Concrete Calculator
Free concrete calculator that turns your slab, footing, or pad dimensions into cubic yards and the number of pre-mix bags you need. Enter length, width, and thickness to estimate ready-mix concrete in seconds.
Calculate concrete volume
How the concrete calculator works
Whether you are pouring a slab, footing, or column, this concrete calculator estimates exactly how much ready-mix concrete you need. It multiplies length by width by thickness, converts the thickness from inches to feet, and divides the result by 27 to give cubic yards. For example, a 10 ft × 10 ft slab at 4 inches thick works out to (10 × 10 × 4 ÷ 12) ÷ 27 = 1.23 cubic yards.
If you are mixing by hand instead of ordering ready-mix, the built-in concrete bag calculator converts that volume into 80 lb and 60 lb pre-mix bags — roughly 45 of the 80 lb bags fill one cubic yard. The concrete pad calculator logic is identical for patios, shed bases, and equipment pads: it is all length × width × depth.
Slabs, footings, columns and blocks
The same volume formula scales to most pours. A concrete pad calculator for a shed uses the slab method above. Footings are long, narrow slabs — measure the length, width, and depth of each run and add them. Columns are easier to think of in cubic feet first, then divide by 27. If you are estimating wall fill, a concrete block calculator instead counts blocks by wall area (a standard 8×8×16 in block covers about 0.89 sq ft of wall face) and then adds mortar and any core fill.
How to use this calculator
- Measure your slab dimensions. Use this concrete slab calculator by measuring the length and width of the area in feet and the thickness in inches. For footings, measure each section separately and add the results.
- Convert to cubic yards. The concrete calculator divides thickness by 12 to convert inches to feet, multiplies length × width × depth, then divides by 27 to express the volume in cubic yards — the unit ready-mix suppliers use.
- Add waste and check bag counts. Add 5–10% waste for spillage and uneven subgrade. If you are DIY-mixing, the concrete bag calculator shows how many 80 lb and 60 lb pre-mix bags cover the same volume.
- Estimate the cost. Enter your local price to estimate the ready-mix cost. The concrete yard calculator multiplies cubic yards by your price per yard so you can compare supplier quotes.
Frequently asked questions
How does the concrete slab calculator handle thickness in inches?
It divides the thickness in inches by 12 to convert to feet before multiplying by length and width, then divides the total cubic feet by 27 to give cubic yards.
How many bags does the concrete bag calculator recommend per cubic yard?
About 45 80 lb bags or 60 60 lb bags of pre-mix fill one cubic yard. The calculator rounds up so you do not come up short.
What waste percentage should I add?
Add about 5% for a clean, well-formed slab and up to 10% for irregular pours, sloped ground, or spillage during placement.
Can I use this as a concrete yard calculator for a ready-mix order?
Yes. Cubic yards is exactly the unit ready-mix suppliers use, so the result is the number you give the plant. Enter your local price to estimate cost.
Are 2026 concrete prices included?
The default of $165 per cubic yard reflects 2026 national-average ready-mix pricing (typical range $130–195). Always enter your own local quote for an accurate cost.
Cubic yards vs bags vs cost
| Pour | Cubic yards | 80 lb bags |
|---|---|---|
| 10×10 slab, 4 in | 1.23 yd³ | ~62 bags |
| 12×12 slab, 4 in | 1.78 yd³ | ~89 bags |
| 20×20 slab, 4 in | 4.94 yd³ | order ready-mix |
Once a pour exceeds about 1 cubic yard (roughly 45–60 bags), ordering ready-mix is usually cheaper and faster than bag-mixing.