Concrete Estimate

Ordering concrete feels like a volume problem until the truck arrives short. The gap between a clean geometric calculation and a real pour is consistent and predictable: uneven subgrade, form flex, and chute waste each steal a slice of your volume. Adding a structured waste factor before you place the order is the standard way practitioners close that gap without guessing.

For a rectangular slab or footing, the calculation is length times width times depth, all in the same unit, divided by 27 to convert cubic feet to cubic yards. For a round column or tube form, it is the circle area of the cross-section (pi times radius squared) multiplied by the height. Both formulas assume the form is perfectly plumb and holds its shape under the weight of wet concrete, which is a reasonable working assumption for standard forming materials.

The waste factor multiplies the entire net volume, not just a fixed allowance. That matters because a 10% allowance on a large commercial pour represents much more concrete than the same percentage on a single footing. Scaling the overage proportionally keeps your order rational across jobs of any size.

Use this tool for any poured-in-place concrete work where you are placing an order: residential slabs, footings, driveways, sidewalks, stairs, deck post bases, and small retaining wall foundations. The estimate is reliable when your forms are built to the entered dimensions and the subgrade is reasonably level and compacted.

This tool is not appropriate for structural design calculations. It tells you how much to order, not whether the thickness and mix design are adequate for the load. For engineered structures — beams, columns carrying point loads, post-tension slabs — consult a structural engineer before pouring. The volume estimate from this tool can still be a useful input to that process, but it does not replace the engineering.

Also reconsider the estimate when pouring over very uneven or soft ground. The tool assumes the bottom of the form is a flat plane. A rough excavation with hollows and humps can add 10 to 20 percent of extra volume that a flat-bottom calculation misses entirely. In that situation, increase the waste factor or measure the actual subgrade depth at multiple points and average them.

Entering thickness in feet instead of inches is the most common input error. A 4-foot-thick slab looks like a reasonable commercial foundation but is almost never what the user meant. The tool takes thickness in inches for imperial and centimetres for metric, so a standard 4-inch residential slab is entered as 4, not 0.33.

Forgetting to multiply for multiple identical pours is a close second. Pouring six deck footings is six times the volume of one, but users frequently calculate a single footing and manually multiply at the supplier — sometimes incorrectly, sometimes forgetting altogether. Use the quantity field to let the tool do that multiplication before applying the waste factor.

Skipping the waste factor entirely because the math already feels conservative. The geometric calculation gives you the theoretical minimum. No real pour hits the theoretical minimum. Even experienced crews using pump trucks lose concrete to line purging, overfill for consolidation, and form variance. Ordering at exactly the net volume means a second smaller delivery or a short pour, both of which cost more than a 10% overage.

For a rectangular slab, the net volume in cubic feet is: V = Length (ft) x Width (ft) x Thickness (ft). Thickness entered in inches is divided by 12 to convert to feet. For a round column: V = pi x r x r x Height (ft), where r is the radius in feet: half the diameter, with inches converted to feet via 12, where diameter in inches is divided by 12 to reach feet, then halved to get the radius.

Net volume in cubic feet is then multiplied by the number of identical pieces and divided by 27 to convert to cubic yards. The order volume applies the waste factor: Order (cu yd) = Net (cu yd) x (1 + Waste% / 100). For the example slab, the net volume is 1.48 cubic yards and the order total with waste is 1.63 cu yd cubic yards.

To convert to cubic metres for metric suppliers, the order volume in cubic yards is multiplied by 0.764555. The bag equivalent uses the standard yield of 0.45 cubic feet per 80 lb bag: bags needed equals the ceiling of (Order in cubic feet divided by 0.45).

The formula assumes constant form dimensions across the full pour depth, which breaks down in tapered footings and bell-bottom piers. For tapered cross-sections, the correct volume is the frustum formula: one-third times height times the sum of the top area, bottom area, and the square root of their product. The slab formula applied to a tapered form will underestimate volume, sometimes by 20 to 30 percent for steeply tapered piers.

The 10% waste factor also hides a mix design assumption: it implicitly assumes you are ordering standard 3000 PSI ready-mix at a standard slump. High-slump mixes (greater than 5 inches) flow into voids more aggressively and can fill uneven subgrade better, which sometimes justifies reducing the waste factor. Self-consolidating concrete (SCC) used in complex architectural forms often requires a slightly higher overage because form pressure is greater and blowouts are more likely.