Concrete Garage Floor Calculator
A standard 24×24 ft 2-car garage slab at 6 inches thick takes 10.67 cubic yards of concrete — about one full ready-mix truck. Enter your garage dimensions and thickness below for volume, bag-equivalent count, and reinforcement guidance.
Pro Tips
- →Garage floors should be 6 inches thick for vehicle weight
- →Include rebar reinforcement in a grid pattern
- →Slope floor slightly toward door for drainage (1-2%)
- →Add a vapor barrier under the slab
- →Consider a higher PSI mix (4,000-5,000) for durability
Includes 10% waste factor
That's typically a professional pour. See costs ↓
4 short emails from Dave: what a fair quote should land at for your slab, the scope changes that swing it ±$500, and whether DIY is actually cheaper at your volume. Reply anytime — he'll review your real quote.
Cost Estimate
Estimated material costs for your project
Recommendation: Ready-Mix Concrete
For projects over 1 cubic yard, ready-mix is typically more economical and easier to work with.
528 bags × 80lb
11.73 cubic yards + delivery
576 sq ft × $3.00–$8.00/sq ft
Prices vary by location and time. Contact local suppliers for accurate quotes.
For general step-by-step instructions, read our complete Garage Floor Thickness and 4 Vs 6 Inch.
Garage Floor Concrete Volume by Size
Garage slab volume is driven by two numbers: footprint and thickness. The table below gives the concrete volume in cubic yards for the most common garage sizes at the 4-, 5-, and 6-inch thicknesses you'll actually pour. Values include no waste — add 10% before ordering (see below).
| Garage Type | Footprint | Square Feet | 4 in (cu yd) | 5 in (cu yd) | 6 in (cu yd) |
|---|---|---|---|---|---|
| 1-car (compact) | 12 × 20 ft | 240 | 2.96 | 3.70 | 4.44 |
| 1-car (standard) | 14 × 22 ft | 308 | 3.80 | 4.75 | 5.70 |
| 2-car (standard) | 20 × 20 ft | 400 | 4.94 | 6.17 | 7.41 |
| 2-car (common) | 20 × 22 ft | 440 | 5.43 | 6.79 | 8.15 |
| 2-car (oversized) | 24 × 24 ft | 576 | 7.11 | 8.89 | 10.67 |
| 3-car (standard) | 30 × 20 ft | 600 | 7.41 | 9.26 | 11.11 |
| 3-car (deep) | 30 × 24 ft | 720 | 8.89 | 11.11 | 13.33 |
A standard 2-car at 24 × 24 ft and 6 inches comes to 10.67 cubic yards — about one full ready-mix truck. Run your exact dimensions through the calculator above, then price the finished job with the companion garage floor cost calculator.
How to Calculate Garage Floor Cubic Yards
The math is the same volume formula for any slab, scaled to garage footprints:
- Area in square feet — length × width. A 24 × 24 ft garage is 576 sq ft.
- Thickness in feet — divide inches by 12. Six inches is 0.5 ft.
- Cubic feet — area × thickness. 576 × 0.5 = 288 cu ft.
- Cubic yards — divide by 27 (cubic feet per cubic yard). 288 ÷ 27 = 10.67 cu yd.
A shortcut that skips the unit juggling: multiply square feet by thickness-in-inches, then divide by 324. For the 576 sq ft garage at 6 in: 576 × 6 ÷ 324 = 10.67 cu yd. For the background on the unit itself, see what is a cubic yard of concrete and the full how to calculate concrete walkthrough.
Choosing Thickness: 4, 5, or 6 Inches
Thickness sets the volume — going from 4 to 6 inches raises a 24 × 24 garage from 7.11 to 10.67 cubic yards, a 50% jump in concrete. Pick by load, not by habit:
| Thickness | Best For | Notes |
|---|---|---|
| 4 in | Parking and storage only | Meets the IRC R506.2.3 minimum residential slab thickness; fine for cars and light trucks |
| 5 in | Heavier trucks, occasional trailers | Middle ground; modest volume increase over 4 in |
| 6 in | Lifts, jack stands, shop equipment | ACI 332-20 residential concrete practice supports thicker slabs for concentrated loads; resists punch-through |
A parked car puts roughly 750–1,000 lbs through each tire patch — well within a 4-inch slab. The reasons to go to 6 inches are concentrated point loads: floor jacks and jack stands push 1,500–4,000 lbs through pads as small as a few square inches, and shop lifts concentrate weight on small base plates. If your garage is purely for parking, 4 inches is correct and saves a third of the concrete. For the full trade-off, read 4 vs 6 inch concrete and the garage floor thickness guide.
Adding Waste and Ordering Ready-Mix
Always add 10% to your calculated volume to cover spillage, uneven subgrade, and slab-edge over-dig. Then round up to the next quarter-yard, because ready-mix is sold in quarter-yard increments.
| Garage (6 in) | Net Volume | +10% Waste | Order As |
|---|---|---|---|
| 1-car (12 × 20) | 4.44 cu yd | 4.88 | 5.00 cu yd |
| 2-car (20 × 20) | 7.41 cu yd | 8.15 | 8.25 cu yd |
| 2-car (24 × 24) | 10.67 cu yd | 11.73 | 11.75 cu yd |
| 3-car (30 × 24) | 13.33 cu yd | 14.67 | 14.75 cu yd |
Every size here exceeds the bag-vs-ready-mix crossover. Even the smallest 1-car garage at 4.44 cubic yards would need roughly 220 bags of 80-lb mix — far past the point where bagged concrete is practical. Order ready-mix for any garage floor. Below about 3 cubic yards, expect a $75–150 short-load fee; a 1-car garage is borderline, while 2- and 3-car pours fill most of a truck. See the bagged vs ready-mix comparison if you're weighing a very small detached bay.
Slope to the Door and Drainage
Slope the entire slab 1–2% toward the garage door so wash water, snowmelt, and rain run out instead of pooling. Set this slope when you screed — it does not change the concrete volume, since the average thickness stays the same. The fall scales with garage depth:
| Garage Depth | Fall at 1% | Fall at 2% |
|---|---|---|
| 20 ft | 2.4 in | 4.8 in |
| 22 ft | 2.6 in | 5.3 in |
| 24 ft | 2.9 in | 5.8 in |
| 30 ft | 3.6 in | 7.2 in |
In snow-belt climates the slope handles slush dripping off vehicles all winter. Skip it and standing water under a parked car becomes a freeze-spall problem within a few seasons.
Base, Vapor Barrier, and Control Joints
The slab is only as good as what's under and around it:
- Gravel base — 4 inches of compacted 3/4-inch crushed stone over native subgrade, compacted in 2-inch lifts. This gives uniform support and breaks capillary moisture rise.
- Vapor barrier — 10-mil polyethylene over the gravel, lapped 12 inches and taped at seams. Essential under heated garages and in humid climates; details in the vapor barrier under concrete guide.
- Reinforcement — #4 (1/2-inch) rebar in a 24-inch grid at mid-slab depth is the residential standard; 6×6 W1.4/W1.4 welded wire mesh is the cheaper alternative for parking-only slabs. See when to use rebar.
- Control joints — saw-cut at 10–12 ft spacing within 24 hours of finishing, to one-quarter the slab depth (1.5 in for a 6-inch slab). For a 24 × 24 garage, two perpendicular cuts dividing the slab into roughly equal quarters keeps random cracking in check.
With volume, thickness, and waste settled, price the finished installation — subgrade prep, vapor barrier, rebar, pour, and finishing — using the garage floor cost calculator, or compare against other project types with the concrete cost calculator.