Concrete Calculator
Calculate the concrete you need for any project — slabs, footings, columns, sonotubes, walls, or stairs. Get the volume in cubic yards and cubic meters, how many 40 lb, 60 lb, or 80 lb bags to buy, and a side-by-side bagged vs ready-mix cost estimate. Worldwide units, with waste allowance built in.
Pick a project shape, enter your dimensions, and the calculator works out the cubic yards (and cubic meters), the number of 40/60/80 lb bags needed, and the cost — comparing bags vs ready-mix delivery.
| Bag Size | Yield per Bag | Bags Needed |
|---|
* Concrete bag yields are approximate (manufacturers list slightly different cubic-foot yields per bag; we use industry-standard averages). Real volumes vary with mix, water content, and pour conditions. Always add a waste allowance (5–10% is typical, 10–15% for irregular pours), measure twice, and confirm quantities and pricing with your supplier before ordering. This tool is for general planning only and is not engineering advice.
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What Is a Concrete Calculator?
A concrete calculator works out how much concrete you need for a project — and turns that volume into the practical numbers you actually buy: cubic yards for ready-mix delivery, the number of 40 lb, 60 lb, or 80 lb bags for smaller jobs, and an estimated cost either way. Get this wrong and you either run out mid-pour (a disaster, because concrete sets fast) or pay for material you'll never use.
Our concrete calculator handles every common shape you'll meet around a home or job site: slabs, footings, columns and sonotubes, walls, and stairs. It shows the volume in both cubic yards and cubic meters, includes a waste allowance, and compares bagged vs ready-mix cost side by side so you can pick the right buying option for your project size.
How to Use This Concrete Calculator
- Pick a project shape — slab, footing, column/sonotube, wall, stairs, or "known volume."
- Choose your units — US (feet + inches) or metric (meters + cm).
- Enter dimensions — length, width, thickness, diameter, or step rise/run.
- Set a waste allowance — 10% is standard for most pours; 15% for irregular ground.
- Pick a bag size — 40 lb, 60 lb, 80 lb, or 25 kg.
- Optional: add prices — per bag and per cubic yard of ready-mix for a cost comparison.
How to Calculate Concrete (The Formulas)
Volume (ft³) = Length × Width × Thickness (all in feet)
Cubic yards = ft³ ÷ 27
Column / sonotube (round):
Volume (ft³) = π × radius² × height (all in feet)
Stairs:
Approx Volume ≈ Width × Run × (½ × Rise × Number of steps + Rise)
Example — a 12 ft × 10 ft patio at 4 inches thick: 4 inches is 0.333 ft, so volume = 12 × 10 × 0.333 = 39.96 ft³, divided by 27 ≈ 1.48 cubic yards. With a 10% waste allowance, order about 1.63 cubic yards — or roughly 74 bags of 80 lb concrete if going the bag route.
How Many Bags of Concrete Do I Need?
Bag yield is industry-standard but approximate. Plan on these averages:
| Bag Size | Yield (ft³) | Bags per Cubic Yard |
|---|---|---|
| 40 lb | ≈ 0.30 | ≈ 90 |
| 60 lb | ≈ 0.45 | ≈ 60 |
| 80 lb | ≈ 0.60 | ≈ 45 |
| 25 kg | ≈ 0.012 m³ | ≈ 83 per m³ |
So a cubic yard of concrete equals roughly 45 × 80 lb bags, 60 × 60 lb bags, or 90 × 40 lb bags. The calculator shows all three at once so you can decide based on how much you can carry and what's in stock.
Bagged Concrete vs Ready-Mix: Which Should I Use?
The break-even point is roughly one cubic yard:
- Under ~1 cubic yard: bags are usually cheaper and more practical — no delivery minimum, no truck waiting fees, and you can pour at your own pace.
- Over ~1 cubic yard: ready-mix delivery is usually faster and works out cheaper per yard, plus the mix is consistent across the whole pour.
Enter both prices in the calculator and it shows the bagged vs ready-mix total side by side, so the decision is obvious for your project.
Common Concrete Project Thicknesses
| Project | Typical Thickness | Notes |
|---|---|---|
| Patio / walkway | 4 inches | Foot traffic only |
| Driveway (car) | 4–5 inches | Add rebar for durability |
| Driveway (truck/RV) | 5–6 inches | Heavier vehicles |
| Garage floor | 4–6 inches | 5″ is common |
| Shed pad | 4 inches | Add 2–4″ of gravel base |
| Footing (residential) | 8–12 inches deep | Below frost line in cold climates |
How Much Waste Should I Add?
Always order more than the bare math says. Standard allowances:
- Standard flat slab on level ground: 5–10%.
- Footings or uneven ground: 10–15% (you can't see what gets absorbed below grade).
- Multiple sonotubes: 10% — over-dig is normal.
- Stairs and irregular shapes: 10–15%.