Calculate how much concrete you need for slabs, footings, pads, walkways and other concrete projects.
Enter the length, width and thickness of your project to estimate the required amount of concrete in cubic feet, cubic yards and bags.
Use the calculator for patios, sidewalks, driveways, equipment pads, footings, fence posts and other projects that require concrete.
Concrete quantities can vary because of excavation depth, subgrade conditions, form dimensions and waste. Use this calculator as an estimating tool and confirm final ready-mix or bag quantities before ordering.
Free Concrete Calculator
Concrete Calculator
Enter your project dimensions to estimate concrete in cubic yards, cubic feet and bags.
Concrete calculations are estimates. Actual requirements vary with excavation depth, forms, subgrade conditions and product yield. Confirm final quantities before ordering.
How Much Concrete Do I Need?
Concrete volume is calculated from the dimensions of the area being poured.
For a rectangular slab:
Length × Width × Thickness = Volume
Because slab thickness is usually measured in inches, first convert the thickness to feet.
For example, a slab measuring:
20 feet × 12 feet × 4 inches
has an area of:
20 × 12 = 240 square feet
Four inches equals approximately 0.333 feet:
240 × 0.333 = 79.9 cubic feet
Then divide by 27 to convert cubic feet to cubic yards:
79.9 ÷ 27 = 2.96 cubic yards
Before ordering concrete, contractors commonly allow additional material for variations in excavation, forms and actual slab thickness.
How to Calculate Concrete Cubic Yards
The basic formula is:
Length (ft) × Width (ft) × Thickness (ft) ÷ 27 = Cubic Yards
If thickness is measured in inches:
Thickness in inches ÷ 12 = Thickness in feet
Then use that number in the volume calculation.
For example:
10 ft × 10 ft × 4 in
Convert 4 inches to feet:
4 ÷ 12 = 0.333 ft
Calculate volume:
10 × 10 × 0.333 = 33.3 cubic feet
Convert to cubic yards:
33.3 ÷ 27 = 1.23 cubic yards
Should I Add Extra Concrete?
Usually, yes.
A mathematical calculation assumes that the forms, excavation and thickness are perfectly uniform. Actual jobsite conditions rarely are.
Extra concrete can help account for:
- Uneven subgrade
- Slightly deeper excavation
- Form variations
- Spillage
- Low areas
- Inaccurate measurements
- Material left in the chute or equipment
A modest overage can help prevent a costly short load or an incomplete pour.
The calculator allows you to adjust the waste percentage rather than assuming every project requires the same amount.
Ready-Mix Concrete vs. Concrete Bags
For larger projects, concrete is usually ordered from a ready-mix supplier by the cubic yard.
Smaller projects may be completed using bags of premixed concrete.
Examples include:
- Fence posts
- Mailbox posts
- Small equipment pads
- Minor repairs
- Steps
- Small footings
- Short walkway sections
The amount of concrete produced by a bag depends on the bag size and product.
Always check the manufacturer's stated yield on the specific concrete mix you are using.
How Many Bags of Concrete Do I Need?
To estimate bags, first determine the project's total volume in cubic feet.
Then divide the required volume by the approximate yield of each bag.
For example:
Required cubic feet ÷ cubic feet produced per bag = Number of bags
Because different products may produce slightly different yields, the calculator lets you choose a common bag size or enter a custom yield from the bag label.
Always round the final number of bags up.
Common Concrete Slab Thicknesses
Concrete thickness depends on the project, soil conditions, reinforcement, expected loads and local requirements.
| Project | Common Thickness |
|---|---|
| Residential walkway | 4 inches |
| Patio | 4 inches |
| Shed or equipment pad | 4–6 inches |
| Residential driveway | 4–6+ inches |
| Garage floor | Project-specific |
| Heavy vehicle area | Engineered/project-specific |
These are general estimating references, not structural specifications. Follow project plans, local requirements and engineering recommendations where applicable.
Concrete Calculator Example
Suppose you're pouring a slab measuring:
24 feet long × 16 feet wide × 4 inches thick
Area:
24 × 16 = 384 square feet
Convert thickness:
4 ÷ 12 = 0.333 feet
Volume:
384 × 0.333 = 127.9 cubic feet
Convert to cubic yards:
127.9 ÷ 27 = 4.74 cubic yards
Adding 10% overage:
4.74 × 1.10 = 5.21 cubic yards
For planning purposes, the estimated order would therefore be approximately:
5.2 cubic yards of concrete
Your ready-mix supplier may have minimum-order quantities, delivery charges and recommended rounding practices.
Frequently Asked Questions
How do I calculate how much concrete I need?
Multiply length by width by thickness to determine volume. Convert thickness to feet if it is measured in inches, then divide cubic feet by 27 to determine cubic yards.
How many cubic feet are in one cubic yard of concrete?
One cubic yard contains 27 cubic feet.
How many square feet does one yard of concrete cover?
Coverage depends on thickness. One cubic yard covers approximately 81 square feet at 4 inches thick, 65 square feet at 5 inches thick, or 54 square feet at 6 inches thick.
Should I order extra concrete?
Usually, some additional concrete is wise because actual slab thickness and excavation conditions vary. The appropriate amount depends on the project and how accurately the forms and subgrade have been prepared.
Should I round concrete orders up?
Generally, yes. Running short during a concrete pour can be expensive. Ready-mix suppliers may also have specific increments or minimum quantities they use when accepting orders.
Can I calculate concrete bags instead of cubic yards?
Yes. Determine the total cubic feet required, then divide by the stated cubic-foot yield of the bagged mix you're using. Always verify the yield printed on the product packaging.
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