Concrete Slab Calculator
Planning a patio, driveway, or garage floor? This concrete slab calculator helps you estimate how much concrete to order based on your measurements, whether you need cubic yards for a ready-mix truck or bag counts for hand mixing.
Last reviewed: June 2026
Standard patio/sidewalk is 4".
10% is recommended for uneven subgrades.
Results update automatically as you enter measurements.
Your Material List
For this project, buy or order approximately
Use these quantities as a shopping list, supplier note, quote check, or quick jobsite material estimate.
Assumes ~0.60 cu ft yield per bag.
Assumes ~0.45 cu ft yield per bag.
Assumes ~0.30 cu ft yield per bag.
How to Use This Calculator
- Measure Length and Width: Measure the layout of your subgrade form. Measurements must be in feet.
- Measure Thickness: The standard thickness for a patio or sidewalk is 4 inches. Enter your slab depth in inches.
- Set Waste Percentage: Usually kept at 10%. Variations in bottom depth and side forms can quickly use up extra concrete.
- Read Output: The calculator returns total cubic yards for a truck delivery and approximate bag counts if you are hand-mixing.
Formula and Assumptions
Formula used
Slab volume in cubic feet = length × width × thickness in feet. Cubic yards = cubic feet ÷ 27. Bag counts are estimated by dividing total cubic feet by the assumed yield for each bag size, then rounding up.
Unit conversions
Inches are converted to feet before volume is calculated. Cubic feet are converted to cubic yards by dividing by 27. Bag counts are rounded up because stores do not sell partial bags.
Waste factor explanation
Waste factor helps account for uneven surfaces, cuts, spills, compaction, settling, and measurement differences. The right buffer depends on your project and material.
Material Assumptions and Disclaimer
Bag yield or density assumptions
- Subgrade is reasonably leveled and compacted.
- Forms are close to the dimensions specified.
- A waste factor accounts for spillage, uneven subgrade, and mixing loss.
Estimate disclaimer
These tools are useful for DIY planning and quick jobsite estimates, but the results are still estimates. Verify quantities for structural work, code requirements, supplier material specs, product labels, or professional requirements before purchasing or quoting a job.
When to be careful
- Concrete weighs about 150 lbs per cubic foot. Ensure your vehicle can safely transport the bags.
- Estimates are mathematical. Ordering slightly more ensures you do not run dry during a pour.
- Properly compact your subgrade and use a gravel base to prevent severe cracking over time.
How Much Waste Factor to Add
No subgrade is magically flat. Over-digging your base or setting forms imperfectly means you will likely use more concrete than the exact mathematical volume.
- Standard pours: Always add 10% extra. It is vastly cheaper than having a truck wait or running short mid-pour.
- Irregular bases (like heavy aggregate): Consider raising your waste to 15%.
- Perfectly boxed deck footings: You can safely drop down to 5%.
Bagged Concrete vs. Ready-Mix Truck
Running out of concrete while the edge is curing creates a structural "cold joint," ruining the surface.
- Use Bagged Concrete when the job requires less than a yard. Anything under 30 bags (roughly 0.70 cubic yards) can reasonably be hand-mixed in a wheelbarrow or portable mixer.
- Use Ready-Mix Trucks for anything over one cubic yard. Hand-mixing 50+ bags of concrete before it cures is physically exhausting and frequently ruins the final finish because the timing is off. A short-load fee from a ready-mix company pays for itself in saved labor and a better finish.
Common DIY Mistakes
- Forgetting the base layer: Slabs poured straight into dirt will crack when the ground freezes and thaws. You need a compacted 4" crushed gravel base.
- Buying only the raw math amount: The math assumes a perfectly flat, geometrically clean form. You will usually spill some material, and forms are rarely perfect.
- Underestimating the weight: A single pallet of 80-lb bags weighs over 3,000 lbs. If your half-ton pickup tries to carry that, you'll ruin your suspension. Take two trips or get it delivered.
When to Call a Professional
Pouring concrete is permanent. Consider hiring a pro if:
- You are pouring a driveway for heavy RVs (which needs an engineered subbase, thickened edges, and rebar).
- You are pouring an attached garage floor where sloping correctly away from the house structure is critical.
- You live somewhere with severe freeze-thaw cycles and require a specialized air-entrained mix.
Example Calculation
For a 10ft x 10ft patio that is 4 inches thick. Using a 10% waste factor:
- Convert thickness to feet: 4 ÷ 12 = 0.333 feet
- Calculate cubic feet: 10 × 10 × 0.333 = 33.33 ft^3
- Add 10% waste: 33.33 × 1.10 = 36.67 ft^3 total
- Convert to cubic yards: 36.67 ÷ 27 = 1.36 yd^3
- Calculate 80-lb bags using 0.60 ft^3 yield: 36.67 ÷ 0.60 = 61.12, then round up to 62 bags
You need 1.36 cubic yards of concrete, or 62 80-lb bags.
Frequently Asked Questions
How thick should a concrete slab be?
For a patio or walkway, 4 inches is standard. For a driveway carrying passenger vehicles, 4 to 6 inches is expected. Heavy equipment or RVs generally require 6 to 8 inches of thickness with reinforcement.
Why do I need a waste factor?
Subgrades are rarely perfectly flat. Variations in depth, forms bulging, and spillage during mixing mean you will almost always use more concrete than the exact mathematical volume. 10% is standard practice.
When should I order a delivery truck vs mixing bags?
The general rule of thumb is that any job requiring more than 1 cubic yard (about forty-five 80-lb bags) is physically demanding and usually cheaper/faster to do with a ready-mix truck delivery.
How many 80lb bags are on a pallet?
Typically, a pallet contains 42 80-lb bags (3,360 lbs total). Be sure your vehicle or trailer can handle this extreme weight before picking it up.
Do I need rebar or wire mesh?
For standard 4-inch patios or walkways, wire mesh or fiber-reinforced concrete is often sufficient. If the slab will bear heavy loads or is in an area with freeze-thaw cycles, #3 or #4 rebar on chairs is highly recommended.
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