How Much Concrete Do I Need? Slab Calculator Guide
For a rectangular slab, concrete volume = length × width × thickness, all in the same units. A 10 × 10 ft slab poured 4 inches thick needs 33.3 cubic feet — 1.24 cubic yards — so order 1.5 cubic yards of ready-mix or buy 56 eighty-pound bags. Below: the full method, a size table, and the waste factor pros use.
What is the concrete volume formula?
The formula is simple: Volume = Length × Width × Depth — but all three measurements must share one unit before you multiply. Thickness is usually given in inches while length and width are in feet, so divide inches by 12 first: a 4-inch slab is 0.333 ft thick, a 6-inch slab is 0.5 ft. Then multiply.
For an L-shaped patio or curved walkway, split the shape into rectangles, calculate each, and add the volumes. Our Concrete Calculator handles the unit conversion, so you can enter feet and inches directly.
How do you convert cubic feet to cubic yards?
Ready-mix is sold by the cubic yard, but your calculation gives cubic feet. Since 1 cubic yard = 27 cubic feet (a 3 × 3 × 3 ft cube), divide by 27: 54 cubic feet ÷ 27 = 2 cubic yards.
For bags, one 80 lb bag yields about 0.6 cubic feet, so divide cubic feet by 0.6 for the bag count (a 60 lb bag yields about 0.45). Measured in meters? Run the dimensions through our Unit Converter first so every unit matches.
Worked example: a 10 × 10 ft slab, 4 inches thick
The most common DIY slab — a 10 by 10 ft patio, 4 inches thick — step by step:
Step 1 — Convert thickness to feet: 4 ÷ 12 = 0.333 ft.
Step 2 — Multiply length × width × depth: 10 × 10 × 0.333 = 33.33 cubic feet.
Step 3 — Convert to cubic yards: 33.33 ÷ 27 = 1.24 cubic yards.
Step 4 — Add a waste factor. Pros add 5–10% for spillage, uneven subgrade, and imperfect forms: 1.24 × 1.10 = 1.36 cubic yards. Ready-mix is ordered in quarter-yard steps, so order 1.5 cubic yards. Running short mid-pour creates a cold joint — a permanent weak seam — so a little extra beats too little.
Step 5 — Or count bags instead: 33.33 ÷ 0.6 = 55.6, so 56 eighty-pound bags before waste; with a 10% waste factor, buy about 62 bags.
You can verify the whole chain in seconds with the Concrete Calculator — enter the same dimensions and compare each intermediate result.
How much concrete do common slab sizes need?
Typical residential slabs below. Bag counts are 80 lb bags, rounded up, before the 5–10% waste factor — always add waste on top.
| Slab size | Thickness | Cubic feet | Cubic yards | 80-lb bags |
|---|---|---|---|---|
| 8 × 8 ft shed pad | 4 in | 21.3 | 0.79 | 36 |
| 10 × 10 ft patio | 4 in | 33.3 | 1.24 | 56 |
| 12 × 12 ft patio | 4 in | 48.0 | 1.78 | 80 |
| 12 × 16 ft patio | 4 in | 64.0 | 2.37 | 107 |
| 16 × 16 ft garage apron | 4 in | 85.3 | 3.16 | 143 |
| 12 × 24 ft driveway section | 4 in | 96.0 | 3.56 | 160 |
| 20 × 20 ft slab | 4 in | 133.3 | 4.94 | 223 |
| 10 × 10 ft, heavy use | 6 in | 50.0 | 1.85 | 84 |
Notice how fast the numbers grow: a 20 × 20 ft slab needs nearly 5 cubic yards. Above roughly 1 cubic yard, ready-mix is usually cheaper than bags — and 56 bags is over two tons to mix by hand.
Should you buy bags or order ready-mix?
Bagged mix suits small jobs: footings, fence posts, or repairs under about half a cubic yard. Buy exactly what you need and mix on the spot — no delivery scheduling.
Past roughly 1 cubic yard — about 45 eighty-pound bags — ready-mix wins on price per yard and labor. The truck mixes, you pour in one continuous placement, and the slab ends up stronger and more uniform than dozens of hand-mixed batches. Warning: most suppliers charge a short-load fee below their minimum (often 3–4 cubic yards), so get the total delivered price, not just the per-yard rate.
Practical tips for a slab that lasts
- Compact the subgrade. Loose soil settles and cracks the slab from below. Tamp gravel in 2-inch lifts until firm and level with your forms.
- Cut control joints. Concrete shrinks as it cures and will crack somewhere — joints decide where. For a 4-inch slab, space them 8–12 ft apart (about 2–3× the thickness in inches, in feet) and cut 1 inch deep.
- Order slightly more, never less. A cold joint — fresh concrete meeting concrete that has started to set — is a permanent weak line. That is what the 5–10% waste factor prevents.
- Pour early and block the wind. Sun and wind dry the surface faster than the interior, causing fine crazing cracks. Morning pours and a windbreak on hot days make finishing far easier.
- Cure it for a week. Concrete gains strength by staying moist, not by drying out. Mist the surface and cover with plastic or wet burlap for at least 7 days — the strength difference is dramatic.
- Slope for drainage. About a quarter inch of fall per foot (roughly 2%) keeps water off the patio and away from the foundation.
Try it yourself
Enter your slab's dimensions to get cubic yards and bag counts instantly.
Frequently asked questions
How thick should a concrete slab be?
Four inches is standard for patios, walkways, and shed pads carrying only foot traffic and light loads. Go five to six inches for driveways and garage floors, and thicken edges to eight inches at the driveway apron. Anything structural — a foundation or load-bearing wall — needs an engineer's specification, not a rule of thumb.
How many 80 lb bags of concrete do I need for a 10×10 slab?
At four inches thick, a 10 × 10 ft slab is 33.3 cubic feet, and each 80 lb bag yields 0.6 cubic feet — 56 bags before waste. Buy 60–62 bags to cover the 5–10% waste factor. At six inches thick the same slab needs 84 bags before waste.
What is the concrete waste factor, and why does it matter?
The waste factor is the extra 5–10% pros add to the theoretical volume. Subgrade is never perfectly level, forms flex, and concrete sticks to the mixer, wheelbarrow, and tools. Skipping it risks running short mid-pour, creating a cold joint — a visible, weaker seam where new concrete meets concrete already setting.
How do I convert cubic feet to cubic yards?
Divide cubic feet by 27, since one cubic yard is a 3 × 3 × 3 ft cube: 54 cubic feet ÷ 27 = 2 cubic yards. Reverse it by multiplying by 27. For bags, divide cubic feet by 0.6 (80 lb bags) or 0.45 (60 lb bags).
Is it cheaper to mix bags or order ready-mix concrete?
Under about half a cubic yard, bags are cheaper and more convenient. Above roughly one cubic yard — about 45 eighty-pound bags — ready-mix usually wins on price per yard and saves enormous labor. Always ask about short-load fees on small orders, since minimum charges can erase the savings.