Concrete Calculator
Use this concrete calculator to estimate concrete for slabs, patios, sidewalks, driveways, footings, and round columns. Enter your project dimensions to calculate cubic feet, cubic yards, ready-mix quantity, bag count, waste factor, and estimated material cost.
How to Use the Concrete Calculator
Use this concrete calculator to estimate how much concrete you need for slabs, patios, posts, footings, and other small home improvement projects. Enter your project measurements, choose your waste factor, and review the estimated cubic yards, bags, and material needs before you buy supplies.
- Choose the project type: slab, footing, or round column.
- Enter the project dimensions.
- Select a concrete bag size or enter a custom bag yield.
- Add a waste or overage factor. Ten percent is a common planning estimate.
- Enter optional price information to estimate cost.
Concrete Calculator Examples
These worked examples show how the calculator turns project dimensions into cubic yards and bag counts. The math is the same the tool runs above — seeing it once makes every estimate easier to check for yourself.
Example 1: A 10 ft × 10 ft Patio Slab
For a patio measuring 10 ft long, 10 ft wide, and 4 inches thick:
- Convert thickness to feet: 4 ÷ 12 = 0.333 ft
- Volume: 10 × 10 × 0.333 = 33.3 cubic feet
- Convert to yards: 33.3 ÷ 27 = 1.23 cubic yards
- Add a 10% waste factor: 1.23 × 1.10 = 1.35 cubic yards
You would order about 1.5 cubic yards of ready-mix. At this size, delivery usually makes more sense than bags — the same slab would take roughly 56 eighty-pound bags.
Example 2: A 20 ft Footing
For a footing 20 ft long, 12 inches wide, and 12 inches deep:
- Convert: 20 ft × 1 ft × 1 ft
- Volume: 20 × 1 × 1 = 20 cubic feet
- Convert to yards: 20 ÷ 27 = 0.74 cubic yards
Under one yard, so bags are reasonable here: about 34 eighty-pound bags.
Example 3: Six Fence Post Holes
Round holes use the cylinder formula. For 6 holes, each 10 inches wide and 24 inches deep:
- Radius = 5 in = 0.417 ft; depth = 24 in = 2 ft
- Per hole: 3.14 × 0.417² × 2 = 1.09 cubic feet
- Six holes: 1.09 × 6 = 6.5 cubic feet
That works out to roughly 18 fifty-pound bags of fast-setting mix.
Common Concrete Thicknesses
| Project Type | Common Thickness | Notes |
|---|---|---|
| Sidewalk | 4 inches | Common for pedestrian traffic |
| Patio | 4 inches | May vary by soil and intended use |
| Driveway | 4–6 inches | Thicker slabs may be needed for heavier vehicles |
| Garage slab | 4–6 inches | Confirm local code and reinforcement needs |
| Footing | Varies | Depends on structure, frost depth, soil, and code |
Bags or Ready-Mix Concrete?
Bagged concrete is sold by weight, but what you actually need is volume. Here is roughly how much each standard bag yields, and how many it takes to fill a cubic yard.
| Bag Size | Approx. Yield | Bags per Cubic Yard |
|---|---|---|
| 40 lb | ~0.30 cu ft | ~90 |
| 50 lb | ~0.375 cu ft | ~72 |
| 60 lb | ~0.45 cu ft | ~60 |
| 80 lb | ~0.60 cu ft | ~45 |
Rule of thumb: under about 1 cubic yard, bags are practical. Above 1 yard, ready-mix delivery is usually cheaper, faster, and easier than mixing dozens of bags by hand. Right around a yard, it is worth pricing both ways. Always confirm the exact yield printed on the bag, since it varies by product.
Concrete Volume Conversion Reference
Concrete volume is calculated in cubic feet, then converted to cubic yards for ordering. There are 27 cubic feet in one cubic yard.
| Cubic Feet | Cubic Yards |
|---|---|
| 27 | 1.0 |
| 40 | 1.5 |
| 54 | 2.0 |
| 81 | 3.0 |
| 108 | 4.0 |
Concrete Estimating Tips
Smart Buying Tips
- Round up concrete orders to avoid running short during the pour.
- Use ready-mix delivery for larger projects when bag mixing becomes unrealistic.
- Check the exact bag yield on the product label before buying.
- Account for uneven subgrade, form variations, and spillage.
Common Mistakes
- Forgetting to convert inches into feet before calculating volume.
- Not adding waste or overage.
- Underestimating the labor required for bag mixing.
- Ignoring reinforcement, base gravel, form boards, and finishing tools.
For general concrete information, you can also review guidance from the National Ready Mixed Concrete Association.
Frequently Asked Questions
How many 80 lb bags of concrete are in a cubic yard?
About 45. Each 80 lb bag yields roughly 0.60 cubic feet, and there are 27 cubic feet in a cubic yard.
How thick should a concrete slab be?
Four inches is common for patios, walkways, and shed floors. Driveways that carry vehicles are often 4 to 6 inches depending on load. Structural work should follow local building code.
Is it cheaper to mix my own concrete or order ready-mix?
For small jobs under about a cubic yard, bags are usually cheaper and more convenient. Above a yard, ready-mix delivery often wins on both cost and effort once you factor in the labor of mixing.
How much extra concrete should I order?
Adding 5 to 10 percent to your calculated volume covers spillage, uneven ground, and waste. Running short during a pour can create a weak seam, so a small surplus is safer than coming up short.
How long can I work with concrete before it sets?
Standard mixes give roughly 30 to 60 minutes of working time, less in hot weather. Plan the pour so you are not mixing more than you can place and finish in that window.
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Concrete Calculator Results Explained
This concrete calculator helps estimate concrete volume for common residential and DIY projects. It calculates cubic feet and cubic yards from your project dimensions, then estimates the number of concrete bags required based on bag yield.
The concrete calculator gives an estimate based on your project dimensions and selected waste factor. Results are meant for planning purposes and should be checked against local building requirements, supplier recommendations, and actual site conditions.
For larger projects, cubic yards are usually the most important number because ready-mix concrete is commonly ordered by the cubic yard. For small projects, bag count may be more useful.
Ready-Mix vs. Bagged Concrete
Bagged concrete is convenient for small repairs, post holes, small pads, and isolated pours. Ready-mix delivery is usually more practical for larger slabs, driveways, patios, and projects where consistent mixing matters.