To find how many yards of concrete you need, multiply length × width in feet by thickness in inches, then divide by 324. A 10 × 10 ft patio 4 inches thick is 1.23 cubic yards. The same calculation gives 33.3 ft³ and 56 bags of 80 lb premix. This concrete yard calculator also handles footings, walls, round columns and steps. It adds a waste allowance and converts the result into cubic feet, cubic meters, bag counts for 40, 50, 60 and 80 lb premix, and the weight you will be hauling or pumping.
It is for homeowners pouring a patio, shed base or post footings, DIYers deciding between bags and a truck, and contractors double-checking an order before calling the plant. You need the dimensions of the pour. The calculator does not need a mix design, because the volume is the same whatever the strength.
How to use the concrete calculator
- Choose the shape. Use "Slab" for patios, driveways and shed bases, and "Slab by square feet" when you know the area but not the sides. Pick "Footing" for a continuous trench footing, "Wall" for a poured wall, "Round column" for piers and tubes, and "Steps" for a solid stair.
- Enter the dimensions. Lengths are in feet. Thickness, footing width and depth, column diameter and step sizes are in inches, which is how forms are usually measured.
- Enter how many identical pours there are, such as 6 deck piers or 2 matching pads.
- Set the extra percentage. The default 10% is our planning assumption, not a standard.
- Read the result. You get cubic yards to order, cubic feet, cubic meters, bag counts and weight. The chart shows how far a yard goes at common slab thicknesses.
Concrete formula: cubic yards from feet and inches
Volume is length × width × thickness, with everything in feet. Since 1 cubic yard is 27 cubic feet (NIST Handbook 44, Appendix C), cubic yards = L × W × (T ÷ 12) ÷ 27. Combining the 12 and the 27 gives the shortcut yd³ = L × W × T ÷ 324 when L and W are in feet and T is in inches. Here is how each shape works:
- Slab or footing: length × width × depth. A footing that is 16 in wide and 8 in deep is 16/12 × 8/12 = 0.889 ft² in cross-section, so every foot of footing holds 0.889 ft³.
- Wall: length × height × thickness.
- Round column: π × (diameter ÷ 2)² × height. A 12 in column holds 0.785 ft³ per foot.
- Steps: each step is a block from the ground to its tread. With n steps the stacked heights add up to rise × n(n + 1) ÷ 2. A top landing adds width × landing depth × full height.
To convert to bags, divide cubic feet by the bag yield and round up. QUIKRETE's Concrete Mix data sheet lists these approximate yields:
| Bag | Yield | Bags per ft³ | Bags per yd³ | Bags for a 10×10 slab, 4 in |
|---|---|---|---|---|
| 40 lb | 0.3 ft³ | 3.33 | 90 | 112 |
| 50 lb | 0.375 ft³ | 2.67 | 72 | 89 |
| 60 lb | 0.45 ft³ | 2.22 | 60 | 75 |
| 80 lb | 0.6 ft³ | 1.67 | 45 | 56 |
How many square feet does a yard of concrete cover?
Divide 324 by the thickness in inches. This table is the quickest check of a ready-mix order: divide your slab's area by the figure for its thickness.
| Thickness | ft² per yd³ | yd³ per 100 ft² | yd³ per 1,000 ft² |
|---|---|---|---|
| 3 in | 108 | 0.926 | 9.26 |
| 3.5 in | 92.6 | 1.08 | 10.8 |
| 4 in | 81 | 1.235 | 12.35 |
| 5 in | 64.8 | 1.543 | 15.43 |
| 6 in | 54 | 1.852 | 18.52 |
| 8 in | 40.5 | 2.469 | 24.69 |
| 10 in | 32.4 | 3.086 | 30.86 |
| 12 in | 27 | 3.704 | 37.04 |
Concrete calculator by square feet: common slab sizes
The figures below are exact volumes with no waste added. Add your own allowance before ordering.
| Slab | Area | 4 in | 5 in | 6 in | 80 lb bags at 4 in |
|---|---|---|---|---|---|
| 8 × 8 ft | 64 ft² | 0.79 yd³ | 0.99 yd³ | 1.19 yd³ | 36 |
| 10 × 10 ft | 100 ft² | 1.23 yd³ | 1.54 yd³ | 1.85 yd³ | 56 |
| 10 × 12 ft | 120 ft² | 1.48 yd³ | 1.85 yd³ | 2.22 yd³ | 67 |
| 12 × 12 ft | 144 ft² | 1.78 yd³ | 2.22 yd³ | 2.67 yd³ | 80 |
| 12 × 16 ft | 192 ft² | 2.37 yd³ | 2.96 yd³ | 3.56 yd³ | 107 |
| 16 × 20 ft | 320 ft² | 3.95 yd³ | 4.94 yd³ | 5.93 yd³ | 178 |
| 20 × 20 ft | 400 ft² | 4.94 yd³ | 6.17 yd³ | 7.41 yd³ | 223 |
| 20 × 24 ft | 480 ft² | 5.93 yd³ | 7.41 yd³ | 8.89 yd³ | 267 |
| 24 × 24 ft | 576 ft² | 7.11 yd³ | 8.89 yd³ | 10.67 yd³ | 320 |
| 30 × 30 ft | 900 ft² | 11.11 yd³ | 13.89 yd³ | 16.67 yd³ | 500 |
| 30 × 40 ft | 1200 ft² | 14.81 yd³ | 18.52 yd³ | 22.22 yd³ | 667 |
Worked examples
Shed base: 12 × 16 ft, 4 in thick
12 × 16 × 4 ÷ 324 = 2.37 yd³ (64 ft³). With 10% extra, 2.61 yd³. In bags, it would take 118 bags of 80 lb mix, so this is a ready-mix job.
Two-car driveway pad: 24 × 20 ft, 5 in thick
24 × 20 × 5 ÷ 324 = 7.41 yd³. At the typical 145 lb/ft³, the concrete weighs about 14.5 US tons, which is why driveways are poured from a truck chute or pump.
House footing: 120 linear ft, 16 in wide × 8 in deep
120 × (16 ÷ 12) × (8 ÷ 12) = 106.7 ft³ = 3.95 yd³. Trenches dug by machine are rarely exact. Measure the real width in a few places and use the average, or raise the waste percentage.
Retaining wall: 30 ft long, 4 ft high, 8 in thick
30 × 4 × (8 ÷ 12) = 80 ft³ = 2.96 yd³, or 2.27 m³.
Six deck piers: 12 in diameter, 4 ft deep
Each pier holds 3.14 ft³, so six hold 18.8 ft³ = 0.7 yd³, which is 32 bags of 80 lb mix. For tube forms with a flared base, use the Sonotube calculator.
Three front steps: 7 in rise, 11 in tread, 4 ft wide
4 × (11 ÷ 12) × (7 ÷ 12) × (1 + 2 + 3) = 12.83 ft³ = 0.475 yd³, or 22 bags of 80 lb mix.
How much does concrete weigh?
FHWA's lightweight-concrete primer puts normal-weight concrete at 140 to 155 lb per cubic foot, with 145 lb/ft³ typical. Per cubic yard that is 3,780 to 4,185 lb, or 1.89 to 2.09 US tons. QUIKRETE reports a unit weight of about 140 lb/ft³ for its Concrete Mix. Weight matters when you plan a trailer load of bags, check what a deck or floor can carry before the concrete cures, or work out how much a mixer truck puts on your driveway.
| Volume | Weight at 140 lb/ft³ | at 145 lb/ft³ | at 155 lb/ft³ |
|---|---|---|---|
| 0.25 yd³ | 945 lb | 979 lb | 1,046 lb |
| 0.5 yd³ | 1,890 lb | 1,958 lb | 2,093 lb |
| 1 yd³ | 3,780 lb | 3,915 lb | 4,185 lb |
| 2 yd³ | 7,560 lb | 7,830 lb | 8,370 lb |
| 3 yd³ | 11,340 lb | 11,745 lb | 12,555 lb |
| 5 yd³ | 18,900 lb | 19,575 lb | 20,925 lb |
| 10 yd³ | 37,800 lb | 39,150 lb | 41,850 lb |
Bags or ready-mix?
Bagged premix is easiest for post holes, small pads and repairs. You can mix it in a wheelbarrow and pour at your own pace. The weight adds up quickly, though. A yard needs 45 bags of 80 lb mix, which is 3,600 lb of dry material to lift twice (onto the cart, then into the mixer). Ready-mix arrives already batched and all at once, so you need enough help and tools to place and finish it before it sets. Many plants charge extra for small loads. Compare the bag count from this calculator with a quote that includes any short-load fee. Our concrete slab cost calculator lays out both options line by line.
Common mistakes
- Entering thickness in feet. A 4 in slab is 0.333 ft, not 4 ft. This calculator takes inches for thickness to prevent that.
- Using the nominal slab thickness on uneven ground. If the subgrade dips, the slab is thicker there. Check depth at several points.
- Forgetting thickened edges. Slabs with turned-down edges need the edge beam added as a footing.
- Rounding bags before totalling. Add all volumes first, then divide by the bag yield and round up once.
- Mixing up bag weight and yield. An 80 lb bag makes 0.60 ft³, not 0.8 or 1 ft³. Check the yield on the bag if you use another brand or product.
Related tools: cubic yard calculator for gravel base and fill under the slab, cinder block calculator for block walls and grout fill, and cubic feet calculator for other shapes.
Frequently asked questions
How many yards of concrete do I need for a 10×10 slab?
A 10 × 10 ft slab 4 inches thick is 10 × 10 × 4 ÷ 12 = 33.3 ft³, or 1.23 cubic yards. With 10% extra, order 1.36 yd³. Most ready-mix plants sell in quarter- or half-yard steps, so round up to the next step they offer.
How many square feet does a yard of concrete cover?
One cubic yard covers 324 ÷ thickness in inches: 81 ft² at 4 in, 64.8 ft² at 5 in and 54 ft² at 6 in. The 324 comes from 27 ft³ × 12 in per ft.
How many 80 lb bags of concrete make a yard?
QUIKRETE lists an 80 lb bag of Concrete Mix as yielding about 0.60 ft³, so a 27 ft³ yard needs 45 bags. That is 60 bags of 60 lb, 72 bags of 50 lb, or 90 bags of 40 lb.
How do I calculate concrete by square feet?
Multiply the square feet by the thickness in inches and divide by 324 to get cubic yards. For 500 ft² at 4 in: 500 × 4 ÷ 324 = 6.17 yd³. Choose "Slab by square feet" in the calculator when you already know the area.
How much does a yard of concrete weigh?
FHWA gives normal-weight concrete a density of 140–155 lb/ft³, typically 145. One cubic yard therefore weighs 3,780–4,185 lb, about 1.96 US tons at the typical figure. QUIKRETE's bagged Concrete Mix tests at about 140 lb/ft³.
Should I order extra concrete?
Yes, a little. Subgrades are never perfectly flat, forms bow, and some concrete stays in the truck, wheelbarrow or pump. This calculator defaults to 10% extra as a planning assumption; reduce it for small, carefully formed bag pours, and raise it for uneven ground or long pump lines. Running short mid-pour is worse than leftover.
How much concrete for a 24×20 driveway?
At 5 inches thick, 24 × 20 × 5 ÷ 12 = 200 ft³ = 7.41 yd³, or 8.15 yd³ with 10% extra. At that size, ready-mix delivery is far more practical than the 334 bags of 80 lb mix it would take.
How many bags of concrete for a post hole or pier?
A 12 in diameter pier 4 ft deep holds 3.14 ft³, which is 6 bags of 80 lb mix. For tube forms, bell footings and post displacement use the Sonotube concrete calculator.
How do I figure concrete for steps?
Treat each step as a block as wide as the stair, one tread deep, and as tall as its height from the ground. Total = width × tread × rise × n(n+1)/2. Three 7 in steps with 11 in treads, 4 ft wide, need 12.83 ft³ = 0.475 yd³ (22 bags of 80 lb mix).
When should I use bags instead of ready-mix?
Do the arithmetic both ways. Half a yard is already 23 bags of 80 lb mix, about 0.9 tons to lift at QUIKRETE's 140 lb/ft³, mixed batch by batch. Many people switch to ready-mix somewhere around a yard, but ask the plant about its minimum order and short-load fee first.
Sources & method
- NIST Handbook 44 (2026), Appendix C: 1 yd³ = 27 ft³ = 0.764 554 857 984 m³
- QUIKRETE Concrete Mix No. 1101 product data sheet: 40, 50, 60, 80 lb bag yields and 140 lb/ft³ unit weight
- FHWA HIF-19-067, Lightweight Concrete Bridge Design Primer (2021): normal-weight concrete 0.140–0.155 kcf
Results are estimates for general information. Found an error? It helps everyone — see our methodology.