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How Much Concrete Do I Need?
Quick answer: To find how much concrete you need, multiply length × width × thickness in feet and divide by 27 for cubic yards — a 10×10 ft slab at 4 inches thick needs about 1.24 cubic yards.
Every concrete estimate comes down to one formula and three decisions: how to measure, whether to buy bags or a ready-mix truck, and how much extra to order. This guide walks through all of it, then points you to the exact calculator for your project.
The one formula that runs everything
Concrete volume is simply length × width × depth. The only trick is keeping your units consistent. Measure length and width in feet, but measure depth (thickness) in inches — then convert that depth to feet by dividing by 12 before you multiply.
For a round footing or post hole, the shape is a cylinder, so the math changes to π × radius² × depth. That is easy to get wrong by hand, which is exactly why a dedicated tool helps.
Step 1: Measure your project
Break the project into simple rectangles and measure each one. For an L-shaped patio, split it into two rectangles, calculate each, and add them together.
- Length & width: measure in feet. For a circular pour, measure the diameter and halve it for the radius.
- Depth: measure in inches. A sidewalk, shed base, or patio is usually 4 inches; a driveway that carries vehicles should be 5 to 6 inches.
Step 2: Convert to cubic yards
Concrete from a supplier is sold by the cubic yard. Since one cubic yard equals 27 cubic feet, divide your total cubic feet by 27:
Bagged concrete, on the other hand, is sold by yield. Here is how the common bag sizes convert:
| Bag size | Yield (per bag) | Bags per cubic yard |
|---|---|---|
| 80 lb | ~0.60 cu ft | ~45 bags |
| 60 lb | ~0.45 cu ft | ~60 bags |
| 40 lb | ~0.30 cu ft | ~90 bags |
Step 3: Bags or a ready-mix truck?
This is the decision that saves the most money and back strain.
- Bags (mix yourself): best for small pours — post holes, small pads, steps, and repairs, roughly up to 1 cubic yard. Beyond that you are mixing 45 to 90 bags, which is a lot of labor.
- Ready-mix truck: best for driveways, large slabs, and foundations. Most suppliers have a minimum load (often 1 yard) and short-load fees, so it pays to know your yardage before you call.
How thick should the slab be?
Thickness drives both your volume and the slab's strength. Quick reference:
| Project | Typical thickness |
|---|---|
| Sidewalk / walkway | 4 in |
| Patio / shed base | 4 in |
| Car driveway | 4–5 in |
| Driveway (trucks/RV) | 5–6 in |
| Garage floor | 4–6 in |
For a deeper look at reinforcement and subbase, see How Thick Should a Concrete Slab Be?
Add a waste factor
Always add 5 to 10% to your calculated volume. Subgrades are never perfectly level, you will lose a little to spillage, and — most importantly — running out mid-pour creates a cold joint that weakens the slab. It is far cheaper to return an unopened bag than to make a second trip halfway through.
Worked example
Say you are pouring a 10 ft × 12 ft patio at 4 inches thick:
- Depth in feet: 4 ÷ 12 = 0.333 ft
- Volume: 10 × 12 × 0.333 = 40 cubic feet
- Cubic yards: 40 ÷ 27 = 1.48 cu yd
- In 80 lb bags: 40 ÷ 0.6 = ~67 bags — add 10% → ~74 bags
At nearly 1.5 yards and 74 bags, this project sits right at the line where a ready-mix truck starts to make sense.
Which calculator should you use?
Skip the arithmetic — pick the tool that matches your project and it applies the waste factor and bag conversions for you:
- Slabs, patios, and post holes in bags → Concrete Bag Calculator
- Cubic yards and installed/ready-mix cost → Concrete Cost & Yardage Calculator
- Round footings, piers, and Sonotubes → Concrete Footing & Sonotube Calculator
- Reinforcing steel for slabs and footings → Rebar Calculator
Frequently asked questions
How do I calculate how much concrete I need?
Multiply length × width × depth in feet to get cubic feet, then divide by 27 for cubic yards. For bags, divide cubic feet by the bag yield (0.6 for 80 lb, 0.45 for 60 lb, 0.30 for 40 lb). Add 5–10% for waste.
How many 80 lb bags are in a cubic yard?
About 45. A cubic yard is 27 cubic feet and an 80 lb bag yields roughly 0.6 cubic feet (27 ÷ 0.6 = 45).
When should I order a ready-mix truck instead of bags?
Once you pass about 1 cubic yard. Bagging a full yard means mixing 45 to 90 bags by hand — a truck is usually cheaper and far less work above that point.
How much extra should I order?
Add 5–10%. Extra bags can be returned, but a second batch poured later will not bond properly to the first.