How Many Bags of Concrete Do I Need for a 10×20 Slab?
The standard 10 × 20 ft patio slab is 4 inches thick. Here is the short answer, with the full table for 4 in and 6 in below.
Exact count before waste is 112 × 80 lb bags. The 123-bag figure includes the standard 10% allowance for spillage, uneven ground, and forms that are not perfectly level. Running short mid-pour leaves a weak cold joint, so the cushion is worth it.
Bags to buy for a 10×20 slab, 4 in thick (with 10% waste)
10×20 Slab Bag Table: 4 in and 6 in, by Bag Size
Exact counts are the mathematical minimum, rounded up. The "with waste" counts multiply the volume by 1.10 before dividing, which is the number to actually buy. Yields: 80 lb bag = 0.60 ft³, 60 lb = 0.45 ft³, 40 lb = 0.30 ft³.
| Thickness | Volume | 80 lb exact | 80 lb +10% | 60 lb exact | 60 lb +10% | 40 lb exact | 40 lb +10% |
|---|---|---|---|---|---|---|---|
| 4 in | 66.7 ft³ (2.47 yd³) | 112 | 123 | 149 | 163 | 223 | 245 |
| 6 in | 100.0 ft³ (3.70 yd³) | 167 | 184 | 223 | 245 | 334 | 367 |
Which thickness? 4 inches is the standard for patios, walkways, and shed pads. Go to 6 inches for driveways, garage floors, and anywhere vehicles park. That jump takes the pour from 2.47 yd³ to 3.70 yd³, and from 123 bags to 184 bags of 80 lb mix with waste.
How the Math Works
A slab is a simple box: length × width × thickness. The only trap is mixed units, so convert the thickness from inches to feet first.
- Convert thickness: divide inches by 12. (4 in = 0.333 ft, 6 in = 0.5 ft.)
- Multiply: 10 ft × 20 ft × thickness in feet = cubic feet.
- Cubic yards: divide cubic feet by 27.
- Add waste: multiply cubic feet by 1.10 for a 10% cushion.
- Bags: divide by the yield printed on the bag and round up.
Volume (ft³) = L × W × (T ÷ 12) · Bags = ft³ ÷ yield, rounded up
Worked example at 4 in: 10 × 20 × (4 ÷ 12) = 66.7 ft³, which is 66.7 ÷ 27 = 2.47 yd³. Bags of 80 lb mix: 66.7 ÷ 0.60 = 111.1, so 112 bags exact. With waste: 66.7 × 1.10 = 73.3 ft³, and 73.3 ÷ 0.60 = 122.2, so 123 bags.
At 6 in: 10 × 20 × 0.5 = 100.0 ft³ (3.70 yd³). Bags: 100 ÷ 0.60 = 166.7, so 167 bags exact, or 184 with the 10% cushion.
When Bags Stop Making Sense: The 1 yd³ Crossover
Bags usually win under about 1 cubic yard. A 10×20 slab is well past that line: 2.47 yd³ at 4 inches, 3.70 yd³ at 6 inches. So for this particular pour, ready-mix is almost always the right call.
Why the crossover exists: ready-mix suppliers bill a 1 yd³ minimum. On a small pour you pay for a full yard you do not use, plus a delivery fee, and often a short-load surcharge on top. That padding is what makes bags cheaper under a yard. Once your pour is genuinely over a yard, you are using what you paid for and the truck's per-yard price beats the per-bag price.
At the example prices used on the main calculator ($6.50 per 80 lb bag, $160 per cubic yard, $75 delivery):
| 10×20 slab | Bags (with waste) | Ready-mix truck | Winner |
|---|---|---|---|
| 4 in thick | 123 bags = $799.50 | 2.72 yd³ billed = $509.57 | Ready-mix by ~$290 |
| 6 in thick | 184 bags = $1,196.00 | 4.07 yd³ billed = $726.85 | Ready-mix by ~$469 |
Then there is labor. 123 bags at 80 lb is 9,840 lb of material you lift, mix, and wheelbarrow; 184 bags is 14,720 lb. That is a multi-day mixing marathon, and concrete does not wait while you catch up — a slow bag-by-bag pour risks cold joints across a slab this size. A truck places the same pour in under an hour. Bags still make sense for a 10×20 when the truck cannot reach the pour (tight backyard access), when you want to work in stages over several days, or when local ready-mix quotes come in far above the example prices. Run your own quotes through the cost comparison on the calculator before deciding.
Pouring a Different Size?
These numbers are only exact for a 10 × 20 slab. For any other footprint, thickness, or shape, use the free calculator: it figures cubic yards, cubic feet, and bag counts live, and compares bag cost against a ready-mix quote with your local prices. Pouring a 10 × 10 instead? See the 10×10 slab bag guide.
10×20 Slab FAQ
How many bags of concrete do I need for a 10x20 slab 4 inches thick?
123 bags of 80 lb mix to buy (112 exact, plus 10% waste). In 60 lb bags: 163 to buy, 149 exact. The slab holds 66.7 ft³, or 2.47 yd³, of concrete.
How many bags do I need for a 10x20 slab 6 inches thick?
184 bags of 80 lb mix to buy (167 exact, plus 10% waste). In 60 lb bags: 245 to buy, 223 exact. At 6 inches the slab holds 100.0 ft³, or 3.70 yd³.
How many cubic yards is a 10x20 slab?
At the standard 4 in thick, a 10 × 20 slab is 2.47 yd³ exactly, or about 2.72 yd³ with the usual 10% waste allowance. At 6 in thick it is 3.70 yd³ exactly, about 4.07 yd³ with waste. Either way it is well past the 1 yd³ point where a ready-mix truck starts beating bags on price.
For a 10x20 slab, is it cheaper to buy bags or order ready-mix?
Almost always the truck. A 10×20 slab is more than double the 1 cubic yard crossover at any standard thickness. At typical prices the 4 in pour runs about $800 in bags versus $510 delivered ready-mix, and the truck also saves you mixing nearly 5 tons of concrete by hand. Bags can still win if a truck cannot reach the site or your local ready-mix prices are unusually high. Check with your own quotes in the cost comparison.
Estimates Only
Results are planning estimates. Actual concrete needs vary with ground conditions, form accuracy, over-excavation, and product yield. Bag yields follow manufacturer figures for standard mixes; your product may differ slightly. Always order a little extra — running short mid-pour creates a weak cold joint. For structural work (foundations, load-bearing footings, retaining walls), consult a licensed professional and follow your local building code.