20x20 Concrete Slab Calculator
Calculated ready-mix cubic yards, 80lb/60lb pre-mixed bags, rebar grid, and base gravel for a 20 foot by 20 foot concrete slab (400 square feet) with standard ASTM C94 waste buffers.
Live Interactive 20x20 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: 20 ft
Width: 20 ft
Depth: 4 inches
Surface Area: 400 sq ft
Ready-Mix Order Quantities
To Order: 5.50 yd³ (4.21 m³)
Net Volume: 5.43 yd³
Waste Factor: 10% (ASTM C94 ground allowance)
If Buying Bags Instead
80 lb Bags: 245 bags
60 lb Bags: 326 bags
Total Cured Weight: 22,000 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)
20x20 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 | 5.50 yd³ | 245 bags | 326 bags | 22,000 lbs |
| 5 Inches | Driveway, Carport | 7.00 yd³ | 306 bags | 408 bags | 27,500 lbs |
| 6 Inches | Heavy RV, Workshop | 8.25 yd³ | 367 bags | 489 bags | 33,000 lbs |
Job-Site Materials Checklist for a 20x20 Concrete Slab
1. Gravel Sub-Base
4 inches of compacted #57 crushed limestone or road base:
5.7 cu yds (≈ 8 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:
601 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:
Control joint grid cut into 10 ft × 10 ft panels
Cut joints within 4 to 12 hours of pouring to 1/4 the slab depth.
Structural Slab Cross-Section & Dimensions
Step-by-Step Calculation: 20' × 20' Concrete Slab (4" Depth)
Suppose you are planning to pour a backyard slab measuring 20 feet long by 20 feet wide with a standard residential depth of 4 inches. Here is the verified 5-step arithmetic:
4" ÷ 12 = 0.333 feet
20' × 20' × 0.333' = 133.33 cubic feet
133.33 cu ft ÷ 27 = 4.938 cubic yards (exact net)
4.938 yd³ × 1.10 = 5.43 cubic yards with waste
- Ready-Mix Delivery: Order 5.50 yd³ (rounded up to nearest 0.25 yd³ increment).
- Pre-mixed 80 lb Bags: Purchase 245 bags (0.60 cu ft yield per bag).
- Pre-mixed 60 lb Bags: Purchase 326 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: 20x20 Concrete Slab
Common contractor questions, bag ratios, thickness standards, and curing guidance.
How many cubic yards of concrete do I need for a 20x20 Concrete Slab at 4 inches thick? ▼
A 20x20 Concrete Slab (4" thick) has a net volume of 4.94 cubic yards. With the standard 10% ASTM C94 waste factor for uneven subgrade and form bulging, you should order 5.50 cubic yards of ready-mix concrete.
How many 80 lb bags of concrete do I need for a 20x20 Concrete Slab? ▼
For a 4-inch deep 20x20 Concrete Slab, you will need 245 bags of 80 lb pre-mixed concrete (or 326 bags of 60 lb mix). If pouring at 6 inches thick, you will need 367 bags of 80 lb mix.
Should a 20x20 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 20x20 Concrete Slab? ▼
Per ACI 302.1R rules (joint spacing in feet = 2 to 3 times thickness in inches), control joints should be cut at Control joint grid cut into 10 ft × 10 ft panels. Joints must be cut to 1/4 the depth of the slab.