Concrete Yardage by Shape
Most concrete yardage guides assume you are pouring a rectangle. Real projects are rarely that tidy. You might be setting round columns on sonotubes, digging strip footings for a foundation, forming a curb, or pouring a circular pad for a fire pit. Every one of these still comes back to the same idea, which is find the volume in cubic feet and divide by 27 to get cubic yards, but the way you find that volume changes with the shape.
This guide walks through the shapes that come up most often, with the formula and a worked example for each. If you want the plain rectangular-slab walkthrough and the reasoning behind the divide-by-27 step, start with how to figure concrete yardage and come back here for the odd shapes.
One rule applies to every calculation below: after you get the cubic-yard figure, add 10 to 15 percent for waste before you order. Uneven ground, bowing forms, and spillage all eat concrete, and coming up short mid-pour is the mistake worth paying to avoid.
Slabs and rectangular pads
A slab is the baseline shape, and the formula is length times width times thickness, all in feet, divided by 27.
Cubic yards = length (ft) x width (ft) x thickness (ft) ÷ 27
Convert the thickness from inches to feet first by dividing by 12, since a slab is usually specified in inches. A 4-inch slab is 0.33 feet, a 6-inch slab is 0.5 feet.
Worked example, a 16 x 20 ft slab at 5 inches:
- Thickness in feet: 5 ÷ 12 = 0.42
- Volume: 16 x 20 x 0.42 = 134.4 cubic feet
- Cubic yards: 134.4 ÷ 27 = 4.98, so about 5 yards before waste
For a full slab this deep and detailed, the concrete slab calculator will run it for you, but the math above is all there is to it.
Calculating slab yardage from square feet
If you already know a slab's area but not its two side lengths, you can still get the yardage. At a 4-inch thickness, one cubic yard covers about 81 square feet, so divide the square footage by 81. At 6 inches, a yard covers about 54 square feet, so divide by 54 instead. The coverage per yard changes with thickness, so match the divisor to the depth you are pouring. This shortcut is handy for irregular slabs where you measured the area but not clean rectangles.
Strip and pad footings
Footings are just long, narrow rectangles below grade, so they use the same slab formula: length times width times depth, divided by 27. The trick with footings is that the width and depth are the small numbers and the length is long, so keep your units straight.
Worked example, a strip footing 40 ft long, 16 in wide, 8 in deep:
- Width in feet: 16 ÷ 12 = 1.33
- Depth in feet: 8 ÷ 12 = 0.67
- Volume: 40 x 1.33 x 0.67 = 35.6 cubic feet
- Cubic yards: 35.6 ÷ 27 = 1.32, so about 1.3 yards before waste
If a foundation has several footing runs, work out each run separately and add the cubic yards together at the end. The concrete footing calculator handles multiple runs in one go.
Round columns and sonotubes
Round shapes trade length times width for the area of a circle, which is π times the radius squared. The radius is half the diameter. The full formula for a column or a sonotube is:
Cubic yards = π x radius (ft)² x height (ft) ÷ 27
Inch Calculator gives this same formula for round footings and columns, noting that for a 4-foot-diameter shape the radius is 2 feet (Inch Calculator).
Worked example, a sonotube 12 in in diameter and 4 ft tall:
- Radius in feet: 6 in = 0.5 ft
- Area of the circle: π x 0.5² = 3.14159 x 0.25 = 0.785 sq ft
- Volume: 0.785 x 4 = 3.14 cubic feet
- Cubic yards: 3.14 ÷ 27 = 0.116, so about 0.12 yards per tube
Columns come in multiples, so figure one and multiply by the count. Six of those sonotubes would be 6 x 0.12 = 0.7 yards, and you would round up and add waste from there. Because each tube is a small fraction of a yard, a run of piers often lands under a full yard, which means a short-load fee is worth planning around.
Round slabs and circular pads
A circular pad, like a base for a fire pit or a grain bin, uses the same circle area as a column but with the pad's thickness as the height.
Cubic yards = π x radius (ft)² x thickness (ft) ÷ 27
Worked example, a 10 ft diameter pad at 4 in thick:
- Radius: 5 ft
- Area: π x 5² = 3.14159 x 25 = 78.5 sq ft
- Volume: 78.5 x 0.33 = 25.9 cubic feet
- Cubic yards: 25.9 ÷ 27 = 0.96, so just under 1 yard before waste
Walls
A wall is a slab stood on its edge, so the formula is length times height times thickness, divided by 27. Keep thickness in feet as always.
Worked example, a wall 24 ft long, 3 ft tall, 8 in thick:
- Thickness in feet: 8 ÷ 12 = 0.67
- Volume: 24 x 3 x 0.67 = 48.2 cubic feet
- Cubic yards: 48.2 ÷ 27 = 1.79, so about 1.8 yards before waste
Curbs and irregular shapes
Curbs, steps, and other odd profiles are best handled by breaking them into simple boxes, figuring each box on its own, and adding the results. A curb with a wide base and a narrower top, for example, splits into a bottom rectangle and a top rectangle. This break-it-into-boxes approach is exactly what experienced hands recommend on contractor forums when someone asks how to figure square-yard needs for an irregular pour (TractorByNet). Stairs work the same way, with each step treated as its own box stacked on the one below.
Shape formulas at a glance
| Shape | Volume formula (cubic feet) | Then |
|---|---|---|
| Slab or pad | length x width x thickness | ÷ 27 |
| Footing | length x width x depth | ÷ 27 |
| Round column or sonotube | π x radius² x height | ÷ 27 |
| Round slab or circular pad | π x radius² x thickness | ÷ 27 |
| Wall | length x height x thickness | ÷ 27 |
| Curb, stairs, irregular | split into boxes, sum each | ÷ 27 |
Every row ends with the same divide-by-27, because every one is turning cubic feet into the cubic yards concrete is ordered in. Get the volume right for the shape and the rest is identical.
Let the calculator handle the geometry
Circles and multi-part shapes are where hand calculations go wrong, usually from squaring a diameter instead of a radius, or forgetting to convert inches to feet. The concrete yardage calculator has the formula for each shape built in, so you pick the shape, enter the dimensions, and it returns the cubic yards with a waste factor already included. For the reasoning behind the core formula and a rectangular walkthrough, see how to figure concrete yardage, and for how a yard translates into bags, trucks, weight, and cost, the concrete yard hub links out to all of it.
Sources
- Inch Calculator, Concrete Footing Calculator - the round-column formula π x radius² x depth ÷ 27 and the 10 to 15 percent overage recommendation.
- Concrete Network, How Much Concrete Do I Need - the divide-square-feet-by-81 coverage shortcut for a 4-inch slab.
- TractorByNet forum, Formula to Figure Concrete Square Yard Needs - contractor discussion on splitting irregular pours into simple shapes.
Frequently asked questions
How do you calculate concrete slab yardage?
Multiply the slab's length, width, and thickness in feet, then divide by 27. Convert an inch thickness to feet first by dividing by 12. A 16 x 20 ft slab at 5 inches works out to about 5 cubic yards before waste.
How do you figure concrete yardage for a round column or sonotube?
Use π times the radius squared times the height, then divide by 27. The radius is half the diameter, so a 12-inch tube has a 0.5-foot radius. A 12-inch tube 4 feet tall is about 0.12 cubic yards.
How do I calculate concrete yardage for footings?
Treat each footing as a long rectangle: length times width times depth in feet, divided by 27. Figure each footing run separately and add the cubic yards together for the total.
How do you figure concrete yardage for a circle?
Find the area of the circle with π times the radius squared, multiply by the thickness in feet, then divide by 27. A 10-foot-diameter pad at 4 inches is just under 1 cubic yard.
Can I calculate concrete yardage from square feet?
Yes, at a fixed thickness. Divide the area by 81 for a 4-inch pour or by 54 for a 6-inch pour, since those are the square feet one cubic yard covers at each depth. For any other thickness, use the full length by width by thickness formula.