Basic Construction Math
Math on the jobsite isn’t just about cutting boards to length. The same skills tell you how far out to set a ladder, how much concrete to order, how steep a trench wall can be, and whether a load is too heavy for your rigging.
- Construction
Fractions and decimals
| Fraction | Decimal |
|---|---|
| 1/16 | 0.0625 |
| 1/8 | 0.125 |
| 1/4 | 0.25 |
| 3/8 | 0.375 |
| 1/2 | 0.5 |
| 3/4 | 0.75 |
To turn inches into decimal feet, divide by 12: 6 inches = 0.5 feet, 9 inches = 0.75 feet.
Area and volume
- Area (square feet) = length × width.
- Volume (cubic feet) = length × width × depth, all in feet.
- Cubic yards = cubic feet ÷ 27.
Example: a slab 20 feet by 10 feet, 4 inches thick: 20 × 10 × 0.333 = 66.7 cubic feet ÷ 27 = about 2.5 cubic yards. Order a little extra for waste.
Square corners: the 3-4-5 rule
Measure 3 feet along one side and 4 feet along the other. If the diagonal between those marks is exactly 5 feet, the corner is square. Use 6-8-10 or 9-12-15 for bigger layouts.
Math that keeps you safe
| Task | Rule | Example |
|---|---|---|
| Extension ladder angle | 1 foot out for every 4 feet of working height (4 to 1) | 16 feet to the support point = base 4 feet out. See Extension Ladders |
| Trench sloping (Type C soil) | 1.5 feet horizontal for every 1 foot of depth (34°) | An 8-foot-deep trench needs each side cut back 12 feet. See Protective Systems |
| Load weight | Volume × weight per cubic foot | Steel weighs about 490 lb per cubic foot; concrete about 150 lb. See Load Charts |
| Roof pitch | Rise in inches per 12 inches of run | A 4/12 roof or less is “low-slope” for OSHA fall rules |
Tips
- Write measurements down; don’t trust memory.
- Double-check math that affects loads, slopes and supports, and ask a competent person if unsure.
- Use a construction calculator or phone app for feet-inch math, but sanity-check the answer.
- Measure twice, cut once.
Standards & references
- See also: Reading Blueprints · Hand Tools
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