Concrete Calculator
Use this concrete calculator to estimate the amount of concrete needed for slabs, driveways, footings, patios, foundations and sidewalks using standard US measurements.
Cubic Yards = Volume ÷ 27 · Always add 10% for waste
1 cubic yard = 27 cu ft · 60 lb bag yields ~0.45 cu ft · 80 lb bag yields ~0.60 cu ft · Ready-mix concrete sold by the cubic yard
Always order 10% more than calculated. Confirm current pricing with your local ready-mix supplier.
How Does the Concrete Calculator Work?
This concrete calculator estimates how many cubic yards of concrete you need for slabs, footings, walls, and columns. It converts your area dimensions and thickness into cubic feet, then divides by 27 to get cubic yards — the standard unit used when ordering ready-mix concrete.
Steps:
- Enter your project length and width in feet.
- Select the concrete thickness — 4 inches for patios and walkways, 6 inches for driveways and garage floors.
- Select your project type — slab, footing/wall, or column/post.
- Get instant results — cubic yards, cubic feet, volume with 10% waste, and equivalent 60 lb bags.
Always order 10% more concrete than calculated to account for spillage, uneven subgrade, and over-excavation. Running short mid-pour creates a cold joint — a serious structural weakness. It's much cheaper to have a small amount left over than to run short.
Concrete Coverage Chart
Use this quick reference chart to estimate how many cubic yards you need based on slab size and thickness.
| Slab Size | 4 inches thick | 6 inches thick | 8 inches thick |
|---|---|---|---|
| 10×10 ft (100 sq ft) | 1.2 cu yd | 1.9 cu yd | 2.5 cu yd |
| 12×12 ft (144 sq ft) | 1.8 cu yd | 2.7 cu yd | 3.6 cu yd |
| 20×20 ft (400 sq ft) | 4.9 cu yd | 7.4 cu yd | 9.9 cu yd |
| 24×24 ft (576 sq ft) | 7.1 cu yd | 10.7 cu yd | 14.2 cu yd |
| 30×30 ft (900 sq ft) | 11.1 cu yd | 16.7 cu yd | 22.2 cu yd |
| 40×40 ft (1,600 sq ft) | 19.8 cu yd | 29.6 cu yd | 39.5 cu yd |
*Amounts shown do not include the 10% waste factor. Always add 10% to the figures above when ordering.
Concrete Mix Types & When to Use Each
Concrete is specified by its compressive strength (PSI) and the application it's designed for. Choosing the right mix is critical for long-term performance.
| Mix / PSI | Best For | Notes |
|---|---|---|
| 2,500 PSI | Sidewalks, patios (non-load) | Minimum residential grade |
| 3,000 PSI | Driveways, slabs, footings | Most common residential mix |
| 3,500 PSI | Garage floors, retaining walls | Good for freeze-thaw climates |
| 4,000 PSI | Foundations, structural slabs | Recommended for cold climates |
| 5,000+ PSI | Commercial, heavy structural | Requires engineered mix design |
Bags (60 or 80 lb): Best for small projects under 1 cubic yard — fence posts, small repairs, steps. Very expensive per cubic yard but no minimum order. Ready-mix truck: Best for any project over 1 cubic yard. Most suppliers have a minimum order of 1 yard with a short-load fee for small orders. Always call 2–3 suppliers for pricing.
Example Calculation
You want to pour a 20 ft × 20 ft concrete slab at 4 inches thick for a patio.
20 ft × 20 ft = 400 sq ft
Step 2 — Convert thickness to feet:4 inches ÷ 12 = 0.333 ft
Step 3 — Calculate volume in cubic feet:400 × 0.333 = 133.3 cubic feet
Step 4 — Convert to cubic yards:133.3 ÷ 27 = 4.9 cubic yards
Step 5 — Add 10% waste:4.9 × 1.10 = 5.4 cubic yards to order
Step 6 — Equivalent in 60 lb bags:133.3 ÷ 0.45 = 296 bags — order ready-mix instead!
Concrete Ordering & Pouring Tips
Before You Order
- Prepare your subgrade — compact the base and ensure 4–6 inches of compacted gravel base for slabs. Soft or organic soil must be removed and replaced.
- Install rebar or wire mesh — use our Rebar Calculator to estimate how much reinforcement you need.
- Build your forms — forms must be straight, level, and properly staked before ordering concrete. Concrete waits for no one.
- Have enough help — plan for at least 1 person per cubic yard when pouring. A 5-yard pour needs 5+ people minimum.
- Check the weather — don't pour concrete if temperatures are below 40°F or above 90°F without special precautions.
When Ordering Ready-Mix
- Call 2–3 local ready-mix suppliers and compare price per cubic yard.
- Confirm their minimum order and short-load fee — typically $50–$150 for orders under 3–5 yards.
- Order for the earliest delivery slot available — concrete has a limited working time (60–90 minutes).
- Confirm truck access — ready-mix trucks weigh 66,000 lbs loaded. Check driveway and access route weight limits.
Never add extra water to concrete to make it easier to work with. Extra water weakens the mix, causes shrinkage cracking, and can cut the strength by 30–50%. If the mix seems stiff, use a plasticizer (water reducer) instead — ask your ready-mix supplier.