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20′ × 20′ • 400 SQ FT

20x20 Concrete Slab Concrete Calculator

A 20x20 slab provides 400 square feet and is the quintessential footprint for two-car residential garages and backyard workshops. Because vehicular traffic creates concentrated point loads, 5-inch or 6-inch thickness is standard practice over a robust gravel subbase.

Surface Area 400 sq ft
Perimeter (Forms) 80 lin ft
Concrete (4″ + 10% extra) 5.43 cu yds
80 lb Bags (4″) 245 bags
Gravel Subbase (4″) 6.9 tons

Custom 20x20 Concrete Slab Interactive Calculator

Pre-populated with 20 ft × 20 ft. Adjust thickness or unit system below for immediate real-time calculations.

ENGINEERING SCHEMATIC
Rectangular Concrete Slab Geometry
Length (L) Width (W) Thickness (T: 4"–6") Surface Area = L × W
💡
Pro Contractor Recommendation: Residential slabs for patios and walkways should be at least 4" thick. Heavy vehicular traffic or storage sheds require 5" to 6" thickness with #4 rebar.
Quick Presets:
Example default values loaded
Estimated Concrete Needed
4.94 cubic yards

Raw volume estimate. Standard practice is to add 5%–10% extra for form deflection, spillage, and uneven subgrade.

Volume in Cubic Feet 133.33 cu ft
Volume in Cubic Meters 3.776 m³
Interactive 2D Blueprint

Rebar Grid & Control Joint Visualizer

Real-time scaled layout calibrated to ACI 318 structural concrete standards.

SCALE: DYNAMIC TO FIT ASPECT: 1:1 COVER: 3.0" PER ACI 318 JOINT DEPTH: T/4 = 1.0"
Slab Perimeter
Rebar Grid
Saw-Cut Control Joint
Tie Intersection
Slab Surface Area
100 sq ft
10.0' × 10.0' × 4"
Rebar Grid Counts
14 total bars
7 Long. • 7 Trans.
Total Rebar Linear Footage
146 linear ft
~98 lbs of #4 rebar (+10% lap)
Crack Control Joints
0 cuts needed
Max 10.0' spacing • 1.0" deep
Site Prep & Subgrade

Ground Excavation & Gravel Subbase

Concrete flatwork requires excavating native soil to accommodate both the compacted gravel base and slab thickness.

Concrete Slab Layer 4.0" Thickness
Compacted Crushed Stone Subbase 4.0" Base Depth
Geotextile Separation Fabric Underlayment
Undisturbed Native Soil Subgrade Tamped & Level
TOTAL DIG DEPTH
8.0"
(4" slab + 4" gravel)
Soil to Excavate & Haul
2.96 cu yds
~3.7 tons loose dirt • 1 dump truck load
Compacted Crushed Stone Base
2.1 tons
~1.5 cu yds (+15% mechanical compaction)
Geotextile Fabric Needed
110 sq ft
Prevents gravel sinking into native clay/subgrade
Ready-Mix Dispatch Helper

Call-a-Batch-Plant Order Cheat Sheet

Concrete dispatchers move fast. Have these exact technical answers ready when placing your ready-mix delivery call.

Copied to clipboard!
1. Volume to Request Rounded to 0.25 yd
1.50 cu yds

Includes +10% waste buffer. Plants meter in 1/4-yard increments.

Under 4.0 cu yds usually incurs a ~$75–$120 short-load fee.
2. Mix Design & Strength Structural Grade
4000 PSI

Air-entrained exterior flatwork mix (freeze-thaw & vehicle rated).

3. Slump & Aggregate Chute Pour
4" Slump • 3/4" Stone

Standard workability for direct chute pours. If using a line pump, specify 3/8" pea gravel mix.

4. Estimated Unload Time No Standby Fee
~30 Mins

Suppliers typically include 5–7 mins/yard free unloading. Additional truck standby costs ~$2.50–$3.50/minute.

Before the Truck Arrives — Site Readiness Checklist:
✓ 10' Clearance: Ensure 10-foot wide driveway and 14-foot overhead branch/wire clearance.
✓ Truck Weight: Fully loaded mixer weighs 60,000+ lbs. Never drive over septic tanks or soft ground.
✓ Washout Station: Prepare a designated dirt hole or tarped bin for driver chute washout.
✓ Crew Ready: Have wheelbarrows, screeds, bull floats, and crew on site 15 mins before scheduled pour.
Contractor Bid & Material Takeoff Sheet

Export or print an executive 1-page PDF job estimate with material checklist.

CONCRETECALCPRO STRUCTURAL ESTIMATION & JOB TAKEOFF
Document: Project Material Takeoff
Date:
Type: Concrete Slab Project

1. Project Dimensions & Area Specifications

Length Width Thickness Surface Area

2. Concrete Volume & Batch Order Requirements

Net Calculated Volume Recommended Order (+10% Waste) 80-lb Pre-Mix Bags 60-lb Pre-Mix Bags

3. Steel Reinforcement & Crack-Control Joint Schedule

Rebar Specification Grid Spacing Bar Count Linear Footage Total Steel Weight Control Saw Cuts

4. Jobsite Material Checklist

Ready-Mix Concrete Truck or Pre-mixed Bags
Deformed Steel Rebar / Welded Wire Mesh
Concrete Dobie Chairs (to suspend rebar at 2" depth)
16-Gauge Tie Wire & Rebar Safety Caps
Formwork Lumber (2×4 or 2×6) & Wooden/Steel Stakes
6-mil or 10-mil Polyethylene Vapor Barrier
1/2" Expansion Joint Filler Strips (at walls/slabs)
Compacted Crushed Stone Subbase (2.1 tons • 4" depth)
Soil Excavation & Hauling (3.0 cu yds loose dirt)
Project / Jobsite Address:
Contractor / Estimator Name:
Client Approval Signature:
Target Pour Date:

Engineering Disclaimer: This takeoff sheet provides estimation guidelines in accordance with ACI 318 & 360R standard residential flatwork practices. Subgrade deflection, form expansion, ground slope, and contractor placing practices will affect final concrete volume. Always confirm structural requirements with local building authorities and structural plans prior to placing orders.

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How It Is Calculated

For a slab measuring 10 ft long × 10 ft wide × 4 in thick:

  1. Normalize Dimensions to Canonical Base Unit (Meters)

    Convert length, width, and thickness to meters using exact NIST international conversion factors.

    Length: 20 ft = 6.096 m | Width: 20 ft = 6.096 m | Thickness: 4 in = 0.1016 m → 6.096 m × 6.096 m × 0.1016 m
  2. Calculate Volume in Cubic Meters

    Multiply normalized length by width by thickness.

    6.096 × 6.096 × 0.1016 → 3.7755795456000003 m³
  3. Convert Volume to Cubic Feet

    Convert cubic meters to cubic feet using exact factor (1 cu ft = 0.028316846592 m³).

    3.7755795456000003 / 0.028316846592 → 133.33333333333334 cu ft
  4. Convert Volume to Cubic Yards

    Convert cubic feet to cubic yards (27 cu ft = 1 cu yd).

    133.33333333333334 / 27 → 4.938271604938272 cu yd

Concrete Volume & Bag Requirements by Depth

Concrete slabs are poured at different depths depending on intended vehicle or structural weight. The table below compares raw volume against recommended order volume (+10% safety buffer for uneven subgrade and form deflection).

Thickness Common Applications Exact Yards Order Yards (+10%) 80 lb Bags 60 lb Bags Ready-Mix Logistics
4″ Standard Patio / Shed / Sidewalk 4.94 yd³ 5.43 yd³ 245 326 1 truckload (54% of 10-yd mixer)
5″ Residential Garage / Light Truck Pad 6.17 yd³ 6.79 yd³ 306 408 1 truckload (68% of 10-yd mixer)
6″ Heavy Equipment / Commercial / RV Pad 7.41 yd³ 8.15 yd³ 367 489 1 truckload (81% of 10-yd mixer)
Standard Ordering Advice: Ready-mix concrete suppliers usually deliver in 0.25 or 0.50 cubic yard increments. If your required volume is 5.43 yd³, always round up to avoid a short-pour emergency.

Ideal Projects for a 20x20 Concrete Slab

  • ✓
    Standard two-car residential garage floor slab
  • ✓
    Large detached woodworking or metalworking shop
  • ✓
    Backyard sports court (pickleball practice court or basketball half-court)
  • ✓
    Large outdoor pavilion or commercial dumpster pad

Gravel Subbase & Ground Prep Specifications

A durable concrete slab requires a solid, well-draining foundation to prevent settlement cracking and frost heave:

Excavation Depth 8 inches (4″ crushed stone base + 4″ concrete slab)
Subbase Thickness 4 inches compacted Crushed #57 stone, dense graded aggregate (DGA), or crusher run
Gravel Volume Required 4.94 cu yds (133 cu ft)
Total Gravel Weight 6.9 tons Compacted density @ ~1.4 tons/yd³

Compacted in 2-inch lifts with a mechanical plate compactor to 95% standard Proctor density. Use our dedicated Gravel & Subbase Material Calculator to estimate base depth for different soil types.

Reinforcement Schedule (#4 Rebar Grid)

Tensile reinforcement resists bending stresses from temperature fluctuations and heavy localized point loads:

Rebar Specification #4 (1/2-inch nominal diameter, Grade 60) Standard ASTM A615 Grade 60
Grid Spacing 18″ on-center Both directions (perpendicular grid)
Total Linear Rebar 616 lin ft Includes 10% lap splice & corner overlap
Standard 20-ft Sticks 31 sticks Weight: ~411 lbs

Support rebar on approximately 67 concrete dobies or plastic rebar chairs to maintain reinforcement in the middle third of the slab. Alternatively, you may install 6x6 – W1.4/W1.4 welded wire reinforcement sheets (with 6" lap allowance). View our Concrete Rebar & Mesh Calculator for comprehensive cut lists.

Contraction Joint Layout (Crack Prevention)

Concrete inevitably shrinks as water evaporates during curing. Control joints create intentional weakened planes that dictate where microscopic hairline cracks occur, keeping them straight and unnoticeable:

Recommended Grid Layout: Perpendicular cross-pattern with one joint centered at 10 feet in both directions, dividing the slab into four equal 10x10 foot quadrants
Resulting Panels: 4 square/near-square panels (aspect ratio ≤ 1.25)
Minimum Cut Depth: 1.25 inches (for 5" slab) or 1.5 inches (for 6" slab)
Saw Timing: Early saw cutting is critical for 20x20 slabs to prevent thermal contraction cracks during the initial 24-hour hydration peak.

Reference: ACI 302.1R Guide for Concrete Floor and Slab Construction recommends maximum joint spacing in feet equal to 2 to 2.5 times slab thickness in inches.

Estimated Cost for a 20x20 Concrete Slab

DIY Material Cost
$1,800 – $2,900
~$4.50 – $7.25 / sq ft

Covers ready-mix concrete delivery, gravel base, #4 rebar, lumber formwork, and stakes.

Turnkey Contractor Installed
$3,200 – $5,400
~$8.00 – $13.50 / sq ft

Includes excavation, subgrade prep, gravel compaction, rebar placement, pour, screeding, and professional broom/trowel finish.

Material costs for a 20x20 slab (ready-mix concrete, 6.9 tons of base gravel, #4 rebar, forms) range from $1,800 to $2,900. Turnkey contractor installation typically runs $8 to $13.50 per sq ft ($3,200 to $5,400). For detailed budget modeling including regional truck delivery fees and pump rentals, use our Concrete Cost & Budget Estimator.

Frequently Asked Questions: 20x20 Concrete Slab

How many yards of concrete for a 20x20 slab?

For a standard 4-inch patio slab, a 20x20 slab requires 4.94 cubic yards (order 5.43 yards with 10% waste). For a 2-car garage floor at 5 inches depth, you need 6.17 cubic yards (order 6.79 yards). If pouring at 6 inches for heavy vehicles or equipment, order 8.15 yards.

Does a 20x20 slab require a full concrete truck?

Yes. A standard ready-mix transit mixer truck carries 9 to 10 cubic yards. A 20x20 slab at 5" thickness (6.79 yards with waste) easily fills the majority of a single truck, allowing you to avoid short-load fees while receiving the entire batch in one single continuous pour.

How much rebar do I need for a 20x20 concrete slab?

For an 18-inch on-center grid, you need 14 bars in each direction (28 bars total). At 20 feet per run plus 10% for lap splices, this requires 616 linear feet of #4 rebar, which is thirty-one 20-foot sticks, supported on 65 rebar chairs.

Explore Other Popular Concrete Slab Dimensions

Need calculations for a different project size? Browse our complete engineering guides for popular slab footprints:

10′ × 10′ 10x10 Concrete Slab 100 sq ft • 1.36 yd³ (4″) 20′ × 10′ 10x20 Concrete Slab 200 sq ft • 2.72 yd³ (4″) 12′ × 12′ 12x12 Concrete Slab 144 sq ft • 1.96 yd³ (4″) 20′ × 12′ 12x20 Concrete Slab 240 sq ft • 3.26 yd³ (4″) 24′ × 12′ 12x24 Concrete Slab 288 sq ft • 3.91 yd³ (4″) 20′ × 14′ 14x20 Concrete Slab 280 sq ft • 3.8 yd³ (4″) 20′ × 16′ 16x20 Concrete Slab 320 sq ft • 4.35 yd³ (4″) 24′ × 16′ 16x24 Concrete Slab 384 sq ft • 5.21 yd³ (4″) 24′ × 20′ 20x24 Concrete Slab 480 sq ft • 6.52 yd³ (4″) 30′ × 20′ 20x30 Concrete Slab 600 sq ft • 8.15 yd³ (4″) 24′ × 24′ 24x24 Concrete Slab 576 sq ft • 7.82 yd³ (4″) 30′ × 24′ 24x30 Concrete Slab 720 sq ft • 9.78 yd³ (4″) 40′ × 30′ 30x40 Concrete Slab 1200 sq ft • 16.3 yd³ (4″) 50′ × 30′ 30x50 Concrete Slab 1500 sq ft • 20.37 yd³ (4″) 60′ × 40′ 40x60 Concrete Slab 2400 sq ft • 32.59 yd³ (4″)
Concrete Slab Calculator General custom dimension volume estimator Garage Floor & Shed Pad Calculator Calculations tailored for vehicle and storage loads Ready-Mix Truck Load Planner Plan fleet delivery intervals and short-load fees Concrete Rebar & Mesh Calculator Comprehensive reinforcement bar schedules