Concrete Calculator
Calculate how much concrete you need for any pour — rectangle, circle, or triangle. Get cubic feet, cubic yards, and bag counts instantly.
Need to estimate project cost? Try the Concrete Estimator →
This concrete calculator takes the guesswork out of any pour. If you've ever asked yourself how much concrete do I need for a project — or needed a concrete calculator in cubic yards to prep a readymix order — this tool covers both. Enter your shape, dimensions, and depth to instantly get the volume in cubic feet, cubic yards, and the exact number of 60lb or 80lb bags required. This concrete volume calculator handles rectangle, circle, and triangle shapes and does the unit conversions automatically. Most projects require at least 10% extra concrete to account for spillage, waste, and uneven subgrade — round up your bag count accordingly.
How to Use the Concrete Calculator
- Select your pour shape — rectangle, circle, or triangle.
- Enter the dimensions in feet: length and width for rectangles, diameter for circles, base and height for triangles.
- Enter the depth or thickness in inches (not feet).
- Click "Calculate Concrete" to see volume in cubic feet, cubic yards, and bag counts.
- Order extra — add at least 10% to account for waste and spillage.
Concrete Calculation Example: 10×10 Slab at 4 Inches
A standard 10-foot by 10-foot patio slab poured at 4 inches thick:
| Measurement | Value |
|---|---|
| Volume | 33.33 cubic feet |
| Cubic yards | 1.23 cu yd |
| 60lb bags needed | 75 bags |
| 80lb bags needed | 56 bags |
If you're asking how many bags of concrete do I need, the calculator above gives you the exact count — but at this volume, readymix ordered by the yard is typically more economical than mixing 75 individual bags. Bags become cost-effective for pours under about half a cubic yard.
When to Use This Calculator
Planning any concrete pour before buying materials. Showing up to the hardware store with exact numbers instead of guessing means you buy the right amount on the first trip. Under-buying means a second trip mid-pour; over-buying means wasted money on bags you return or discard.
Deciding between bags and readymix. At volumes below 1 cubic yard, bags are typically more convenient. Above 1–2 cubic yards, a readymix truck delivery becomes more cost-effective per cubic foot. This calculator gives you the volume you need to make that call.
Calculating concrete for non-standard shapes. Circular footings, curved garden borders, and triangular fill areas all require different formulas. Selecting the right shape here handles the math automatically.
Frequently Asked Questions
How do you calculate how much concrete you need?
Multiply length × width × depth (depth converted to feet) for the volume in cubic feet, then divide by 27 to get cubic yards. Knowing how to calculate concrete for a slab: a 10×10 at 4 inches thick is 10 × 10 × (4/12) = 33.33 cubic feet, or 1.23 cubic yards. For circles use π × r² × depth, and for triangles use 0.5 × base × height × depth. Always add 10% for waste.
How many bags of concrete do I need for a 10x10 slab?
A 10×10 slab at 4 inches thick requires 33.33 cubic feet. Using 60lb bags, which cover 0.45 cubic feet each, you need 75 bags. Using 80lb bags, which cover 0.60 cubic feet each, you need 56 bags. At this volume most contractors would use readymix instead, which is cheaper and far less labor-intensive than mixing 56–75 individual bags.
How many cubic yards is a 10x10 concrete slab?
A 10×10 slab at 4 inches thick equals 1.23 cubic yards. At 6 inches thick it is 1.85 cubic yards. At 3.5 inches — the thinnest common residential slab — it is 1.08 cubic yards. To convert cubic feet to cubic yards, divide by 27. This calculator does that conversion automatically.
How do I calculate concrete for a circular area?
Use the formula: volume = π × (diameter ÷ 2)² × depth in feet. An 8-foot diameter circular pad at 4 inches thick equals π × 4² × 0.333 = 16.76 cubic feet, or 0.62 cubic yards — about 38 60lb bags. Select "Circle" in this calculator and enter the diameter to get the result automatically without doing the math manually.
How much does a cubic yard of concrete cover?
One cubic yard covers 81 square feet at 4 inches thick, 54 square feet at 6 inches, and 108 square feet at 3 inches. At 4 inches — the most common residential slab thickness — a single cubic yard covers roughly a 9×9 foot area. Knowing this lets you quickly sanity-check your calculated volume before ordering.