Measure the longest dimension of your slab.Please enter a valid length greater than 0.
The shorter dimension of your slab.Please enter a valid width greater than 0.
Standard residential slab: 4 inches. Driveways: 6 inches. Heavy load: 8 inches.
Please enter a valid thickness greater than 0.
Add 5–10% for standard jobs. Add 10–15% for complex shapes or first-time pours.
$
Leave blank to skip cost estimate. US average: $100–$150/yd³ for ready-mix.
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Your Concrete Estimate
Concrete Volume (with waste)
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Cubic Yards (yd³)
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Cubic Feet (ft³)
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Cubic Meters (m³)
Bags Required (includes waste)
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40 lb bags
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60 lb bags
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80 lb bags
—Area (sq ft)
—Area (m²)
—Thickness
—Waste Factor
Estimated Material Cost
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Concrete material cost only. Add labor ($2–$5/ft²), delivery ($100–$300), forming, and finishing for a full project budget. Use our Full Project Estimator for a complete breakdown.
Step 1: Convert all dimensions to feet
Step 2: Volume (ft³) = Length (ft) × Width (ft) × Thickness (ft)
Step 3: Cubic Yards = ft³ ÷ 27
Step 4: Final Volume = Volume × (1 + waste% ÷ 100)
Step 5: Bags = CEIL(Final ft³ ÷ bag yield) — never round down
Measure your slab dimensions.
Use a tape measure to get the length and width of the area you plan to pour. For irregular shapes, break the area into rectangles and run separate calculations. Record your measurements in whatever unit is most convenient — the calculator handles the conversion.
Select your units and enter values.
Choose feet, inches, meters, or centimeters from the dropdown next to each field. Use the quick-select buttons — [4 in], [6 in], [8 in] — to fill in the most common slab thicknesses without typing. Make sure each field uses the correct unit before calculating.
Set a waste factor.
The default is 10%, which is appropriate for most straightforward rectangular slabs. Increase to 15% for complex shapes, curved edges, or if this is your first time pouring. Never reduce below 5% — concrete plants often require a minimum order and you cannot top up a pour mid-job without creating a structural cold joint.
Use your results to order materials.
The cubic yards figure is what you give the ready-mix supplier when ordering by truck. The bag counts tell you how many bags to buy at a hardware store. If you entered a price per cubic yard, the cost estimate shows your material-only budget. Always confirm final quantities with your concrete supplier.
⚠ Pro Tip: Always order a minimum of 10% extra concrete. Running short mid-pour forces a cold joint — a structural weak point that can crack within a year. The small cost of extra concrete is nothing compared to cutting out and repouring a section of failed slab.
Concrete Slab Volume Formula
The calculation follows the standard volumetric formula used across the industry and aligned with ACI 318 guidelines. Here's the process step by step:
Step
Formula
Example (10 × 10 ft, 4 in)
1. Convert thickness to feet
inches ÷ 12
4 ÷ 12 = 0.333 ft
2. Volume in cubic feet
L × W × T
10 × 10 × 0.333 = 33.33 ft³
3. Convert to cubic yards
ft³ ÷ 27
33.33 ÷ 27 = 1.235 yd³
4. Add waste factor (10%)
Volume × 1.10
1.235 × 1.10 = 1.358 yd³
Common Slab Size Reference Table
Concrete volumes and bag counts — no waste factor applied. Add 10% for real-world ordering.
Slab Size
Thickness
Cubic Yards
60 lb Bags
80 lb Bags
10 × 10 ft
4 in
1.24 yd³
92 bags
69 bags
12 × 12 ft
4 in
1.78 yd³
132 bags
99 bags
20 × 20 ft
4 in
4.94 yd³
366 bags
274 bags
10 × 10 ft
6 in
1.85 yd³
137 bags
103 bags
24 × 24 ft
6 in
10.67 yd³
790 bags
592 bags
30 × 30 ft
6 in
16.67 yd³
1,234 bags
926 bags
10 × 10 ft
8 in
2.47 yd³
183 bags
137 bags
Bag counts assume no waste factor. Add 10% for real-world ordering.
What Thickness Does My Concrete Slab Need?
Slab thickness is one of the most important decisions you'll make — and one of the most commonly underestimated. Thicker slabs handle loads better, resist freeze-thaw cycles, and last longer. The table below reflects industry-standard recommendations.
Recommended concrete slab thickness by application type.
Slab Type
Recommended Thickness
PSI Strength
Notes
Sidewalk / Walkway
4 inches
3,000 PSI
Standard foot traffic
Residential Patio
4 inches
3,000 PSI
Add wire mesh or rebar
Residential Driveway
6 inches
3,500 PSI
Cars and light trucks
Commercial Driveway
6–8 inches
4,000 PSI
SUVs, delivery trucks
Garage Floor
6 inches
3,500 PSI
Rebar recommended
RV / Heavy Vehicle Pad
8 inches
4,000 PSI
Thicken edges to 12 in
Structural Floor Slab
8–12 inches
4,500+ PSI
Engineer approval required
When in doubt, go thicker. The cost difference between a 4-inch and 6-inch slab is minor — typically $1–2 per square foot in materials. The cost of cutting out, removing, and repouring a cracked slab is not.
Common Mistakes When Ordering Concrete
⚠️
Not adding a waste factor.
Ordering exactly what your calculations show is a rookie mistake. Spills happen, subgrade is rarely perfectly flat, and ready-mix plants have minimum delivery charges. Always order at least 10% more than your calculated volume — it's cheap insurance.
📐
Using exterior dimensions instead of the actual pour area.
If your slab sits inside a wood form, measure the inside of the form — not the outside of the lumber. Using exterior dimensions adds fractions of an inch that compound into significant over-ordering on large pours.
📦
Mixing up cubic feet and cubic yards.
Ready-mix is priced and delivered by the cubic yard. Bagged concrete yields are listed in cubic feet. Confusing these two units is the most common ordering error — 1 cubic yard is 27 cubic feet, not 3. Use this calculator to avoid the confusion entirely.
🔩
Forgetting to account for rebar or wire mesh displacement.
Steel reinforcement displaces a small volume of concrete. For most residential slabs with #4 rebar on 12-inch spacing, the displacement is under 1% — generally within your waste factor. For heavily reinforced structural slabs, consult an engineer for adjusted volumes.
📅
Ordering on a tight timeline.
Ready-mix concrete plants often need 24–48 hours notice for standard orders. During peak season (spring and summer), lead times can stretch to several days. Lock in your delivery date before your forms are built, not after.
Frequently Asked Questions
Multiply the slab's length by width by thickness — all in feet — to get cubic feet. Divide by 27 to convert to cubic yards (there are 27 cubic feet per cubic yard). Then multiply by 1.10 to add a 10% waste factor. For example, a 10 ft × 10 ft slab at 4 inches (0.333 ft) thick = 33.3 ft³ ÷ 27 = 1.23 yd³. With 10% waste, you'd order 1.36 yd³. This calculator does all of that instantly.
A 10 ft × 10 ft slab at 4 inches thick requires about 33.3 cubic feet of concrete before waste. With a 10% waste factor, you need approximately 36.7 ft³ total. That works out to 82 bags of 80 lb concrete (0.60 ft³ yield each) or 82 bags of 60 lb (0.45 ft³ yield each, so 82 bags). Use the calculator above for the exact figures. Most experienced contractors recommend ready-mix delivery for any job over 1 cubic yard — mixing that many bags by hand or in a rented mixer is exhausting and can lead to inconsistent mix quality.
Ready-mix concrete arrives in a truck fully mixed and ready to pour. You specify the volume in cubic yards, the PSI strength, and delivery time. Bagged concrete (Quikrete, Sakrete, etc.) is a dry mix you blend with water yourself, one bag at a time. Ready-mix is more efficient and consistent for jobs over 1 cubic yard. Bagged concrete is practical for small repairs, post holes, or pours under half a yard where a truck delivery isn't economical. The cost per cubic yard of bagged concrete is typically 2–3x more than ready-mix once you factor in bag costs and your time.
80 lb bags yield more concrete per bag (0.60 ft³ vs 0.45 ft³ for a 60 lb bag), so you need fewer bags and spend less money overall. However, 80 lb bags are heavy — too heavy for many people to safely carry and maneuver repeatedly. If you're working alone or on a raised site, 60 lb bags are far more manageable. For large jobs, 40 lb bags exist but are rarely economical. Most contractors use 80 lb bags with a helper; most homeowners prefer 60 lb bags.
Nationally, professionally installed concrete slabs average $6–$12 per square foot in the USA, covering concrete materials, labor, forming, and basic finishing. A plain 4-inch residential driveway or patio typically lands at $6–$8 per square foot. Decorative finishes like exposed aggregate, broom finish, or stamped concrete add $3–$10 per square foot on top. Geographic location has a major impact — labor rates in California or New York can double the cost compared to rural Midwest markets. Always get at least 3 competitive quotes from licensed local contractors.
A cold joint forms when fresh concrete is poured against concrete that has already begun to set. The two layers don't bond properly — the surface between them is essentially a seam with reduced tensile and shear strength. Cold joints are a leading cause of slab cracking and are almost always the result of running short on concrete mid-pour and having to wait for a second delivery. This is why ordering 10% extra is non-negotiable: the cost of the extra concrete is trivial compared to cutting out and repouring a failed section.
Technically yes, but it's almost always a mistake. Pouring directly on loose, uncompacted, or organic-rich soil leads to uneven settling, cracking, and heaving — especially in freeze-thaw climates. Best practice is to excavate to stable subgrade, compact thoroughly with a plate compactor, and add a 4-inch layer of compacted gravel base (crushed stone or road base). The gravel improves drainage under the slab and provides a stable, uniform bearing surface. Skipping the gravel base is the #1 reason residential slabs fail prematurely.
Concrete reaches initial set in 6–8 hours and is typically safe to walk on after 24–48 hours. However, it reaches 70% of its design strength at 7 days and full strength at 28 days. Avoid parking vehicles on a fresh slab for at least 7 days, and avoid heavy loads for 28 days. Curing is the process of keeping the slab moist and at a stable temperature to allow proper hydration — covering with plastic sheeting or applying a curing compound after finishing is standard practice and significantly improves final strength.
It depends on your municipality and the slab's purpose. Small decorative patios under a certain square footage (often 200 sq ft) typically don't require permits in most US jurisdictions. Driveways, garage floors, and any slab attached to a structure almost always require a permit. Slabs in flood zones or with specific drainage implications may have additional requirements. Always check with your local building department before breaking ground — working without a permit when one is required can result in fines and mandatory removal.
The minimum recommended compressive strength for a residential driveway is 3,500 PSI. In freeze-thaw climates (most of the northern USA, Canada, and UK), specify at least 4,000 PSI with air entrainment (typically 5–7% air) — entrained air creates microscopic bubbles that give the concrete room to expand during freezing without cracking the surface. Using 3,000 PSI or lower on a driveway is a false economy that typically results in scaling and surface deterioration within 5–10 years.
