Concrete Calculator
Estimate the concrete volume for slabs, patios, driveways, sidewalks, and footings. The calculator converts your dimensions to cubic feet and cubic yards, adds a waste allowance, recommends a ready-mix order size rounded to the half yard, and shows the equivalent number of bags for small pours. All formulas are documented below so you can verify every number by hand.
Project Dimensions
Cost (optional)
Results
Recommended ready-mix order
– cubic yards
Planning estimate only. Structural slabs, loading conditions, and local codes dictate actual design requirements - see our safety disclaimer.
The formula this calculator uses
Order volume = Volume × (1 + Waste%) , rounded up to the nearest 0.5 yd³
Concrete is sold by the cubic yard (27 cubic feet). The divide-by-12 step converts thickness from inches to feet so all three dimensions are in the same unit - skipping that conversion is the single most common estimating error in flatwork.
Worked example: 12 × 20 ft driveway, 4 in thick
- Convert thickness: 4 in ÷ 12 = 0.333 ft
- Volume: 12 × 20 × 0.333 = 80 ft³ = 2.96 yd³
- Add 10% waste: 2.96 × 1.10 = 3.26 yd³
- Order: round up to 3.5 cubic yards
Common project values
| Project | Size | Thickness | Volume + 10% waste | Suggested order |
|---|---|---|---|---|
| Small patio | 10 × 10 ft | 4 in | 1.36 yd³ | 1.5 yd³ (or 62 × 80 lb bags) |
| Large patio | 15 × 20 ft | 4 in | 4.07 yd³ | 4.5 yd³ |
| Single-car driveway | 12 × 20 ft | 4 in | 3.26 yd³ | 3.5 yd³ |
| Two-car driveway | 20 × 24 ft | 5 in | 8.15 yd³ | 8.5 yd³ |
| Sidewalk | 4 × 50 ft | 4 in | 2.72 yd³ | 3 yd³ |
| Garage slab | 24 × 24 ft | 5 in | 9.78 yd³ | 10 yd³ |
| Shed slab | 8 × 12 ft | 4 in | 1.30 yd³ | 1.5 yd³ |
Common mistakes to avoid
Mixing units. Feet times feet times inches gives a meaningless number. Convert thickness to feet first - the calculator does this automatically.
Skipping the waste factor. Forms bow outward, subgrades are never perfectly level, and some concrete always stays in the chute and wheelbarrow. A pour that comes up half a yard short means a cold joint and a second delivery fee.
Ignoring thickened edges and footings. Many slabs thicken from 4 in to 8–12 in at the perimeter. Calculate the thickened band as a separate rectangle and add it.
Ordering exact volume. Suppliers batch in 0.5 yd³ steps and charge short-load fees under 3–4 yards. Round up - leftover concrete costs a few dollars; a short pour can ruin the slab.
When to call a professional
This tool covers non-structural flatwork estimating. Bring in a licensed contractor or engineer for suspended slabs, slabs supporting vehicles heavier than passenger cars, foundation work, retaining structures, or any pour on expansive or uncompacted soil. Building departments in most U.S. jurisdictions require permits for s
Assumptions used in this calculator
- Standard-weight concrete at 4,050 lb per cubic yard
- Bag yields: 80 lb = 0.60 ft³, 60 lb = 0.45 ft³, 40 lb = 0.30 ft³ (manufacturer-published)
- Default waste 10%, adjustable 0-30%
- Ready-mix order rounded up to 0.5 yd³ batching increments
- Cost = order volume × your price + delivery fee
Change the adjustable inputs above where your project differs. Supplier figures and local codes govern; see our methodology.
Built for planning, not blind ordering. Verify dimensions, supplier conversions, local codes, and site conditions before construction.
Frequently Asked Questions
How much concrete do I need for a 10x10 slab at 4 inches?
A 10 ft x 10 ft slab at 4 inches thick requires 33.3 cubic feet, or 1.23 cubic yards. With 10% waste added, order 1.5 cubic yards of ready-mix, or about 62 bags of 80 lb concrete mix.
How many 80 lb bags of concrete make a cubic yard?
One 80 lb bag yields about 0.60 cubic feet, so a cubic yard (27 cubic feet) requires 45 bags. At that volume, ready-mix delivery is almost always cheaper and far less work.
How much does a cubic yard of concrete weigh?
Standard-weight concrete weighs about 4,050 lb per cubic yard (roughly 2 tons), or 150 lb per cubic foot. Verify your subgrade and formwork can handle the load before pouring.
What waste percentage should I use for concrete?
Use 5% for simple square slabs on a firm, level base; 10% for typical residential work with normal form and subgrade variation; and 10-15% for irregular shapes, sloped sites, or rough excavation.
Should I round up my ready-mix order?
Yes. Suppliers batch in half-yard increments, and running short mid-pour creates a cold joint that permanently weakens the slab. Round up to the nearest half yard after adding waste.