″ − 4′ 7¾″

Borrow 1 ft = 12″: 12′ 3⅛″ → 11′ 15⅛″. 15⅛ − 7¾ = 15⅛ − 7⅝ = 7½″. Feet: 11−4 = 7. Result = 7′ 7½″.

Common mistakes

• Rounding each piece to the nearest ⅛″ before adding a long list.
• Forgetting to borrow 12″ when subtracting larger inches from smaller.

Practical notes

• Story poles beat repeated tape additions for repetitive layout.
• Use the free feet-fraction calculator when multiplying/dividing lengths by counts.

Related calculator

Use the free Feet & Fraction Calculator to run the same math with your field numbers.

Related tools

• Feet & Fraction Calculator — Add, subtract, multiply, and divide construction measurements.
• Board Foot Calculator — Lumber board feet, linear feet, and volume from dimensions.

Related guides

• How to Calculate Board Feet — Use the classic lumber board-foot formula.