16x20 Concrete Slab Calculator
Calculated ready-mix cubic yards, 80lb/60lb pre-mixed bags, rebar grid, and base gravel for a 20 foot by 16 foot concrete slab (320 square feet) with standard ASTM C94 waste buffers.
Live Interactive 16x20 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: 16 ft
Depth: 4 inches
Surface Area: 320 sq ft
Ready-Mix Order Quantities
To Order: 4.50 yd³ (3.44 m³)
Net Volume: 4.35 yd³
Waste Factor: 10% (ASTM C94 ground allowance)
If Buying Bags Instead
80 lb Bags: 196 bags
60 lb Bags: 261 bags
Total Cured Weight: 17,600 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)
16x20 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 | 4.50 yd³ | 196 bags | 261 bags | 17,600 lbs |
| 5 Inches | Driveway, Carport | 5.50 yd³ | 245 bags | 326 bags | 22,000 lbs |
| 6 Inches | Heavy RV, Workshop | 6.75 yd³ | 294 bags | 392 bags | 26,400 lbs |
Job-Site Materials Checklist for a 16x20 Concrete Slab
1. Gravel Sub-Base
4 inches of compacted #57 crushed limestone or road base:
4.5 cu yds (≈ 7 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:
496 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 at 8 ft and 10 ft grid intervals
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' × 16' Concrete Slab (4" Depth)
Suppose you are planning to pour a backyard slab measuring 20 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
20' × 16' × 0.333' = 106.67 cubic feet
106.67 cu ft ÷ 27 = 3.951 cubic yards (exact net)
3.951 yd³ × 1.10 = 4.35 cubic yards with waste
- Ready-Mix Delivery: Order 4.50 yd³ (rounded up to nearest 0.25 yd³ increment).
- Pre-mixed 80 lb Bags: Purchase 196 bags (0.60 cu ft yield per bag).
- Pre-mixed 60 lb Bags: Purchase 261 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: 16x20 Concrete Slab
Common contractor questions, bag ratios, thickness standards, and curing guidance.
How many cubic yards of concrete do I need for a 16x20 Concrete Slab at 4 inches thick? ▼
A 16x20 Concrete Slab (4" thick) has a net volume of 3.95 cubic yards. With the standard 10% ASTM C94 waste factor for uneven subgrade and form bulging, you should order 4.50 cubic yards of ready-mix concrete.
How many 80 lb bags of concrete do I need for a 16x20 Concrete Slab? ▼
For a 4-inch deep 16x20 Concrete Slab, you will need 196 bags of 80 lb pre-mixed concrete (or 261 bags of 60 lb mix). If pouring at 6 inches thick, you will need 294 bags of 80 lb mix.
Should a 16x20 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 16x20 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 at 8 ft and 10 ft grid intervals. Joints must be cut to 1/4 the depth of the slab.