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20′ × 12′ • 240 SQ FT

12x20 Concrete Slab Concrete Calculator

A 12x20 concrete slab offers 240 square feet of functional area. Spanning 12 feet wide by 20 feet long, it accommodates a single car or trailer with 2 feet of walking room on either side, making it an extremely popular residential footprint.

Surface Area 240 sq ft
Perimeter (Forms) 64 lin ft
Concrete (4″ + 10% extra) 3.26 cu yds
80 lb Bags (4″) 147 bags
Gravel Subbase (4″) 4.1 tons

Custom 12x20 Concrete Slab Interactive Calculator

Pre-populated with 20 ft × 12 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
2.96 cubic yards

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

Volume in Cubic Feet 80.00 cu ft
Volume in Cubic Meters 2.265 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: 12 ft = 3.6576000000000004 m | Thickness: 4 in = 0.1016 m → 6.096 m × 3.6576000000000004 m × 0.1016 m
  2. Calculate Volume in Cubic Meters

    Multiply normalized length by width by thickness.

    6.096 × 3.6576000000000004 × 0.1016 → 2.26534772736 m³
  3. Convert Volume to Cubic Feet

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

    2.26534772736 / 0.028316846592 → 80 cu ft
  4. Convert Volume to Cubic Yards

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

    80 / 27 → 2.962962962962963 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 2.96 yd³ 3.26 yd³ 147 196 3.26 yd³ (Short truck or small load)
5″ Residential Garage / Light Truck Pad 3.7 yd³ 4.07 yd³ 184 245 1 truckload (41% of 10-yd mixer)
6″ Heavy Equipment / Commercial / RV Pad 4.44 yd³ 4.89 yd³ 220 294 1 truckload (49% 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 3.26 yd³, always round up to avoid a short-pour emergency.

Ideal Projects for a 12x20 Concrete Slab

  • ✓
    Carport base, boat parking pad, or oversized single-car driveway extension
  • ✓
    Spacious backyard covered patio with outdoor kitchen area
  • ✓
    Medium storage shed, detached hobby studio, or lawn tractor garage
  • ✓
    Enclosed greenhouse base or swim-spa foundation

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 2.96 cu yds (80 cu ft)
Total Gravel Weight 4.1 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 383 lin ft Includes 10% lap splice & corner overlap
Standard 20-ft Sticks 20 sticks Weight: ~256 lbs

Support rebar on approximately 40 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: One transverse contraction joint across the 12-ft width at the 10-ft mark, creating two 12x10 panels
Resulting Panels: 2 square/near-square panels (aspect ratio ≤ 1.25)
Minimum Cut Depth: 1.0 inch for 4" slab; 1.25 inches for 5" slab
Saw Timing: Saw-cut within 4 to 12 hours after finishing.

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 12x20 Concrete Slab

DIY Material Cost
$1,050 – $1,650
~$4.38 – $6.88 / sq ft

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

Turnkey Contractor Installed
$1,900 – $3,100
~$7.92 – $12.92 / sq ft

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

DIY materials (concrete, 4" gravel base, #4 rebar, forms) range from $1,050 to $1,650. Professional contractor installation averages $8 to $13 per sq ft ($1,920 to $3,120). For detailed budget modeling including regional truck delivery fees and pump rentals, use our Concrete Cost & Budget Estimator.

Frequently Asked Questions: 12x20 Concrete Slab

How many yards of concrete for a 12x20 slab?

A 12x20 concrete slab at 4 inches thick requires 2.96 cubic yards of concrete (order 3.26 yards with 10% waste). At 5 inches, you need 3.70 cubic yards (order 4.07 yards). At 6 inches, order 4.89 yards.

How many 80 lb bags of concrete for a 12x20 slab?

At 4 inches thick with a 10% safety margin, a 12x20 slab requires 147 bags of 80 lb concrete (or 196 bags of 60 lb concrete). Ready-mix delivery by truck is strongly recommended.

How much gravel base for a 12x20 slab?

A 4-inch compacted gravel subbase requires 2.96 cubic yards of crushed stone, which weighs approximately 4.1 to 4.5 tons.

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″) 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″) 20′ × 20′ 20x20 Concrete Slab 400 sq ft • 5.43 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