Concrete Slab Calculator
Calculate the exact concrete volume, bag count, and cost for any slab. Free concrete slab calculator with imperial and metric units, waste factor, and bag-size selection.
Pouring a slab? This concrete slab calculator tells you exactly how much concrete you need: cubic yards, cubic meters, and number of bags, all based on your slab’s length, width, and thickness. Whether you’re estimating a backyard patio, a garage floor, a shed pad, or a driveway section, the math is the same. Length × width × thickness gives you volume, and the calculator handles the unit conversions and bag math for you.
Enter your dimensions below to get an instant estimate. For larger pours, you’ll also see whether ready-mix delivery makes more sense than buying bags.
Pouring a slab? This concrete slab calculator tells you exactly how much concrete you need: cubic yards, cubic meters, and number of bags, all based on your slab’s length, width, and thickness. Whether you’re estimating a backyard patio, a garage floor, a shed pad, or a driveway section, the math is the same. Length × width × thickness gives you volume, and the calculator handles the unit conversions and bag math for you.
Enter your dimensions below to get an instant estimate. For larger pours, you’ll also see whether ready-mix delivery makes more sense than buying bags.
How to Use This Calculator
How to use the concrete slab calculator
The calculator works in two modes: a simple default mode for quick estimates, and an advanced mode for bag count, waste factor, and cost. Here’s how to use both.
Step 1. Enter your slab dimensions
Type the length and width of your slab. You can use any unit (feet, meters, inches, centimeters, or yards) by selecting it from the unit dropdown next to each input. For thickness, the default is inches, which is what most US contractors use. Typical residential slabs are 4 inches for patios and floors, 6 inches for driveways and garages.
Step 2. Set quantity if you’re pouring multiple slabs
If you’re pouring two identical pads (for example, two shed bases), set quantity to 2. The calculator multiplies the volume by your quantity automatically.
Step 3. Open advanced options for bag count and cost
Click “Show advanced options” to access the waste factor slider, bag size selector, and cost-per-bag input. The advanced section is where the calculator becomes a full concrete slab estimator instead of just a volume tool.
Step 4. Adjust the waste factor
The default 10% waste factor accounts for spillage, uneven subgrade, and slight over-pour. Increase to 15% for irregular shapes or DIY pours. Drop to 5% for tightly controlled commercial pours.
Step 5. Pick your bag size
Drag the bag size slider to match what your supplier stocks, anywhere from 20 lb to 100 lb bags. The calculator updates the bag count immediately.
Step 6. Enter cost per bag (optional)
If you want a cost estimate, enter what you’re paying per bag at your supplier. The calculator multiplies bag count by price for an estimated cost. Leave blank if you only need volume.
Step 7. Read the results
Three result cards appear: Concrete Volume (always shown), Bags Required (when advanced is open), and Estimated Cost (only if you entered a price).
How the Calculation Works
The formula behind the concrete slab calculator
Calculating concrete for a slab comes down to one equation:
Volume = Length × Width × Thickness
The catch is that all three measurements need to be in the same unit before you multiply. Since slab length and width are usually in feet but thickness is in inches, you have to convert thickness to feet first by dividing by 12.
Worked formula in feet (Imperial)
Cubic feet = Length(ft) × Width(ft) × (Thickness(in) ÷ 12)
Cubic yards = Cubic feet ÷ 27
Cubic meters = Cubic feet × 0.0283
Why divide by 27 for cubic yards?
One cubic yard equals 3 ft × 3 ft × 3 ft, which is 27 cubic feet. Concrete is sold by the cubic yard in the US, so this conversion is the one you’ll use most often when ordering ready-mix.
Bag count formula
Pre-mixed concrete bags have a known yield (volume per bag):
- 40 lb bag yields approximately 0.30 cubic feet
- 60 lb bag yields approximately 0.45 cubic feet
- 80 lb bag yields approximately 0.60 cubic feet
To get the bag count, divide your total cubic feet by the yield of your chosen bag size and round up. The calculator does this automatically once you select a bag size in the advanced options.
Worked Example
Worked example: 12 ft × 12 ft × 4 in concrete patio slab
Let’s walk through a real example. Say you’re pouring a 12 ft × 12 ft concrete patio at 4 inches thick. Here’s exactly what the concrete slab calculator does behind the scenes.
Step 1. Convert thickness to feet
4 inches ÷ 12 = 0.333 feet
Step 2. Calculate cubic feet
12 ft × 12 ft × 0.333 ft = 48 cubic feet
Step 3. Convert to cubic yards
48 ÷ 27 = 1.78 cubic yards
Step 4. Add waste factor (10%)
1.78 × 1.10 = 1.96 cubic yards. Round up to 2 yards when ordering ready-mix.
Step 5. Bag count for 80 lb bags
48 cubic feet × 1.10 (waste) = 52.8 cubic feet
52.8 ÷ 0.60 (yield per 80 lb bag) = 88 bags
Step 6. Estimated cost at $5.50 per bag
88 bags × $5.50 = $484
Practical takeaway: At 88 bags for a small patio, this is right at the threshold where ready-mix delivery becomes cost-competitive. A 2-yard mini-mix delivery typically runs $300 to $450 in most US markets, plus delivery fee. For pours over 1 cubic yard, get a quote both ways.
Understanding the Variables
Variables explained
Each input in the concrete slab calculator affects the final volume calculation. Understanding what each one means, and how to measure it correctly, is the difference between an accurate estimate and ordering 30% too much concrete.
Length and Width
The two horizontal dimensions of your slab. The calculator accepts feet, meters, inches, centimeters, and yards. Pick whatever unit your tape measure shows from the dropdown next to each field. For rectangular slabs, length is typically the longer side and width is the shorter side, though the calculator doesn’t care which is which because multiplication is commutative. Measure the actual finished slab dimensions, not the form board outside-to-outside.
Thickness
The depth of the concrete pour. Default is inches, with metric (cm), feet, and meters also available. This is the most commonly under-estimated input. Typical residential slab thicknesses:
- 4 inches for patios, sidewalks, light-duty floors
- 5 inches for slabs supporting interior walls or moderate loads
- 6 inches for driveways, garages, sheds with vehicles
- 8 inches and above for heavy equipment pads or structural slabs
Always check local code requirements before pouring. The calculator gives you volume, not engineering specifications.
Quantity
The number of identical slabs you’re pouring. Set this to 1 for a single slab, or higher if you’re pouring multiple pads with the same dimensions (for example, three identical shed bases). The volume is multiplied by quantity, and bag count is also recalculated for the total.
Waste Factor (advanced)
A percentage added to the calculated volume to account for spillage, uneven subgrade, slight over-pour at edges, and concrete that gets stuck in the mixer or wheelbarrow. Industry standard is 10%. Use 15% for DIY or irregular shapes, 5 to 7% for tightly controlled commercial pours with experienced crews.
Bag Size (advanced)
Drag the bag size slider to match what you’re buying. The slider supports any bag weight from 20 lb to 100 lb, in 5 lb increments. The most common US sizes are 40, 60, and 80 lb. For metric bags, 25 kg is roughly 55 lb and 30 kg is about 66 lb.
Cost per Bag (advanced)
The retail price you’re paying per bag at your local supplier. Prices vary regionally and by retailer. Expect $4 to $7 per 60 lb bag and $5 to $9 per 80 lb bag at major retailers. Leave blank if you only need volume. No cost estimate will be shown.
Industry Standards & References
Industry standards referenced
The math in this calculator is based on volumetric geometry, but real-world slab construction is governed by published industry standards. Here are the relevant references for residential and light commercial slabs.
ACI 318: Building Code Requirements for Structural Concrete
The American Concrete Institute’s ACI 318 is the foundational code for concrete construction in the US. While ACI 318 covers structural concrete (not most residential slabs-on-grade), it provides minimum cover, reinforcement, and curing standards that inform residential best practice.
ACI 332: Residential Concrete Code
ACI 332 specifically addresses residential concrete construction, including slab thickness recommendations, joint spacing, and reinforcement requirements for slabs-on-grade.
IRC Section R506: Concrete Floors on Ground
The International Residential Code, Section R506, sets minimum slab thickness at 3.5 inches for residential floors on ground, with vapor retarder, base material, and reinforcement requirements.
ASTM C94: Standard Specification for Ready-Mixed Concrete
For ready-mix orders, ASTM C94 governs how the concrete is batched, delivered, and tested. Reference this when ordering ready-mix to ensure you get specified strength (typically 3,000 to 4,000 psi for residential slabs).
Note: This concrete slab calculator handles volume math only. Reinforcement design, joint placement, vapor barrier specs, and structural requirements are outside the calculator’s scope. Consult ACI 332 or IRC R506, or hire a structural engineer for engineered slabs.
Frequently Asked Questions
At 4 inches thick, about 1.78 cubic yards (close to 2 yards once you add a 10% waste allowance). Enter your exact length, width, and thickness in the calculator above for any size. For a full breakdown across common slab sizes, see our how much concrete do I need guide.
It depends on your slab volume and the bag size you buy. Select a bag size in the calculator above and it returns the exact count. For bag counts by project type, a 40 vs 60 vs 80 lb comparison, and the bag count formula, see our complete guide on how many bags of concrete you need.
Cubic yards equals cubic feet divided by 27. To get cubic feet, multiply length × width × thickness, but make sure all three dimensions are in feet first. Convert thickness from inches to feet by dividing by 12. So a 10 ft × 10 ft × 4 inch slab is 10 × 10 × 0.333 = 33.3 cubic feet, then 33.3 ÷ 27 = 1.23 cubic yards.
4 inches for patios and interior floors, 5 inches for slabs supporting walls, and 6 inches for driveways or shed pads with vehicle traffic. The IRC code minimum is 3.5 inches for floors on ground. For a full breakdown by use case and load, see our concrete slab thickness guide.
For most pours over 1 cubic yard, ready-mix is more economical and far less labor than mixing dozens of bags by hand. Mini-mix and short-load deliveries cover 1 to 3 yard pours in most US markets, often $200 to $450 per yard plus delivery. For the full bags vs ready-mix comparison and exact bag counts, see our how many bags of concrete guide.
Yes. Standard practice is to add 10% to your calculated volume to account for spillage, uneven subgrade, and slight over-pour at slab edges. For DIY pours or irregular shapes, increase to 15%. For tightly controlled commercial pours, you can drop to 5 to 7%. The calculator's advanced mode lets you set this directly.
Yes. A concrete pad and a concrete slab use the same volume calculation: length × width × thickness. The calculator works equally well as a concrete pad calculator for shed bases, AC unit pads, hot tub pads, or patios. Just enter the pad dimensions and the math is identical.
The volume calculation is mathematically exact (within rounding). Real-world accuracy depends on how carefully you measured your slab dimensions and how level your subgrade is. A slab that's supposed to be 4 inches thick but averages 4.5 inches because of an uneven base will need about 12% more concrete than calculated. That's why the 10% waste factor is standard practice.