Concrete Garage Floor Calculator
Enter your garage floor’s length, width, and slab thickness to get the cubic yards of ready-mix concrete to order, the bag equivalent, estimated weight, and total cost.
Covers sub-base irregularities and spillage. 5–10% is typical.
Assumes standard-weight concrete at about 150 lb per cubic foot and includes your waste allowance. Bag counts are shown for reference — most garage floors this size are poured with ready-mix delivered by truck, not bagged concrete.
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Concrete Garage Floor Calculator
Short answer: multiply length × width × thickness (converted to feet), divide by 27 to get cubic yards, then add 5–10% for waste. A typical 20×20 ft, 4-inch garage floor needs about 5 cubic yards of ready-mix concrete after waste.
Formula
Thickness is usually given in inches, so divide it by 12 before multiplying. Ready-mix suppliers sell by the cubic yard, and most require orders in quarter-yard increments with a minimum load, so round your final number up to the nearest supplier increment.
Reference table: typical garage floor volumes
| Garage size | 4″ thick | 5″ thick | 6″ thick |
|---|---|---|---|
| 12 × 20 ft (1-car) | 2.96 yd³ | 3.70 yd³ | 4.44 yd³ |
| 20 × 20 ft (2-car) | 4.94 yd³ | 6.17 yd³ | 7.41 yd³ |
| 24 × 24 ft (2-car, deep) | 7.11 yd³ | 8.89 yd³ | 10.67 yd³ |
| 30 × 30 ft (3-car) | 11.11 yd³ | 13.89 yd³ | 16.67 yd³ |
Figures above are pure geometric volume with no waste allowance added — use the calculator for a number that matches your exact dimensions.
Worked example
- A 24 × 22 ft garage floor, poured 4 inches thick: 24 × 22 = 528 ft² of slab area.
- Convert thickness to feet: 4 ÷ 12 = 0.333 ft.
- Volume: 528 × 0.333 = 175.9 ft³.
- Convert to cubic yards: 175.9 ÷ 27 = 6.51 yd³.
- Add 5% waste: 6.51 × 1.05 = 6.84 yd³, which rounds up to about 7 cubic yards to order.
Result: order roughly 7 cubic yards of ready-mix concrete, or budget around 293 bags of 80 lb pre-mix if you were pouring by hand — which is exactly why floors this size are almost always poured with a ready-mix truck instead.
Assumptions this calculator makes
- Concrete weighs approximately 150 lb per cubic foot (standard-weight mix, no lightweight aggregate).
- 80 lb bags yield about 0.6 ft³ of concrete each; 60 lb bags yield about 0.45 ft³ each.
- The slab is a uniform rectangle at a single, constant thickness — thickened edges, footings, or a curb are not included.
- Your waste percentage covers sub-base irregularities and normal spillage, not major excavation errors.
Common mistakes
- Forgetting to convert thickness to feet. Thickness is almost always given in inches — leaving it in inches in the volume formula inflates the result by 12x.
- Skipping the waste allowance. Uneven sub-base and spillage routinely use 5–10% more concrete than the flat geometric calculation.
- Ordering in fractional cubic yards. Most ready-mix suppliers sell in quarter-yard increments and may charge a short-load fee below a minimum order — round up and confirm with your supplier.
- Ignoring thickened edges. If your garage floor has a thickened perimeter footing (common where the slab doubles as a foundation), that extra concrete isn’t captured by a flat-slab calculation and should be added separately.
FAQ
How thick should a garage floor slab be?
Most residential garage floors are poured 4 inches thick. If you plan to park heavier vehicles, RVs, or lifts on the slab, many contractors recommend 5 to 6 inches with reinforcement.
How many bags of concrete do I need for a garage floor?
Bagged concrete is rarely practical for a full garage floor because of the volume involved. For a typical 20×20 ft, 4-inch slab you would need well over 200 bags, so most garage floors are poured with ready-mix delivered by truck instead.
Should I add extra concrete for waste?
Yes. Sub-base irregularities, spillage, and slightly uneven excavation typically use 5 to 10 percent more concrete than the pure geometric volume, so most estimators add a waste allowance before ordering.
Do I need to order concrete in full cubic yard increments?
Ready-mix suppliers usually sell in quarter-yard increments and often charge a short-load fee below a minimum order, commonly around 1 cubic yard, so it helps to confirm your supplier’s minimum before finalizing an order.