Units & Conversions
The trades use a mix of units — being fluent with them saves time and prevents mistakes.
Feet and inches
- 12 inches = 1 foot. Measurements are written like 5'-6" (five feet, six inches).
- Decimal feet — for calculations and many plans, inches convert to decimals of a foot: 6 inches = 0.5 ft; 3 inches = 0.25 ft. (Divide the inches by 12.)
Converting
- Inches to decimal feet: inches ÷ 12.
- Decimal feet to inches: decimal × 12.
- Know the metric basics too — 1 inch ≈ 25.4 mm — for metric products and international work.
Going Deeper (Intermediate)
Construction constantly converts units: inches↔feet (×/÷12), sq ft↔sq yd (÷9), cu ft↔cu yd (÷27), decimal feet↔feet-inches, and imperial↔metric (25.4 mm = 1 in). Getting a conversion wrong throws off material orders by a lot.
Advanced / Pro-Level
The conversions pros do daily:
- Decimal feet for survey/grading (0.5′ = 6″, 0.25′ = 3″) — and back.
- Concrete: cubic feet ÷ 27 = cubic yards; concrete weighs ~150 pcf (~4,050 lb/CY).
- Flooring: square feet ÷ 9 = square yards; roofing "squares" = 100 SF.
- Steel and lumber by weight/foot.
- The pro habit: carry units through the calculation (dimensional analysis) so the units cancel to what you want — that alone catches most arithmetic errors.
Practice Challenge
You pour a slab needing 270 cubic feet of concrete. How many cubic yards do you order, and why round up? (Answer: 270 ÷ 27 = 10 CY; you'd order ~11 CY — round up for waste, spillage, and uneven subgrade, because a short pour creates a cold joint and you can't easily add to a partial slab.)
In Practice
A plan dimension reads 8.5 feet. That's not 8 feet 5 inches — 0.5 ft × 12 = 6 inches, so it's 8'-6". Mixing up decimal feet and feet-and-inches is a classic, costly error.
Common Mistakes to Avoid
- Confusing decimal feet with feet-and-inches
- Forgetting to convert units before calculating
- Assuming a plan is in feet when a detail is in inches
Takeaway: Convert inches to decimal feet (inches ÷ 12) for calculations — and know metric for international or metric specs.
Educational overview — confirm structural and layout specifics with the project plans and engineer.