Calculate exactly how much concrete you need for your slab, footing, post hole, driveway, patio, walkway, or other concrete project. Enter the dimensions of your project to estimate the required concrete volume in cubic yards, cubic feet, or cubic meters.
Use the calculator above to estimate the amount of concrete required for your project.
For a rectangular slab, the basic calculation is:
Length × Width × Thickness = Concrete Volume
For example, a slab that is 20 feet long, 10 feet wide, and 4 inches thick requires approximately 6.67 cubic yards of concrete before adding a waste allowance.
How the Concrete Calculator Works
The calculator determines the volume of concrete based on three main measurements:
- Length – the total length of the area
- Width – the total width of the area
- Thickness – the depth of the concrete
Because concrete is commonly ordered by cubic yards in the United States, the calculator converts the measurements into the appropriate unit.
Concrete Volume Formula
For a rectangular slab:
Volume = Length × Width × Thickness
If your measurements are in feet, convert the thickness from inches to feet first.
For example:
- Length: 20 ft
- Width: 10 ft
- Thickness: 4 in
- Thickness in feet: 4 ÷ 12 = 0.333 ft
Therefore:
20 × 10 × 0.333 = 66.67 cubic feet
To convert cubic feet to cubic yards:
66.67 ÷ 27 = 2.47 cubic yards
So a 20 × 10-foot slab that is 4 inches thick requires about 2.47 cubic yards of concrete, before adding extra material for waste and variations.
How Much Concrete Do I Need?
The amount of concrete depends on the size and thickness of your project.
4-Inch Concrete Slab
A 4-inch slab is commonly used for many residential applications, although the appropriate thickness depends on the project’s purpose, soil conditions, reinforcement, loads, and local requirements.
5-Inch Concrete Slab
Increasing the thickness to 5 inches increases the concrete volume by 25% compared with a 4-inch slab of the same length and width.
6-Inch Concrete Slab
A 6-inch slab requires 50% more concrete than a 4-inch slab with the same surface dimensions.
Concrete Calculator by Project Type
The same basic volume principle can be used for many concrete projects, but irregular shapes require different calculations.
Concrete Slab Calculator
For patios, garage floors, sheds, floors, and other rectangular slabs:
Length × Width × Thickness
Measure the length and width in feet and the thickness in inches.
Concrete Driveway Calculator
A driveway may consist of a rectangular or irregular area. For a simple rectangular driveway, calculate the slab volume first.
For an irregular driveway, divide the area into smaller rectangles or sections, calculate each section separately, and add the volumes together.
Concrete Footing Calculator
Footings are commonly calculated using:
Length × Width × Depth
If a footing contains multiple sections, calculate each section separately and add them together.
Concrete Post Hole Calculator
For cylindrical post holes, use the cylinder formula:
Volume = π × Radius² × Depth
You need the diameter and depth of the hole to determine the volume.
Concrete Round Slab Calculator
For circular concrete areas:
Volume = π × Radius² × Thickness
Because the radius is half the diameter:
Radius = Diameter ÷ 2
Concrete Steps Calculator
Concrete steps can have more complicated shapes. Break the structure into simple rectangular or triangular sections, calculate each section, and add the results.
For complex stairs, a professional contractor or structural professional may be needed to determine the appropriate dimensions and reinforcement.
How to Calculate Concrete in Cubic Yards
Concrete suppliers in the United States commonly sell ready-mix concrete by the cubic yard.
To convert cubic feet to cubic yards:
Cubic Yards = Cubic Feet ÷ 27
For example, if your project requires 81 cubic feet:
81 ÷ 27 = 3 cubic yards
Therefore, the theoretical concrete volume is 3 cubic yards.
How to Calculate Concrete in Cubic Feet
If the dimensions are measured in feet:
Cubic Feet = Length × Width × Thickness
Remember to convert inches into feet.
For example:
6 inches = 6 ÷ 12 = 0.5 feet
A 12 × 10-foot slab that is 6 inches thick requires:
12 × 10 × 0.5 = 60 cubic feet
How to Calculate Concrete in Cubic Meters
If you’re working with metric measurements, use:
Volume = Length × Width × Thickness
When all dimensions are measured in meters, the result is cubic meters.
For example:
- Length = 6 m
- Width = 4 m
- Thickness = 0.10 m
6 × 4 × 0.10 = 2.4 m³
The project requires approximately 2.4 cubic meters of concrete before accounting for waste.
Concrete Waste Allowance
It is usually wise to allow some extra concrete rather than ordering exactly the theoretical calculated volume.
The actual amount needed can vary because of:
- Uneven ground
- Variations in slab thickness
- Formwork dimensions
- Spillage
- Measuring inaccuracies
- Irregular excavation
- Site conditions
A common planning approach is to add approximately 5% to 10% to the calculated volume, depending on the project and conditions.
For example, if the calculated requirement is 5 cubic yards and you add 10%:
5 × 1.10 = 5.5 cubic yards
The appropriate amount to order depends on the supplier’s policies and the specifics of the project.
Concrete Calculator Example
Suppose you are pouring a patio that measures:
- Length: 24 feet
- Width: 12 feet
- Thickness: 4 inches
Convert thickness:
4 ÷ 12 = 0.333 feet
Calculate volume:
24 × 12 × 0.333 = 96 cubic feet
Convert to cubic yards:
96 ÷ 27 = 3.56 cubic yards
With a 10% allowance:
3.56 × 1.10 = 3.92 cubic yards
You would therefore plan for approximately 3.9 cubic yards, subject to the supplier’s ordering increments and project conditions.
Concrete Bag Calculator
If you’re mixing concrete yourself instead of ordering ready-mix concrete, you’ll also need to determine how many bags are required.
The number of bags depends on:
- Bag size
- Manufacturer
- Yield per bag
- Required concrete volume
Do not assume that every 50-lb or 60-lb bag produces the same volume. Always check the manufacturer’s stated yield on the packaging.
The general calculation is:
Number of Bags = Required Concrete Volume ÷ Yield Per Bag
Round the result up to a whole bag.
Ready-Mix Concrete vs. Bagged Concrete
For small projects, bagged concrete may be convenient because you can mix it as needed.
For larger projects, ready-mix concrete is often more practical because a concrete truck can deliver a large quantity already mixed to the required consistency.
Bagged Concrete
Advantages:
- Convenient for small projects
- No concrete truck required
- Easy to purchase in smaller quantities
- Useful for repairs and small footings
Disadvantages:
- Labor-intensive for large projects
- Requires mixing equipment
- Many bags may be needed
- Larger projects can take considerable time
Ready-Mix Concrete
Advantages:
- Suitable for larger pours
- Faster installation
- Consistent professionally mixed material
- Less manual mixing
Disadvantages:
- Delivery charges may apply
- Minimum order quantities may apply
- The site needs suitable truck access
- Timing is important because concrete has a limited working window
How Thick Should Concrete Be?
The required concrete thickness depends on what the slab will be used for, expected loads, soil conditions, reinforcement, climate, and applicable building requirements.
There is no single thickness that is appropriate for every concrete project.
A residential walkway may have different requirements from a garage floor or vehicle driveway. Heavy loads may require a thicker slab and additional structural design.
For structural or load-bearing projects, follow local building requirements and professional specifications rather than relying solely on a general calculator.
Common Concrete Calculation Mistakes
Forgetting to Convert Inches to Feet
If your length and width are in feet but thickness is in inches, convert the thickness before multiplying.
Ordering Exactly the Calculated Amount
The theoretical volume may not account for site variations or waste. Consider an appropriate allowance.
Measuring the Excavation Instead of the Finished Form
The dimensions used in the calculation should correspond to the volume that will actually be filled with concrete.
Ignoring Irregular Shapes
An L-shaped or irregular area should generally be divided into simpler sections before calculating the total volume.
Assuming Every Concrete Bag Has the Same Yield
Bag yields vary by product. Check the manufacturer’s specifications.
Frequently Asked Questions
What is a concrete calculator?
A concrete calculator estimates how much concrete is required for a project based on its dimensions. Depending on the calculator, the result may be provided in cubic feet, cubic yards, cubic meters, or the estimated number of concrete bags.
How do I calculate how much concrete I need?
For a rectangular slab, multiply the length by the width and thickness. Make sure all measurements use the same unit before calculating the volume.
How many cubic yards of concrete do I need?
Calculate the volume in cubic feet and divide by 27 to convert cubic feet to cubic yards.
How much extra concrete should I order?
A waste allowance of around 5% to 10% is commonly used for planning, but the appropriate allowance depends on the shape, site conditions, and accuracy of the measurements.
How do I calculate concrete for a circular area?
Use the cylinder formula:
π × radius² × thickness
Make sure all dimensions are expressed in the same unit.
Can I use this calculator for a driveway?
Yes. A concrete calculator can estimate the volume required for a driveway. For irregular driveways, divide the area into manageable sections and calculate each section separately.
How many bags of concrete do I need?
Divide the required volume by the yield stated on the concrete bag. Because products have different yields, always use the manufacturer’s specification.
How many cubic feet are in a cubic yard of concrete?
One cubic yard contains 27 cubic feet.
How do I convert cubic feet to cubic yards?
Divide the number of cubic feet by 27.
Cubic Yards = Cubic Feet ÷ 27
Does concrete thickness affect the amount needed?
Yes. Increasing the thickness directly increases the required concrete volume. For the same length and width, a 6-inch slab requires 50% more concrete than a 4-inch slab.
Final Thoughts
A concrete calculator makes it easier to estimate the amount of material required before starting a project. Measure the project carefully, convert all dimensions to the same units, calculate the volume, and consider an appropriate allowance for waste and variations.
For structural projects, heavy-load applications, or projects subject to building regulations, use the calculator as an estimate and follow the applicable professional and local requirements.