Enter your garage dimensions and thickness to get exact concrete volume in cubic yards, bag counts, floor area, and a cost estimate — ready to hand to your supplier.
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Formulas verified against ACI 318
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✓ 1, 2 & 3-car garage presets✓ Bag counts (60 lb & 80 lb)✓ Cost estimator included✓ Last verified May 2026
Interior dimension, door to back wall. Typical 1-car: 20–22 ft. 2-car: 20–24 ft.Please enter a valid length greater than 0.
Interior wall-to-wall. 1-car: 12–14 ft. 2-car: 18–24 ft. 3-car: 28–36 ft.Please enter a valid width greater than 0.
Passenger cars: 4 in minimum. Trucks, SUVs, lifts: 6 in standard.
Please enter a valid thickness greater than 0.
Use 10% for standard rectangular floors. Use 15% for the first pour or L-shaped footprints.
$
Leave blank to skip cost estimate. US average: $110–$155/yd³ for ready-mix (2025).
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Your Garage Floor Concrete Estimate
Concrete Volume (with waste)
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Cubic Yards (yd³)
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Cubic Feet (ft³)
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Cubic Meters (m³)
Bags Required (includes waste) — for patches & small pours only
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60 lb bags
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80 lb bags
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40 lb bags
⚠ Garage floors almost always require ready-mix delivery. Bag counts are shown for reference only.
—Floor Area (ft²)
—Floor Area (m²)
—Thickness
—Waste Factor
Estimated Material Cost
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Concrete material cost only. Add forming ($1–$2/ft²), labor ($3–$6/ft²), delivery ($100–$350), vapor barrier, rebar, and finishing for a full project budget. Use our Full Project Estimator for a complete breakdown.
Step 1: Convert all dimensions to feet
Step 2: Volume (ft³) = Length (ft) × Width (ft) × Thickness (ft)
Step 3: Cubic Yards = ft³ ÷ 27
Step 4: Final Volume = Volume × (1 + waste% ÷ 100)
Step 5: Bags = CEIL(Final ft³ ÷ bag yield) — never round down
Measure the interior floor area.
Measure the inside of your garage — wall to wall for the width, and from the door threshold to the back wall for the length. These are your pour dimensions. Do not use exterior footprint dimensions or include wall thickness. For L-shaped garages or garages with a bump-out, break the floor into rectangles and run separate calculations, then add the results.
Choose your slab thickness.
Use the quick-select buttons to pick your thickness — 4 inches for passenger cars only, 6 inches for trucks, SUVs, or vehicles over 6,000 lbs, and 8 inches for a vehicle lift or commercial use. If you're unsure, go with 6 inches. It costs roughly $1.50–$2 per square foot more in concrete and dramatically improves durability and load capacity over the life of the slab.
Set your waste factor and enter a price (optional).
Leave the waste factor at 10% for a standard rectangular garage. Bump it to 15% if this is your first pour, if your subgrade is uneven, or if the garage has a drain or step-down footing. Enter your local ready-mix price per cubic yard if you want a material cost estimate — your supplier will quote this when you call.
Use your cubic yards figure to order ready-mix.
The cubic yards result is the number you give your ready-mix supplier. Give them this figure and specify the PSI (3,500 minimum; 4,000 in freeze-thaw climates) and whether you need air entrainment. Schedule delivery for a day when the full crew is on-site — a garage floor cannot be paused mid-pour without creating a cold joint at the stopping point.
⚠ Pro Tip: For any garage floor over 400 sq ft, order by ready-mix truck — not bags. A 20 × 22 ft floor at 6 inches requires over 9 cubic yards, which is more than 400 bags of 80 lb concrete. Mixing that volume by hand or with a rented mixer produces inconsistent results, creates dangerous cold joints, and takes days instead of hours. The truck delivery fee ($100–$350) is trivial against the cost of the slab.
Concrete Garage Floor Volume Formula
Garage floor concrete is calculated using the same rectangular slab formula used across all flatwork. The key variables are interior floor dimensions and slab thickness, all converted to feet before computing volume:
Step
Formula
Example (20 × 22 ft, 6 in)
1. Convert thickness to feet
inches ÷ 12
6 ÷ 12 = 0.500 ft
2. Volume in cubic feet
L × W × T
22 × 20 × 0.500 = 220.00 ft³
3. Convert to cubic yards
ft³ ÷ 27
220.00 ÷ 27 = 8.148 yd³
4. Add waste factor (10%)
Volume × 1.10
8.148 × 1.10 = 8.963 yd³
5. Order quantity
Round up to next 0.25 yd³
Order 9.0 yd³
Common Garage Floor Size Reference Table
Concrete volumes at exact dimensions — no waste factor applied. Add 10% when ordering.
Garage Type
Floor Size
Thickness
Cubic Yards (exact)
Order (10% waste)
1-Car
14 × 22 ft
4 in
4.52 yd³
4.97 yd³
1-Car
14 × 22 ft
6 in
6.78 yd³
7.46 yd³
2-Car Standard
20 × 22 ft
4 in
6.46 yd³
7.10 yd³
2-Car Standard
20 × 22 ft
6 in
9.63 yd³
10.59 yd³
2-Car Large
24 × 24 ft
4 in
8.53 yd³
9.38 yd³
2-Car Large
24 × 24 ft
6 in
12.80 yd³
14.08 yd³
3-Car
30 × 24 ft
6 in
16.00 yd³
17.60 yd³
3-Car Wide
36 × 24 ft
6 in
19.20 yd³
21.12 yd³
Volumes calculated as L × W × T ÷ 27. Figures assume uniform thickness. Thickened edge beams add volume — calculate them separately as a perimeter strip.
What Thickness Does a Concrete Garage Floor Need?
Slab thickness is the single most important decision in garage floor design. Thicker slabs resist point loads better, flex less under dynamic loads (vehicles driving in), and survive freeze-thaw cycles and deicing salts with less surface damage. The cost difference between 4 and 6 inches is roughly $1.50–$2 per square foot in concrete material — a minor investment for a floor that lasts 40+ years.
Recommended garage slab thickness by vehicle and use type.
Garage Use
Recommended Thickness
Min. PSI
Reinforcement
Notes
Passenger cars only
4 inches
3,500 PSI
Wire mesh or #3 rebar
Absolute minimum — upgrade if budget allows
SUVs & light trucks
5–6 inches
3,500 PSI
#4 rebar @ 18 in o.c.
Most common residential spec
Full-size trucks & vans
6 inches
4,000 PSI
#4 rebar @ 12–18 in o.c.
Recommended standard for all new garages
Vehicle lift (2-post or 4-post)
6 inches min.
4,000 PSI
#4 rebar @ 12 in o.c.
Lift manufacturer may require 6+ in; check specs
RV or motorhome
6–8 inches
4,000 PSI
#4–#5 rebar @ 12 in o.c.
Thicken entry transition and edge beams
Commercial / fleet vehicles
8 inches
4,500 PSI
#5 rebar @ 12 in o.c.
Engineer review recommended
Shop crane or overhead lift
8–12 inches
4,500+ PSI
Engineer-designed
Structural engineering required
On thickened edge beams: Regardless of interior slab thickness, the perimeter edge of a garage floor should be thickened to 8–12 inches and widened to 12–16 inches to act as a continuous grade beam. This edge beam provides frost protection, prevents the slab edge from cracking under the weight of walls, and gives anchor bolts adequate embedment. Calculate the edge beam volume separately and add it to your interior slab volume.
Common Mistakes When Estimating a Garage Floor
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Using exterior garage dimensions instead of interior floor dimensions.
The poured area is the interior of the walls, not the outside footprint. For a wood-framed garage with 2×6 walls, the exterior is 11 inches larger per side than the pour area. This error leads to significant over-ordering on large floors — up to 1.5 yards extra on a 3-car garage.
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Forgetting to calculate the thickened edge beam separately.
The perimeter grade beam is a different thickness than the interior slab (typically 8–12 in deep vs. 4–6 in interior). If you calculate the floor as uniform thickness, you will under-order the concrete. Calculate the interior slab and the edge beam as two separate rectangular volumes, then add them together.
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Specifying too-low a PSI to save money.
Using 3,000 PSI instead of 3,500 PSI saves maybe $3–$5 per cubic yard on the mix but results in a softer surface that dusts, scales, and stains more easily — particularly in garages where deicing salt is tracked in from vehicles. The concrete cost difference on a 2-car garage is under $50. The cost of resurfacing a scaled floor is $2,000–$6,000.
🔩
Placing rebar or wire mesh directly on the ground.
Reinforcement sitting on the dirt or gravel provides zero structural benefit — it must be embedded in the lower third of the slab to work in tension. Use plastic rebar chairs (dobies) to hold mesh and rebar at the correct height before pouring. For a 4-inch slab, rebar should sit at 1.5 inches from the bottom. For 6 inches, target 2 inches from the bottom.
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Not establishing the correct floor slope before pouring.
A garage floor must pitch 1/8 inch per foot toward the door or drain — or water will pool permanently at the back wall. This slope must be established during subgrade preparation and forming. Once the concrete is poured and cured, correcting a flat or back-sloped garage floor requires grinding down the high areas or applying a self-leveling overlay, both costly operations.
Frequently Asked Questions
The minimum recommended thickness for a residential garage floor is 4 inches for passenger vehicles. However, 6 inches is the professional standard and is required if you park trucks, SUVs, or store heavy equipment. Any garage used for a lift, a heavy vehicle, or commercial purposes should be 6 inches minimum, with thickened edge beams at 8–12 inches around the perimeter.
A standard 2-car garage floor measuring 20 × 22 feet at 4 inches thick requires approximately 6.46 cubic yards before waste. With a standard 10% waste factor, you should order 7.10 cubic yards. At 6 inches thick, that same floor requires 9.63 cubic yards without waste, or 10.59 cubic yards with 10% overage. Use the calculator above to get exact figures for your specific dimensions.
3,500 PSI is the minimum for a residential garage floor, and it is the most widely specified strength for this application. In freeze-thaw climates, specify 4,000 PSI with 5–7% air entrainment to resist surface scaling from deicing salts and freeze-thaw cycles. Using 3,000 PSI on a garage floor is undersizing it and will likely result in surface dusting and scaling within 10 years.
For a residential garage floor, #4 rebar on 18-inch centers in both directions, or 6×6 W1.4/W1.4 welded wire mesh, is standard. Rebar is structurally superior because it actively holds cracks closed if they form; wire mesh mainly helps keep small cracks from widening. Any garage that will see a vehicle lift, heavy equipment, or loads above 8,000 lbs should use rebar. The reinforcement must be placed in the lower third of the slab thickness, not on the ground where it provides no structural benefit.
Calculate the full rectangular volume first, then subtract the drain opening volume (typically negligible for a standard 4-inch floor drain). More important is accounting for the slope: a garage floor should pitch 1/8 inch per foot toward the drain or door. This slope slightly increases the average thickness at the high end versus the drain end. For most residential garages, the effect is under 2% of total volume and falls within your waste factor.
Almost always use ready-mix for a garage floor. Even a 1-car garage floor at 4 inches requires 4–5 cubic yards of concrete — that is 180+ bags of 80 lb concrete mixed by hand, which is impractical and produces inconsistent quality. Ready-mix arrives in a truck, fully mixed to your spec, with a quality guarantee. Bagged concrete is only practical for very small pours under 0.5 cubic yards or for repairs and patches.
Standard US garage dimensions are: 1-car garage — 12 × 20 ft to 14 × 22 ft (minimum drivable floor space); 2-car garage — 20 × 20 ft to 24 × 24 ft (most common); 3-car garage — 30 × 22 ft to 36 × 24 ft. These are interior floor dimensions. Add 6 inches to each dimension if you want to account for wall thickness when calculating exterior dimensions. Most contractors recommend going as large as your lot allows — extra garage space is almost always used.
Yes. A 10-mil (or thicker) polyethylene vapor barrier should be placed over the compacted gravel base before pouring. Without it, moisture migrates up through the slab year-round, causing efflorescence, coating adhesion failures, and accelerated corrosion of any steel touching the floor. In high-moisture areas, use 15-mil barrier. Lap seams by 12 inches and tape them with vapor-barrier tape. This is one of the cheapest and most impactful steps in garage slab construction.
A freshly poured garage floor should be kept free of foot traffic for 24–48 hours and vehicle traffic for a minimum of 7 days. However, the slab continues gaining strength for 28 days — reaching 70% of design strength at 7 days and full strength at 28 days. Avoid parking heavy trucks or placing concentrated loads on the slab for the full 28-day cure period. Keep the slab moist with wet curing or a curing compound for the first 7 days for maximum strength development.
A residential garage floor should slope 1/8 inch per foot toward the garage door or floor drain to allow water to run off. For a 20-foot-deep garage, this means the back wall is 2.5 inches higher than the door threshold. This slope is barely perceptible when standing in the garage but is critical for drainage. Failing to establish the correct slope during forming is the most common garage floor finishing error — it cannot be corrected after the concrete sets without grinding or overlaying.
