🧱Concrete Calculator
Calculate cubic yards of concrete and the number of 40, 60 and 80 lb bags for a slab, footing, pad or round pier
Last updated: October 22, 2026
How to Use the Concrete Calculator
How to calculate how much concrete you need
- Choose a shape. Use Slab / footing / wall for anything rectangular, or Round pad / pier for circular pours.
- Enter the dimensions. Length and width (or diameter) are in feet. Thickness is in inches.
- Set the quantity. If you are pouring several identical piers or slabs, enter how many.
- Keep the waste allowance. The default 10% extra is a sensible buffer. Lower it if your site is perfectly level and your forms are precise.
- Read the results. The total is shown in cubic yards, cubic feet and cubic meters, plus the number of 40, 60 and 80 lb bags.
For L-shaped or irregular pours, split the area into rectangles, calculate each, and add the cubic yards together. Concrete is hard to top up, so it is better to order slightly more than you need.
Formula & Methodology
Concrete volume formula
Cubic yards = Length (ft) × Width (ft) × Thickness (ft) ÷ 27
Thickness is usually measured in inches, so divide it by 12 first. For a round pad or pier, replace length × width with the circle's area, π × (diameter ÷ 2)².
Worked example
A patio slab is 10 ft × 10 ft and 4 inches thick, with 10% extra.
- Thickness in feet: 4 ÷ 12 = 0.333 ft
- Volume: 10 × 10 × 0.333 = 33.3 cubic feet
- Add 10%: 33.3 × 1.10 = 36.7 cubic feet
- Cubic yards: 36.7 ÷ 27 = 1.36 cubic yards
How many bags?
Each pre-mixed bag yields a fixed volume, so divide the cubic feet you need by the yield and round up:
| Bag size | Approximate yield | |---|---| | 80 lb | 0.60 cu ft | | 60 lb | 0.45 cu ft | | 40 lb | 0.30 cu ft |
One cubic yard is 27 cubic feet, which is about 45 bags of 80 lb. Bag yields vary slightly by brand, so check the label on the product you buy.
Bags or ready-mix?
Bags are practical for small jobs. For roughly a cubic yard or more, ordering ready-mix concrete delivered by truck is usually faster and often cheaper per cubic yard.
About Concrete Calculator
Our concrete calculator is a free, easy-to-use tool designed to help youcalculate cubic yards of concrete and the number of 40, 60 and 80 lb bags for a slab, footing, pad or round pier. For professionals or those who need quick calculations, our calculator provides accurate results instantly.
Frequently Asked Questions
How do I calculate how much concrete I need?
Multiply length by width by thickness, all in feet, then divide by 27 to get cubic yards. A 10 ft by 10 ft slab that is 4 inches (0.333 ft) thick needs 10 × 10 × 0.333 ÷ 27 = 1.23 cubic yards before any waste allowance.
How many bags of concrete are in a cubic yard?
A cubic yard is 27 cubic feet. An 80 lb bag yields about 0.6 cubic feet, so you need roughly 45 bags per cubic yard. That is about 60 bags of 60 lb or 90 bags of 40 lb.
How thick should a concrete slab be?
Four inches is typical for patios, walkways and shed bases. Driveways and garage floors usually use 5 to 6 inches, or more for heavy vehicles. Local building codes and your soil conditions can change the requirement, so check before you pour.
Should I order extra concrete?
Yes. Most contractors add 5 to 10% for spillage, uneven ground and over-excavation. Running short mid-pour can leave a cold joint, so a little extra is cheap insurance.
How much concrete do I need for a 10x10 slab?
At 4 inches thick, a 10 ft by 10 ft slab is 33.3 cubic feet, or 1.23 cubic yards. With a 10% allowance, plan on about 1.36 cubic yards, which is roughly 62 bags of 80 lb concrete.
How do I calculate concrete for a round hole or pier?
Find the circle area with π × (diameter ÷ 2)², multiply by the depth in feet, and divide by 27 for cubic yards. For a 1 ft diameter, 3 ft deep pier, that is 0.785 × 3 = 2.36 cubic feet, or about 4 bags of 80 lb.
Is this enough for reinforcement and gravel base?
No. This tool calculates only the concrete volume. A gravel base, rebar or wire mesh, and forms are separate materials. Use the gravel calculator to estimate the base layer.
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