Concrete Calculator — Estimate Yards, Bags & Volume
Enter your slab or column dimensions to instantly calculate the concrete volume in cubic feet, cubic yards, and cubic meters, plus the exact number of 60 lb and 80 lb premix bags needed. Supports rectangular slabs and circular columns with common depth presets.
Concrete Calculator
Estimate volume and bags needed for slabs and columns
Frequently Asked Questions
How do I calculate how much concrete I need for a slab?
Multiply the length (ft) × width (ft) × depth (ft) to get cubic feet. Convert depth from inches to feet by dividing by 12. Then divide cubic feet by 27 to get cubic yards. For a 10×10 ft slab at 4 inches thick: 10 × 10 × (4/12) = 33.33 ft³ = 1.23 cubic yards. Always add 5–10% extra for waste.
How many 80 lb bags of concrete do I need for a 10x10 slab?
For a 10×10 foot slab at 4 inches thick, you need about 33.33 cubic feet of concrete. Each 80 lb bag covers 0.60 cubic feet, so you need Math.ceil(33.33 / 0.60) = 56 bags of 80 lb concrete. At 6 inches thick (50 ft³), you would need 84 bags of 80 lb concrete.
How many bags of concrete do I need for a 4x4 post hole?
A 4×4 post typically uses a 10–12 inch diameter hole that is 2–3 feet deep. For a 12-inch diameter, 3-foot deep column: Volume = π × (0.5 ft)² × 3 ft = 2.36 ft³. At 0.60 ft³ per 80 lb bag, you need 4 bags. For a 10-inch hole at 3 ft deep, 3 bags of 80 lb concrete is sufficient.
What is the difference between cubic yards and cubic feet of concrete?
1 cubic yard equals 27 cubic feet. Ready-mix concrete is ordered and priced by the cubic yard, while bag concrete is sold by volume per bag (0.45 ft³ for 60 lb, 0.60 ft³ for 80 lb). To convert cubic feet to cubic yards, divide by 27. A 100 ft³ pour equals 3.70 cubic yards.
How many cubic yards of concrete do I need for a 20x20 slab at 4 inches?
For a 20×20 foot slab at 4 inches thick: Volume = 20 × 20 × (4/12) = 133.33 ft³ = 4.94 cubic yards. Add 10% waste: 5.43 cubic yards. You would need approximately 5.5 yards from a ready-mix supplier, or 223 bags of 80 lb premix. Ready-mix is strongly recommended for pours over 1 cubic yard.
Can I mix concrete by hand or do I need a mixer?
Hand mixing in a wheelbarrow or tub is practical for up to 3–4 bags at a time (roughly 2 ft³). A rented electric mixer handles 8–10 bags per batch and reduces labor significantly. For most projects requiring 10 or more bags, a mixer is recommended. For anything over 1 cubic yard (about 45 bags of 80 lb), ordering ready-mix concrete is more cost-effective and consistent.
How long does concrete take to cure?
Concrete reaches initial set in 24–48 hours (foot traffic possible), structural strength at 7 days (light loads), and full design strength at 28 days. Keep concrete moist for the first 7 days (curing) to prevent cracking and ensure full strength. Avoid heavy loads, freezing temperatures, and direct hot sun during the first 72 hours.
What PSI concrete should I use for a driveway versus a sidewalk?
Use 3,000 PSI (standard mix) for sidewalks, patios, and walkways with foot traffic only. Use 4,000 PSI for driveways, garage floors, and areas with vehicle traffic. Use 5,000 PSI or higher for structural foundations, retaining walls, and commercial applications. Higher PSI concrete is denser, more durable, and more resistant to freeze-thaw cycles.
Concrete Calculator: Volume, Yards & Bags Guide
This concrete calculator estimates the volume of concrete needed for slabs and columns in cubic feet, cubic yards, and cubic meters. It also calculates how many 60 lb and 80 lb premix bags you need to complete your project.
Concrete Volume Formulas
Slab Formula
For rectangular or square slabs (patios, driveways, floors):
Note: Depth must be converted from inches to feet by dividing by 12.
Column Formula
For circular columns, posts, and piers:
Where r = radius in feet = (diameter in inches / 2) / 12
How to Calculate Concrete for a Slab
- Measure the length and width of your slab in feet.
- Determine the required depth in inches (4" for residential slabs, 6" for driveways, 8" for heavy loads).
- Convert depth to feet: divide inches by 12.
- Multiply length × width × depth (in feet) to get cubic feet.
- Divide cubic feet by 27 to get cubic yards.
- Add 5–10% for waste and spillage.
Depth = 4 / 12 = 0.333 ft
Volume = 10 × 10 × 0.333 = 33.33 ft³ = 1.23 yd³
With 10% waste: 1.35 yd³
Column Volume Formula
Columns use the cylinder volume formula. The diameter is entered in inches for convenience, then converted to feet internally.
Radius = (12 / 2) / 12 = 0.5 ft
Volume = π × 0.5² × 4 = 3.14 ft³ = 0.116 yd³
Common column diameters: 8" (fence posts), 10" (deck piers), 12" (structural columns), 16" (large footings).
Bag Coverage Table
Pre-mixed concrete bags (Quikrete, Sakrete, etc.) are available in 40 lb, 60 lb, and 80 lb sizes. The 60 lb and 80 lb bags are most common for structural work.
| Bag Size | Volume per Bag | Bags per Cubic Foot | Bags per Cubic Yard |
|---|---|---|---|
| 40 lb | 0.30 ft³ | 3.33 | 90 |
| 60 lb | 0.45 ft³ | 2.22 | 60 |
| 80 lb | 0.60 ft³ | 1.67 | 45 |
Recommended Slab Thickness
| Application | Thickness | Notes |
|---|---|---|
| Sidewalk / walkway | 4 inches | Foot traffic only |
| Patio / pool deck | 4 inches | Light outdoor use |
| Residential driveway | 4–6 inches | Passenger vehicles |
| Garage floor | 4–6 inches | Cars and light trucks |
| Shed / workshop floor | 4 inches | Tools, light equipment |
| Commercial / heavy loads | 6–8 inches | Heavy vehicles, forklifts |
Tips for Ordering Concrete
- Add 5–10% waste factor to account for spillage, uneven subgrade, and over-excavation. Use 10% for irregular shapes.
- Minimum ready-mix order is typically 1 cubic yard from most suppliers. For less, use bags or ask about short-load fees.
- Order slightly more rather than less — running short mid-pour creates cold joints that weaken the slab.
- Specify PSI strength: 3,000 PSI for residential slabs, 4,000 PSI for driveways, 5,000 PSI for structural applications.
- Check truck access before ordering — a concrete truck requires about 10 ft width and 12 ft clearance height.
- Have extra hands ready — ready-mix must be placed and finished within 90 minutes of mixing.
Volume Unit Conversions
1 cubic yard (yd³) = 27 cubic feet (ft³)
1 cubic meter (m³) = 35.3147 cubic feet (ft³)
1 cubic meter (m³) = 1.30795 cubic yards (yd³)
1 cubic foot (ft³) = 0.0370370 cubic yards (yd³)
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