Concrete Slab Calculator
Estimate exactly how many cubic yards or bags of concrete you need for your slab, patio, or driveway. Enter your dimensions below to instantly calculate volume, weight, and project cost.
Slab Dimensions
Enter your project measurements to calculate your estimate.
Total Volume Required
Includes 10% waste allowance.
Or Using Premixed Bags
How to Use the Concrete Slab Calculator
Whether you’re building a new patio, pouring a shed base, or framing a driveway, knowing exactly how to calculate yards of concrete is crucial. Ordering too little means a compromised pour, while ordering too much wastes money.
- Measure your dimensions: Determine the length, width, and thickness of your slab.
- Select your units: Our concrete dimensions tool supports feet, inches, meters, and centimeters. You don’t need to manually convert inches to feet.
- Set a waste factor: A 10% waste allowance is standard to account for subgrade settling, uneven terrain, and spillage.
- Review the results: The calculator instantly outputs the total cubic yards needed for a ready-mix truck, as well as the exact number of premixed bags (80 lb, 60 lb, etc.) if you choose to mix it yourself.
How the Calculation Works
Concrete volume is determined by multiplying the three primary dimensions together to find the total cubic volume. Because concrete is sold and delivered in cubic yards (in the US), you must convert your measurements.
The Formula
Volume (Cubic Yards) = (Length in ft × Width in ft × Depth in ft) ÷ 27
Note: We divide by 27 because there are 27 cubic feet in one cubic yard (3ft × 3ft × 3ft).
Worked Example: 20×30 Concrete Slab Calculator
Let’s say you are building a 20-foot by 30-foot garage floor, poured 4 inches thick.
- Length: 20 feet
- Width: 30 feet
- Thickness: 4 inches (which is 4/12 = 0.333 feet)
Calculation: 20 × 30 × 0.333 = 200 cubic feet.
Next, convert to yards: 200 ÷ 27 = 7.41 cubic yards.
Finally, add a 10% waste margin: 7.41 × 1.10 = 8.15 cubic yards. (You would typically order 8.25 or 8.5 yards from your ready-mix supplier).
Understanding the Results
Our tool functions as a complete concrete slab estimator by providing multiple formats for your answer:
- Cubic Yards: The standard measurement used by ready-mix delivery companies. A standard mixer truck holds about 9 to 11 cubic yards of concrete.
- Premixed Bags: If you are pouring a small cement pad, buying bags is often cheaper. The calculator outputs bag counts based on standard yields (e.g., an 80lb bag yields ~0.60 cubic feet).
- Estimated Cost: By expanding the advanced settings, you can input your local prices to calculate the concrete pad cost instantly.
Important Variables for Slab Calculation
Concrete Pad Thickness
Choosing the correct concrete pad thickness is critical for structural integrity. For comprehensive structural standards and best practices, you can refer to guidelines published by the American Concrete Institute (ACI):
| Project Type | Recommended Thickness | Load Bearing Capacity |
|---|---|---|
| Sidewalks & Patios | 4 inches | Foot traffic, light patio furniture |
| Standard Driveways | 4 to 5 inches | Passenger cars, light trucks |
| Heavy Duty Garage/Driveway | 6 inches | RVs, large trucks, heavy equipment |
Common Mistakes to Avoid
- Ignoring the Subgrade: A slab calculation assumes your base is perfectly flat. If your gravel base has dips and ruts, you will end up using more concrete than estimated. Always order extra to compensate.
- Forgetting to calculate in feet: A common error when trying to calculate for concrete slab manually is multiplying Feet × Feet × Inches without converting the inches to a decimal foot first.
- Mixing too much by hand: Generally, if your concrete slab calculation requires more than 1 to 1.5 cubic yards (roughly 45 to 68 80lb bags), it is vastly easier and often cheaper to order a short-load delivery truck rather than mixing bags by hand.
Frequently Asked Questions
How much does a 20×20 slab cost?
A standard 20×20 slab poured 4 inches thick requires roughly 5 cubic yards of concrete. At an average ready-mix cost of $130 to $160 per yard, the concrete alone will cost between $650 and $800. Labor, gravel, and forms will increase the total project cost to $1,500 to $3,500 depending on your region.
How do I calculate concrete for a slab?
To calculate concrete for a slab, multiply the length by the width by the thickness to find the volume. Ensure all measurements are in feet (convert inches to feet by dividing by 12) to get total cubic feet. Finally, divide the cubic feet by 27 to find the total cubic yards needed.
How much does a 30x40x4 concrete slab cost?
A 30×40 slab at 4 inches thick requires about 14.8 cubic yards of concrete (with a 10% waste factor, round up to 16 yards). The concrete alone will cost roughly $2,100 to $2,500. Total installed cost usually ranges from $6,000 to $10,000 depending on local labor rates and site prep requirements.
How many bags of concrete do I need for a 4×8 slab?
A 4×8 foot slab poured 4 inches thick equals 10.67 cubic feet. A standard 80lb bag yields about 0.60 cubic feet. Therefore, you will need roughly 18 bags of 80lb concrete to complete this slab.
Do I need gravel under a concrete slab?
Yes, in most cases. A 4 to 6-inch layer of compacted gravel or crushed stone provides a stable, drainable base that prevents the concrete slab from cracking due to frost heave, soil expansion, or water pooling underneath.
Can I mix a 50lb bag of concrete in a 5 gallon bucket?
Yes, you can comfortably mix a 50lb bag in a standard 5-gallon bucket using a heavy-duty drill and a paddle attachment. It is much harder to mix a 60lb or 80lb bag in a bucket due to the volume of dry material and the torque required.
Is it cheaper to buy bags of concrete or mix your own?
For small projects under 1 cubic yard, buying premixed bags is usually cheaper and more practical. For projects requiring over 1.5 yards, ordering ready-mix concrete delivered by a truck is significantly cheaper, faster, and ensures a more consistent pour.
What is poor man’s concrete?
“Poor man’s concrete” usually refers to soil cement or stabilized earth—a mixture of natural subsoil blended with a small amount of Portland cement and water, then compacted. It creates a hard surface but lacks the structural strength and durability of true concrete.
Can I pour concrete straight onto soil?
You can, but it is highly discouraged. Pouring directly onto dirt can lead to uneven settling, cracking, and moisture wicking. Unless it is a very temporary or low-stakes pad (like a small trash can pad in a warm climate), always use a compacted gravel subbase.
What happens if you pour concrete without gravel?
Without a gravel base, the concrete is subject to shifting soil and poor drainage. Water can pool beneath the slab, leading to severe frost heave in winter, rapid cracking, sinking, and a drastically shortened lifespan for the concrete.
Why put sand under concrete?
Sand is sometimes used to create a perfectly flat grading layer over top of coarse gravel, and to protect vapor barriers from being punctured by sharp rocks before the concrete is poured. However, sand alone without a gravel base can wash away easily.
Should I wet dirt before pouring concrete?
Yes, gently misting the subgrade or gravel base before pouring is a best practice. If the ground is bone dry, it will rapidly suck moisture out of the fresh concrete mix, causing premature curing, weakening the concrete, and increasing the risk of shrinkage cracks.
Disclaimer: This calculator provides planning estimates only. Always verify critical quantities, structural requirements, and local building codes before purchasing materials or beginning construction.
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