Concrete is unforgiving — once you start the pour, you can’t pause or fix a major shortage. Get the volume right and the day goes smoothly; get it wrong and you’re either short mid-pour or paying for material that goes to waste. This guide shows you exactly how to calculate concrete for slabs, footings, columns, walls and stairs, how to translate cubic yards into bags, and when ready-mix beats bags. For instant numbers, use our free Concrete Calculator.

The Core Concrete Formula

For any rectangular pour — slab, footing, wall — the volume math is the same:

Volume (ft³) = Length × Width × Thickness (all in feet)
Cubic yards = Volume in ft³ ÷ 27

Worked example: a 12 ft × 10 ft patio at 4 inches thick. Convert 4 inches to feet (4 ÷ 12 = 0.333), then 12 × 10 × 0.333 = 39.96 ft³, divided by 27 ≈ 1.48 cubic yards. Add 10% waste and order about 1.63 yd³ — or roughly 74 bags of 80 lb concrete.

How Many Bags of Concrete Do I Need?

Bag yields are industry-standard but always approximate:

• 80 lb bag ≈ 0.60 ft³ → about 45 bags per cubic yard
• 60 lb bag ≈ 0.45 ft³ → about 60 bags per cubic yard
• 40 lb bag ≈ 0.30 ft³ → about 90 bags per cubic yard
• 25 kg bag ≈ 0.0125 m³ → about 80 bags per cubic meter

Our Concrete Calculator shows the bag count for all four sizes at once, so you can pick based on what’s in stock and what you can physically carry.

Slabs: How Thick Should They Be?

• Patios & walkways: 4 inches.
• Car driveway: 4–5 inches (5″ is safer).
• Truck or RV driveway: 5–6 inches.
• Garage floor: 4–6 inches (5″ is common).
• Shed pad: 4 inches, with a 2–4″ gravel base under it.

Add rebar or mesh for any slab that will see vehicle traffic.

Footings: Going Below Grade

For a strip footing, use the same length × width × depth formula, but everything below grade hides under the soil — which means you can’t see how much extra is being absorbed. Always add at least 10–15% waste for footings, especially over uneven or soft ground.

Columns and Sonotubes (Round Pours)

For round pours like deck posts or column footings, use the cylinder formula:

Volume = π × radius² × height

A 12-inch diameter sonotube 4 ft deep is π × 0.5² × 4 ≈ 3.14 ft³, or about 0.12 cubic yards per tube. For four tubes, multiply by 4 — and our calculator’s “Number of Columns” field handles that for you.

Stairs: The Tricky Shape

Stairs are essentially a series of stacked rectangles. A simple approximation: for N steps each with rise R and run T, and width W, the total volume is roughly:

Volume ≈ Width × Run × Rise × N(N+1)/2

So 4 steps with 7″ rise and 11″ run at 3 ft wide is about 16 ft³. The calculator does this for you when you switch to the stairs shape.

Bagged vs Ready-Mix: Which Should You Use?

The break-even point sits roughly at one cubic yard:

• Under ~1 cubic yard: bags usually win. No delivery minimum, no truck waiting fees, and you can pour at your own pace. Bag pricing is higher per cubic foot but you only buy what you need.
• Over ~1 cubic yard: ready-mix delivery is normally cheaper per yard, the mix is consistent across the whole pour, and you finish far faster.

Enter both prices in our Concrete Calculator and it shows bagged vs ready-mix side by side, with the difference highlighted.

How Much Waste to Add

Always order more than the bare math says. Common allowances:

• Standard flat slab on level ground: 5–10%.
• Footings & uneven ground: 10–15%.
• Multiple sonotubes: 10% — over-dig is normal.
• Stairs and irregular shapes: 10–15%.

You’d rather have an extra wheelbarrow than be short mid-pour.

Quick Reference