The conversions every contractor must know cold
- 1 cubic yard = 27 cubic feet
- 1 square = 100 square feet (roofing)
- 1 board foot = 1 inch thick × 12 inches wide × 12 inches long
- 1 acre = 43,560 square feet
- 1 ton = 2,000 pounds
- 1 yard of concrete weighs ~4,000 lb (2 tons)
Board feet, worked example
Formula: Board Feet = (T × W × L) ÷ 12, where T and W are in inches and L is in feet.
Example: How many board feet are in 25 pieces of 2×6 lumber, each 16 feet long?
Per piece: (2 × 6 × 16) ÷ 12 = 16 BF. Total: 25 × 16 = 400 BF.
Concrete yardage, worked example
Formula: Cubic yards = (L × W × D in feet) ÷ 27. Add 5-10% for waste.
Example: Garage slab 24 ft × 20 ft, 4 inches thick.
- Convert depth: 4 in ÷ 12 = 0.333 ft
- Volume: 24 × 20 × 0.333 = 160 cubic feet
- Cubic yards: 160 ÷ 27 = 5.93 yards
- With 10% waste: 5.93 × 1.10 ≈ 6.5 yards
Roofing squares, worked example
Formula: Roofing squares = total roof area (sq ft) ÷ 100.
Example: A 30 ft × 50 ft simple gable roof with a 5:12 slope.
- Plan area: 30 × 50 = 1,500 sq ft
- Slope factor for 5:12 ≈ 1.083
- Actual roof area: 1,500 × 1.083 = 1,625 sq ft
- Squares: 1,625 ÷ 100 = 16.25 squares (round up; order 17)
Markup vs margin, the easy way to lose money
Markup is added to cost. Margin is a percentage of the sale price. They are not the same number.
| Cost | 25% Markup | 25% Margin |
|---|---|---|
| $1,000 | $1,250 | $1,333.33 |
| $10,000 | $12,500 | $13,333.33 |
| $50,000 | $62,500 | $66,666.67 |
If a customer asks for "your costs plus 25%," that's markup. If your accountant says you need "25% gross margin," that's a different (higher) number.
Estimating in real bids
A complete bid has four components:
- Direct costs, materials, labor, equipment, subs.
- Job overhead, permits, temporary utilities, port-a-potty, site security.
- General overhead, your office rent, insurance, vehicles, qualifying individual's time, slow-season float.
- Profit, what you earn after everything else is paid.
Practice with exam-format questions
A good way to lock in these formulas is to drill them in the same multiple-choice format used for contractor exam prep. Paid courses include estimating practice with explanations. Or start with the free practice test to see which topics need review.