30x40 Concrete Slab Calculator
Calculated ready-mix cubic yards, 80lb/60lb pre-mixed bags, rebar grid, and base gravel for a 40 foot by 30 foot concrete slab (1200 square feet) with standard ASTM C94 waste buffers.
Live Interactive 30x40 Concrete Slab Calculator
Adjust depth slider below or switch to metric units to see instant volume updates.
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: 40 ft
Width: 30 ft
Depth: 4 inches
Surface Area: 1200 sq ft
Ready-Mix Order Quantities
To Order: 16.50 yd³ (12.62 m³)
Net Volume: 16.30 yd³
Waste Factor: 10% (ASTM C94 ground allowance)
If Buying Bags Instead
80 lb Bags: 734 bags
60 lb Bags: 978 bags
Total Cured Weight: 66,000 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)
30x40 Concrete Slab Concrete Requirements by Thickness
Pre-calculated volume, bags, and weight including 10% ASTM C94 waste factor.
| Thickness | Application | Ready-Mix Order (yd³) | 80 lb Bags | 60 lb Bags | Cured Weight |
|---|---|---|---|---|---|
| 4 Inches | Patio, Walkway, Shed | 16.50 yd³ | 734 bags | 978 bags | 66,000 lbs |
| 5 Inches | Driveway, Carport | 20.50 yd³ | 917 bags | 1223 bags | 82,500 lbs |
| 6 Inches | Heavy RV, Workshop | 24.50 yd³ | 1100 bags | 1467 bags | 99,000 lbs |
Job-Site Materials Checklist for a 30x40 Concrete Slab
1. Gravel Sub-Base
4 inches of compacted #57 crushed limestone or road base:
17.0 cu yds (≈ 23 tons)
Tamp with a mechanical plate compactor to eliminate settling voids.
2. Steel Reinforcement
#4 Rebar (1/2" dia.) placed in an 18" on-center grid:
1821 linear feet (≈ sticks)
Elevate on 2" plastic rebar chairs so steel floats in the middle third.
3. Contraction Joint Layout
ACI 302.1R maximum spacing rules:
Joints cut into 10 ft × 10 ft bays within 12 hours of pouring
Cut joints within 4 to 12 hours of pouring to 1/4 the slab depth.
Structural Slab Cross-Section & Dimensions
Step-by-Step Calculation: 40' × 30' Concrete Slab (4" Depth)
Suppose you are planning to pour a backyard slab measuring 40 feet long by 30 feet wide with a standard residential depth of 4 inches. Here is the verified 5-step arithmetic:
4" ÷ 12 = 0.333 feet
40' × 30' × 0.333' = 400.00 cubic feet
400.00 cu ft ÷ 27 = 14.815 cubic yards (exact net)
14.815 yd³ × 1.10 = 16.30 cubic yards with waste
- Ready-Mix Delivery: Order 16.50 yd³ (rounded up to nearest 0.25 yd³ increment).
- Pre-mixed 80 lb Bags: Purchase 734 bags (0.60 cu ft yield per bag).
- Pre-mixed 60 lb Bags: Purchase 978 bags (0.45 cu ft yield per bag).
Explore Other Common Slab Sizes
Select another dimension to view pre-calculated materials and truck orders:
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.
Frequently Asked Questions: 30x40 Concrete Slab
Common contractor questions, bag ratios, thickness standards, and curing guidance.
How many cubic yards of concrete do I need for a 30x40 Concrete Slab at 4 inches thick? ▼
A 30x40 Concrete Slab (4" thick) has a net volume of 14.81 cubic yards. With the standard 10% ASTM C94 waste factor for uneven subgrade and form bulging, you should order 16.50 cubic yards of ready-mix concrete.
How many 80 lb bags of concrete do I need for a 30x40 Concrete Slab? ▼
For a 4-inch deep 30x40 Concrete Slab, you will need 734 bags of 80 lb pre-mixed concrete (or 978 bags of 60 lb mix). If pouring at 6 inches thick, you will need 1100 bags of 80 lb mix.
Should a 30x40 Concrete Slab be 4 inches or 6 inches thick? ▼
Pour at 4 inches thick if the slab is intended for pedestrian use, outdoor patios, hot tubs, or light storage sheds. Pour at 5 to 6 inches thick if it will park passenger vehicles, trucks, or support heavy workshop machinery.
Where should control joints be cut on a 30x40 Concrete Slab? ▼
Per ACI 302.1R rules (joint spacing in feet = 2 to 3 times thickness in inches), control joints should be cut at Joints cut into 10 ft × 10 ft bays within 12 hours of pouring. Joints must be cut to 1/4 the depth of the slab.