Concrete for a 20×20 Garage Slab: Yards, Bags, Subbase & Cost Takeoff
A comprehensive, field-calibrated engineering takeoff and procurement guide for a standard 2-car 20′ × 20′ garage slab (400 sq ft). Calculate exact ready-mix cubic yards, commercial 80-lb pre-mix bag counts, crushed stone subbase tonnage, rebar reinforcement grids, and delivery cost economics.
20′ × 20′ Slab Parameters
20×20 Takeoff Breakdown
400 SQ FT SLABHow Many Yards of Concrete for a 20×20 Slab?
A standard 20′ × 20′ slab has a surface footprint of exactly 400 square feet. Determining the required ready-mix volume requires converting all dimensions into feet, calculating total cubic feet, and dividing by 27 to obtain cubic yards.
Commercial Estimating Rule: Never order the exact net volume of 4.94 cubic yards. Subgrade elevation tolerances, form deflection under hydrostatic pressure, and contractor edge spillage demand an 8% to 10% safety buffer. For ready-mix batch plants, round up to 5.5 cubic yards. Running short on a 400 sq ft monolithic slab by even 1/3 cubic yard causes emergency short-load dispatch surcharges ($150–$250) and leaves an unsightly cold-joint weakness under vehicle tire tracks.
| Slab Thickness | Structural Application | Net Volume | With 10% Waste | 80-lb Bags | Transit Truckloads |
|---|---|---|---|---|---|
| 4.0 Inches | Standard passenger vehicles, sedans, compact crossovers | 133.3 cu ft (4.94 yd³) | 5.43 cu yds | 245 Bags | 0.6 Truckload (1 Truck) |
| 5.0 Inches | Full-size pickups (F-150, Silverado), large SUVs, work vans | 166.7 cu ft (6.17 yd³) | 6.79 cu yds | 309 Bags | 0.8 Truckload (1 Truck) |
| 6.0 Inches | Commercial shop, diesel trucks, 2-post car lift anchors | 200.0 cu ft (7.41 yd³) | 8.15 cu yds | 371 Bags | 0.9 Truckload (1 Truck) |
How Many Bags of Concrete for a 20×20 Slab?
Pre-mixed dry concrete bags (such as Quikrete or Sakrete) are rated by yield volume per bag:
- 80-lb Bag: Yields 0.60 cubic feet of placed concrete (requires 45 bags per cubic yard).
- 60-lb Bag: Yields 0.45 cubic feet of placed concrete (requires 60 bags per cubic yard).
- 40-lb Bag: Yields 0.30 cubic feet of placed concrete (requires 90 bags per cubic yard).
| Slab Depth | Order Volume | 80-lb Bags | 60-lb Bags | 40-lb Bags | Total Placed Weight |
|---|---|---|---|---|---|
| 4.0″ Slab | 5.43 yds | 245 bags | 326 bags | 489 bags | 19,600 lbs (9.80 T) |
| 5.0″ Slab | 6.79 yds | 309 bags | 412 bags | 618 bags | 24,720 lbs (12.36 T) |
| 6.0″ Slab | 8.15 yds | 371 bags | 494 bags | 741 bags | 29,680 lbs (14.84 T) |
Safety & Structural Warning: Hand-mixing 245 bags of 80-lb concrete means handling over 9.80 tons (19,600 lbs) of raw aggregate and cement, requiring approximately 184 gallons of clean potable water. Concrete takes 60 to 90 minutes to begin its initial set. Continuous manual hand-mixing over 8 to 12 hours guarantees that the first section poured will begin hardening before subsequent batches are placed, causing severe cold joints, structural cracking, and uneven curing under load.
Cost to Pour a 20×20 Concrete Slab (2026 Price Index)
Material and turnkey contractor costs vary based on whether you handle excavation, forming, and finishing yourself or hire a licensed concrete finishing crew:
| Procurement Route | Components Included | Estimated Total Cost | Cost per Sq. Ft. |
|---|---|---|---|
| Ready-Mix Concrete Delivery | 5.5 cubic yards @ 145–175/yd delivered + environmental fees | 850 –1,050 | 2.12 –2.62 / sq ft |
| Subbase & Forming Materials | 7 tons crushed aggregate + 2×4 forms + 10-mil barrier + rebar | 650 –950 | 1.62 –2.38 / sq ft |
| DIY Finishing Tool Rentals | Bull float, edger, mag floats, early-entry saw, vibratory compactor | 200 –350 | 0.50 –0.88 / sq ft |
| Turnkey Contractor (Installed) | Complete excavation, base, forming, #4 rebar grid, 4,000 PSI pour & finish | 2,800 –4,400 | 7.00 –11.00 / sq ft |
Verdict: Ready-mix concrete is approximately $500 cheaper in direct materials than bags, while delivering a monolithic, high-strength certified pour in 45 minutes instead of 10 grueling hours of back-breaking hand mixing.
Thickness Guide: 4-Inch vs 5-Inch vs 6-Inch for a 20×20 Slab
Choosing the correct thickness depends entirely on the design loads, vehicle weights, or structural applications planned for the pad:
- 4-Inch Slab (Standard Urban 2-Car Garage): Ideal for standard passenger sedans, crossovers, and compact sports cars. Requires 5.43 cubic yards (5.5 yds ordered). Requires 3,500 PSI mix and #4 rebar on 18-inch centers.
- 5-Inch Slab (Heavy Trucks & Utility Trailers): Recommended if you park full-size 1/2-ton or 3/4-ton pickups, commercial service vans, or loaded dual-axle landscape trailers. Requires 6.79 cubic yards (7.0 yds ordered). Provides 56% greater shear and point-load resistance than a 4-inch pour.
- 6-Inch Slab (Automotive Shop & 2-Post Hoists): Mandatory if you plan to install a 2-post or 4-post automotive car hoist (which requires 4 to 6 inches of 3,000+ PSI crack-free concrete for wedge anchor embedment). Requires 8.15 cubic yards (8.25 yds ordered).
Subbase Prep, Gravel Tonnage & Reinforcement Schedule
A concrete slab is only as durable as the ground underneath it. Poor subgrade preparation is the primary cause of settlement, uneven cracking, and frost-heave damage.
1. Subbase Excavation & Crushed Stone Base
Remove all organic topsoil down to stable subgrade. Place a minimum 4-inch compacted gravel base using #57 crushed limestone, dense-graded aggregate (crusher run), or 3/4″ road base.
- Subbase Volume: 400 sq ft × 0.333 ft = 133.3 cu ft = 4.94 cubic yards.
- Gravel Tonnage Required: At 1.4 tons per cubic yard compacted: 4.94 × 1.4 = 6.91 Tons (order approximately 7.0 to 8.0 Tons).
- Compaction: Compact in 2-inch lifts using a mechanical vibratory plate compactor to achieve 95% Standard Proctor density.
Slope the garage slab 1/8 inch per foot towards the overhead vehicle door opening (approximately 2.5 inches drop over the 20-foot depth) to ensure melting snow, slush, and car wash runoff drains outward rather than pooling against rear walls.
2. Vapor Retarder (Moisture Barrier)
Install a heavy 10-mil or 15-mil true polyolefin vapor barrier (ASTM E1745 Class A) directly over the compacted stone base. Overlap seams by 6 inches and tape securely. A vapor barrier prevents hydrostatic groundwater migration, avoiding floor dampness and coating peeling.
3. Steel Rebar & Reinforcement Schedule
Unreinforced concrete cracks as it shrinks during hydration. To maintain structural tensile strength and prevent cracks from widening:
- Rebar Size: #4 (1/2-inch diameter) Grade 60 steel rebar.
- Grid Spacing: 18 inches on-center in both directions. Requires approximately 22 sticks of 20-foot rebar (~440 linear feet including 30-bar-diameter lap splices).
- Placement: Support the steel grid on 2-inch plastic rebar chairs or precast concrete dobies. Never place rebar directly on the gravel base.
4. Control Joint Layout (Prevent Random Cracking)
Concrete shrinks roughly 1/16 inch per 10 feet as moisture evaporates during curing. Control joints introduce weakened planes so shrinkage cracks occur cleanly beneath saw cuts rather than meandering across the surface:
- ACI 302.1R guidelines require control joint spacing not to exceed 2 to 2.5 times slab thickness in inches. For a 4″ slab, joints must be spaced 8 to 10 feet.
- For a 20′ × 20′ slab, cut two saw joints intersecting at the dead center (one at 10 feet along the length, one at 10 feet along the width).
- This creates four equal 10′ × 10′ panels (100 sq ft each).
- Depth: Saw cut to a depth of exactly 1 inch using an early-entry concrete saw within 12 to 18 hours after finishing.
Frequently Asked Questions About Pouring a 20×20 Concrete Slab
A standard 4-inch thick 20×20 foot garage slab requires 4.94 cubic yards of concrete net. Adding the industry standard 10% allowance for subbase grading variations and formwork deflection, you should order exactly 5.43 cubic yards (rounded to 5.5 cubic yards for ready-mix truck dispatch).
A 20×20 slab at 4 inches thick requires 223 eighty-pound bags net, or 245 eighty-pound bags with a 10% waste buffer. This represents approximately 5.8 full pallets (42 bags per pallet) and a total weight of 19,600 lbs (9.8 tons). Hand-mixing this quantity is not recommended due to severe cold-joint risks.
In 2026, the complete installed cost for a professional turnkey 20×20 concrete slab ranges between $2,800 and $4,400 ($7.00 to $11.00 per square foot) including excavation, compacted gravel base, vapor barrier, #4 rebar grid, ready-mix concrete, and professional trowel finishing. If handling excavation and forming DIY and ordering ready-mix delivery only, materials cost approximately $1,500 to $2,000.
Specify a minimum 3,500 PSI mix for moderate climates, or a 4,000 PSI mix with 5% to 7% air entrainment for northern freeze-thaw climates where road de-icing salts dripping from vehicle tires can cause surface scaling and spalling.
Results are estimates based on the dimensions and material assumptions entered. Actual material requirements can vary due to site conditions, material density, compaction, waste, supplier specifications, and construction methods. Confirm quantities with your supplier or project professional before ordering.