Concrete Slab Calculator
Accurately estimate ready-mix cubic yards, pre-mixed bag quantities (80 lb, 60 lb), rebar requirements, and cured weight for concrete slabs, patios, walkways, garage floors, and shed pads.
Rectangular Slab & Pad Estimator
Enter slab dimensions in feet and thickness in inches to get instant ready-mix order volume.
Pre-mixed Concrete Bags Includes 10% waste
Projects over 2 cu yds are almost always cheaper with ready-mix.
THE CONCRETE CALCULATOR – DISPATCH ORDER TICKET
Standards-Grounded ASTM C94 & ACI 318 Job-Site Summary
Project Dimensions
Length: 12 ft
Width: 12 ft
Depth: 4 inches
Surface Area: 144 sq ft
Ready-Mix Order Quantities
To Order: 2.00 yd³ (1.53 m³)
Net Volume: 1.96 yd³
Waste Factor: 10% (ASTM C94 ground allowance)
If Buying Bags Instead
80 lb Bags: 89 bags
60 lb Bags: 118 bags
Total Cured Weight: 7,920 lbs
Job-Site Placement Notes
• Recommended Slump: 3 to 5 inches (ASTM C143)
• Reinforcement: #4 Rebar @ 18" OC on 2" chairs
• Sub-Base: 4" compacted crushed gravel
• Discharge Limit: 90 mins / 300 drum revs (ASTM C94)
Structural Slab Cross-Section & Dimensions
Slab Depth Specifications by Project Type
Concrete thickness dictates structural load capacity. Never pour a vehicle slab at walkway thickness.
Backyard Patios & Sidewalks
4" DepthEngineered for foot traffic, patio furniture, grills, and small lawn equipment. Requires 4 inches of compacted gravel sub-base and wire mesh or #3 rebar at 24" spacing.
Driveways & Garage Slabs
5" to 6" DepthEngineered for passenger cars, SUVs, and delivery trucks. 5 inches is standard for passenger cars; pour 6 inches for RVs or heavy pickup trucks. Requires #4 rebar on 18" centers.
Common Concrete Slab Dimensions & Ready-Mix Orders
Includes standard 10% safety margin for subgrade unevenness.
| Slab Size (Ft) | Depth | Surface Area | Ready-Mix Order (Yds) | 80 lb Bags | 60 lb Bags |
|---|---|---|---|---|---|
| 4' × 4' | 4" | 16 sq ft | 0.25 yd³ | 10 bags | 14 bags |
| 4' × 4' | 6" | 16 sq ft | 0.50 yd³ | 15 bags | 20 bags |
| 6' × 6' | 4" | 36 sq ft | 0.50 yd³ | 23 bags | 30 bags |
| 6' × 6' | 6" | 36 sq ft | 0.75 yd³ | 33 bags | 44 bags |
| 8' × 8' | 4" | 64 sq ft | 1.00 yd³ | 40 bags | 53 bags |
| 8' × 8' | 6" | 64 sq ft | 1.50 yd³ | 59 bags | 79 bags |
| 10' × 10' | 4" | 100 sq ft | 1.50 yd³ | 62 bags | 82 bags |
| 10' × 10' | 6" | 100 sq ft | 2.25 yd³ | 92 bags | 123 bags |
| 10' × 12' | 4" | 120 sq ft | 1.75 yd³ | 74 bags | 98 bags |
| 10' × 12' | 6" | 120 sq ft | 2.50 yd³ | 110 bags | 147 bags |
| 12' × 12' | 4" | 144 sq ft | 2.00 yd³ | 89 bags | 118 bags |
| 12' × 12' | 6" | 144 sq ft | 3.00 yd³ | 132 bags | 176 bags |
| 12' × 16' | 4" | 192 sq ft | 2.75 yd³ | 118 bags | 157 bags |
| 12' × 16' | 6" | 192 sq ft | 4.00 yd³ | 177 bags | 235 bags |
| 12' × 20' | 4" | 240 sq ft | 3.50 yd³ | 147 bags | 196 bags |
| 12' × 20' | 6" | 240 sq ft | 5.00 yd³ | 220 bags | 294 bags |
| 14' × 14' | 4" | 196 sq ft | 2.75 yd³ | 120 bags | 160 bags |
| 14' × 14' | 6" | 196 sq ft | 4.00 yd³ | 180 bags | 240 bags |
| 16' × 16' | 4" | 256 sq ft | 3.50 yd³ | 157 bags | 209 bags |
| 16' × 16' | 6" | 256 sq ft | 5.25 yd³ | 235 bags | 313 bags |
| 20' × 20' | 4" | 400 sq ft | 5.50 yd³ | 245 bags | 326 bags |
| 20' × 20' | 6" | 400 sq ft | 8.25 yd³ | 367 bags | 489 bags |
| 24' × 24' | 4" | 576 sq ft | 8.00 yd³ | 353 bags | 470 bags |
| 24' × 24' | 6" | 576 sq ft | 11.75 yd³ | 528 bags | 704 bags |
Worked Example: 12' × 16' Shed Foundation Slab (4 Inches Thick)
Suppose you are planning to pour a backyard slab measuring 12 feet long by 16 feet wide with a standard residential depth of 4 inches. Here is the verified 5-step arithmetic:
4" ÷ 12 = 0.333 feet
12' × 16' × 0.333' = 64.00 cubic feet
64.00 cu ft ÷ 27 = 2.370 cubic yards (exact net)
2.370 yd³ × 1.10 = 2.61 cubic yards with waste
- Ready-Mix Delivery: Order 2.75 yd³ (rounded up to nearest 0.25 yd³ increment).
- Pre-mixed 80 lb Bags: Purchase 118 bags (0.60 cu ft yield per bag).
- Pre-mixed 60 lb Bags: Purchase 157 bags (0.45 cu ft yield per bag).
How a Professional Concrete Pour Works: 4-Step Field Checklist
Proper estimating is only half the battle. Here is what happens from site excavation to final broom finish on a real job site.
Excavation & Base
Excavate 8 inches total depth: 4 inches for a compacted gravel base (crushed limestone) and 4 inches for concrete. Tamping the base prevents settling cracks.
Formwork & Rebar
Construct 2x4 forms with stakes every 3 feet. Elevate #4 rebar on plastic "chairs" so the steel sits in the center of the slab rather than on the dirt.
Pour & Consolidate
Deposit concrete starting at the farthest corner. Tap form sides with a hammer to eliminate air pockets and consolidate the mix around edge forms.
Screed, Float & Cure
Screed flat with a straight 2x4, run a bull float to bring paste to the surface, apply a slip-resistant broom finish, and keep moist for 7 days to reach full PSI strength.
Calculation Methodology & Estimating Formulas
Standards-compliant math based on ASTM C94 and American Concrete Institute (ACI 318) guidelines.
1. The Core Volume Formula
Concrete volume is measured in three dimensions and converted to cubic yards (1 cubic yard = 27 cubic feet):
Volume (yd³) = [Length (ft) × Width (ft) × (Thickness (in) ÷ 12)] ÷ 27
Cylinder / Pier / Sonotube:
Volume (yd³) = [π × (Diameter (in) ÷ 24)² × Depth (ft)] ÷ 27
2. Why We Add a 10% Waste Margin
Ordering the exact geometric volume almost always results in a short pour. A minimum 10% waste buffer is critical for three physical reasons:
- Subgrade Unevenness: Just 1/4 inch of extra grade depth across a 400 sq ft slab requires an additional 0.31 cubic yards.
- Form Deflection: Wooden 2×4 or 2×6 forms bulge outward under 150 lb/cu ft liquid concrete pressure.
- Equipment Residuals: Concrete sticks to truck chutes, pump hoses, wheelbarrows, and screed boards.
3. Bagged Concrete Yield Constants
Pre-mixed bagged concrete (e.g., Quikrete, Sakrete) is pre-measured with Portland cement, sand, and gravel. Standard yields:
| Bag Size | Cubic Feet Yield | Bags Per 1 yd³ |
|---|---|---|
| 80 lb bag | 0.60 cu ft | 45 bags |
| 60 lb bag | 0.45 cu ft | 60 bags |
| 50 lb bag | 0.375 cu ft | 72 bags |
| 40 lb bag | 0.30 cu ft | 90 bags |
4. Structural Weight & Load Assumptions
Cured normal-weight structural concrete is modeled at 150 pounds per cubic foot (ASTM C94 benchmark). This means:
- 1 cubic yard weighs approximately 4,050 pounds (2.025 US tons).
- For sub-base preparation, always lay 4 inches of compacted crushed gravel (dense grade aggregate) under slabs to prevent frost heave and settlement cracking.
Written & Reviewed by Maddy
✓ ACI & ASTM VerifiedConstruction Estimating Specialist & Civil Calculation Modeler
Governing Industry Standards: This calculator executes mathematical models grounded strictly in ASTM C94 (Standard Specification for Ready-Mixed Concrete), ACI 318-19 (Building Code Requirements for Structural Concrete), and NRMCA volumetric batch tolerances.
Need to verify our references and derivation? Explore our dedicated Engineering Sources & Specifications directory or read our Calculation Formulas guide.
Concrete Slab Frequently Asked Questions
Common contractor questions, bag ratios, thickness standards, and curing guidance.
How thick should a concrete slab be? ▼
A standard concrete slab for residential patios, sidewalks, and shed foundations should be at least 4 inches thick. For residential driveways supporting passenger vehicles, 5 inches is recommended. For heavy-duty driveways supporting RVs, boats, or delivery trucks, pour 6 inches with a #4 rebar grid spaced at 18 inches.
How much concrete do I need for a 12x12 slab at 4 inches thick? ▼
A 12x12 foot slab at 4 inches depth has a volume of 48 cubic feet (1.78 cubic yards net). Adding a standard 10% safety margin gives 1.96 cubic yards. You should order 2.0 cubic yards of ready-mix concrete, or purchase 88 bags of 80 lb concrete (or 118 bags of 60 lb concrete).
Do I need gravel under my concrete slab? ▼
Yes, you should always lay at least 4 inches of well-compacted crushed gravel (such as crushed limestone or crushed stone base) under a slab. The gravel sub-base provides uniform structural support, allows stormwater drainage, and prevents frost-heave and settling cracks.
How far apart should control joints be placed in a slab? ▼
As a rule of thumb from the American Concrete Institute (ACI), contraction joints should be spaced at a distance in feet equal to 2 to 3 times the slab thickness in inches. For a 4-inch slab, space joints every 8 to 10 feet in both directions.
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