Construction Calculators
Estimate concrete volume, calculate board feet, and determine slope grades for your projects.
Approximate Bags Needed
Concrete Volume Guide
Concrete is ordered and delivered by the cubic yard (or cubic meter in metric countries). One cubic yard equals 27 cubic feet and covers approximately 81 square feet at 4 inches thick — roughly the size of a single-car driveway pad.
When ordering ready-mix concrete, add 5-10% extra to account for waste, spillage, uneven subgrade, and form irregularities. For small jobs using bags of premixed concrete, note that an 80 lb bag yields about 0.6 cubic feet and a 60 lb bag yields about 0.45 cubic feet.
Standard residential slab thickness is 4 inches (10 cm) for patios and sidewalks, and 6 inches (15 cm) for driveways and garage floors. Always check local building codes for minimum requirements.
Common Concrete Coverage (4" thick)
| Area (sq ft) | Cubic Yards | 80 lb Bags |
|---|---|---|
| 50 | 0.62 | 28 |
| 100 | 1.23 | 56 |
| 200 | 2.47 | 111 |
| 500 | 6.17 | 278 |
| 1,000 | 12.35 | 556 |
Understanding Board Feet
A board foot is the standard unit for measuring hardwood lumber in the United States. One board foot equals a piece of wood 1 inch thick, 12 inches wide, and 1 foot long (144 cubic inches). Hardwood lumber is sold by the board foot, unlike softwood dimensional lumber (2x4s, 2x6s) which is sold by the linear foot.
The formula is simple: BF = (Thickness″ × Width″ × Length′) ÷ 12. Note that thickness and width are in inches, but length is in feet. Nominal dimensions (stated size) are used for pricing, not actual dimensions — a board sold as 1″ thick is actually 13/16″ after surfacing.
Slope and Grade in Construction
Slope can be expressed three ways: as a ratio (1:12), a percentage (8.33%), or in degrees (4.76°). In construction, percentage is the most common format for drainage and road grades, while ratio is standard for ADA ramp requirements.
- Minimum drainage slope: 1-2% (1/8″ to 1/4″ per foot) for patios, sidewalks, and flat roofs.
- ADA ramp maximum: 1:12 ratio (8.33%), meaning 1 inch of rise per 12 inches of run.
- Residential driveway maximum: Typically 15-25% depending on climate and local codes.
- Stairway angle: 30-35° (58-70% slope) for comfortable residential stairs.