Ordering ready-mix by the truck or buying bags at the store both start with the same question: how many cubic yards of concrete do you need? This concrete yardage calculator works out the volume for a slab, footing, wall, or round column, adds an optional waste allowance, and shows the equivalent number of 40, 60, and 80 lb bags — so you can order with confidence.
Built by Anchor AI Tools, it uses the standard volume formula (length × width × thickness ÷ 27) and shows the working, so you understand the number rather than just copying it.
Quick Answer: To find concrete yardage, multiply length × width × thickness in feet, then divide by 27 to convert cubic feet to cubic yards. For example, a 10 ft × 10 ft slab at 4 inches thick is 10 × 10 × 0.333 ÷ 27 = 1.23 cubic yards. Always add about 10% for waste and spillage. The formula is Cubic Yards = (L × W × T) ÷ 27, with all measurements in feet.
Pick the shape you are pouring, enter the dimensions, and the calculator returns the volume in cubic yards and cubic feet, plus estimated bag counts. Tick the waste box to add a 10% allowance for spillage and uneven subgrade.
Volume in cubic yards for slabs, footings, walls, and round columns.
Measuring an irregular area first? Use the square footage calculator to get the area, then enter it here as length × width. For mixed projects, the main concrete calculator covers additional shapes.
These are the most-ordered slab sizes at a standard 4-inch thickness, shown before any waste allowance. Add roughly 10% when you place the order. To convert any other size, divide length × width × thickness (in feet) by 27.
| Slab Size | Thickness | Cubic Yards | +10% Waste |
|---|---|---|---|
| 8 × 8 ft | 4 in | 0.79 | 0.87 |
| 10 × 10 ft | 4 in | 1.23 | 1.36 |
| 12 × 12 ft | 4 in | 1.78 | 1.96 |
| 10 × 20 ft | 4 in | 2.47 | 2.72 |
| 20 × 20 ft | 4 in | 4.94 | 5.43 |
| 20 × 24 ft (2-car) | 4 in | 5.93 | 6.52 |
| 24 × 24 ft | 4 in | 7.11 | 7.82 |
| 12 × 12 ft | 6 in | 2.67 | 2.94 |
| 20 × 30 ft | 6 in | 11.11 | 12.22 |
A 10 × 10 slab at 4 inches is the most common single-pour lookup: it needs 1.23 cubic yards of concrete, or about 1.36 cubic yards with waste included.
Cubic Yards = (L × W × T) ÷ 27
All measurements in feet · 27 cubic feet = 1 cubic yardConcrete is ordered by the cubic yard, but slabs are measured in feet and inches. There are 27 cubic feet in one cubic yard, so once you have the volume in cubic feet you divide by 27. The only trick is converting thickness: 4 inches is 4 ÷ 12 = 0.333 feet.
For a round column or footing (like a Sonotube), the volume uses π: Cubic Feet = π × radius² × height, then divide by 27. The calculator above handles this automatically when you choose the Round Column shape.
For small pours it is often cheaper to buy bags than to order a ready-mix truck. Each bag lists a yield — the volume of wet concrete it produces. Standard yields are 0.60 cubic feet for an 80 lb bag, 0.45 for a 60 lb bag, and 0.30 for a 40 lb bag.
| Bag Size | Yield (cu ft) | Bags per cu yd | Best For |
|---|---|---|---|
| 80 lb | 0.60 | 45 | Larger DIY pours |
| 60 lb | 0.45 | 60 | General repairs |
| 40 lb | 0.30 | 90 | Small posts & patches |
Rule of thumb: it takes 45 bags of 80 lb concrete to make one cubic yard. Above roughly 1 cubic yard (about 45 bags), a ready-mix delivery is usually cheaper and far less labour than mixing by hand.
The calculator converts your cubic yards into 40, 60, and 80 lb bag counts automatically.
📋 12 × 12 ft slab at 4 inches
Convert thickness: 4 ÷ 12 = 0.333 ft. Volume: 12 × 12 × 0.333 = 48 cu ft. Yardage: 48 ÷ 27 = 1.78 cu yd. With 10% waste: about 1.96 cu yd.
📋 20 × 24 ft slab at 4 inches
Volume: 20 × 24 × 0.333 = 160 cu ft. Yardage: 160 ÷ 27 = 5.93 cu yd. Add 10%: order about 6.52 cu yd, which most suppliers round to 6.5 or 7 yards.
🛢️ Four 12-inch Sonotubes, 4 ft tall
Radius = 6 in = 0.5 ft. One tube: π × 0.5² × 4 = 3.14 cu ft. Four tubes: 12.57 cu ft ÷ 27 = 0.47 cu yd. Small enough to pour from bags — about 21 × 80 lb bags.
The formula gives the exact theoretical volume, but real pours never match it perfectly. Subgrade is rarely dead level, forms bow slightly, some concrete sticks in the chute or wheelbarrow, and a little always spills. Ordering the exact figure risks running short — and a second small delivery costs far more per yard than the first.
A 10% allowance covers most residential slabs. For rough or hand-dug subgrade, footings against uneven soil, or your first pour, 10–15% is safer. The calculator adds 10% when the waste box is ticked; untick it to see the exact theoretical volume.
| Project Type | Suggested Waste | Why |
|---|---|---|
| Formed slab, level base | 10% | Standard spillage |
| Footings on dug soil | 10–15% | Uneven trench walls |
| Round columns / tubes | 5–10% | Contained forms |
| First-time DIY pour | 15% | Extra margin for error |
This tool is used by homeowners pouring a patio, shed base, or driveway; DIY builders estimating bag counts before a hardware run; contractors and concrete finishers confirming a ready-mix order; landscapers setting posts and footings; and estimators pricing labour and materials. Anyone placing a concrete order needs the yardage first, because ready-mix is sold by the cubic yard and bags are sold by yield.
Planning the wider project too? Measure the area with the square footage calculator, and use the percentage calculator to add tax or a contingency to your materials budget.
Need to measure the area before you pour?
Use the Square Footage Calculator →Concrete Calculator
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Square Footage Calculator
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