📦 Concrete Volume

Written and maintained by the PhDino author · Last reviewed 21 September 2026 · Checked against 2 independent reference calculations · how PhDino checks its numbers

Converting a slab or footing's dimensions into the cubic yards ready-mix concrete is actually sold in.

Ready-mix concrete in the US is ordered and delivered by the cubic yard, not the cubic foot — so the first real step in planning any pour is converting a slab or footing's length, width, and thickness into that unit. It sounds trivial, but it's the single most common source of an underestimated order.

A waste margin matters more than it looks like it should. Subgrade is rarely perfectly flat, formwork is never perfectly plumb, and some concrete is lost to spillage and to filling the low spots a laser level didn't catch. Coming up short mid-pour means a second truck, a cold joint where the two placements meet, and a rushed call to the batch plant — all avoidable with a modest margin built in up front.

Key formula

Volume (ft³) = Length × Width × Thickness
Volume (yd³) = Volume (ft³) ÷ 27

How to use the Concrete Volume calculator

Use this to turn the dimensions of a slab, pad, footing or step into the amount of concrete to order, in cubic feet and in cubic yards, which is the unit ready-mix suppliers sell by. It also adds a margin for the concrete you will inevitably lose, so the number you get is the number to phone in.

Measure the length and width of the finished shape, enter the thickness in inches, and choose a waste margin. For a shape that is not a rectangle, split it into rectangles and add the results, or use π × radius² for a round pad.

Length (ft)

The length of the pour in feet, measured to the outside of the forms. For an L-shaped slab, calculate each rectangle separately.

Width (ft)

The width of the pour in feet, again to the outside of the forms.

Thickness (in)

The slab thickness in inches. Typical values are 4 in for a patio or walkway and 5 to 6 in for a garage floor or driveway; heavier loads and thickened edges need more, and your plans or local practice decide.

Waste/Spill Margin (%)

An allowance for spillage, over-excavated or uneven subgrade, and form leakage. Five to ten percent is usual. Uneven ground and hand-placed pours sit at the high end.

Worked example: a garage slab

A single-car-and-workshop garage slab measures 24 ft by 20 ft and is 5 in thick. The subgrade is graded but not perfectly flat, so a 7% allowance is used. How much ready-mix should be ordered?

Worked by hand:

1. Thickness in feet. 5 in ÷ 12 = 0.417 ft.
2. Volume in cubic feet. 24 × 20 × 0.417 = 200 ft³.
3. Convert to cubic yards. One cubic yard is 27 ft³, so 200 ÷ 27 = 7.41 yd³.
4. Add the waste allowance. 7.41 × 7% more = 7.93 yd³.
5. For comparison, bags. An 80 lb bag of concrete mix yields about 0.6 ft³, so 200 ft³ would take about 333 bags.

Order 8 cubic yards. Ready-mix is delivered in truckloads and priced by the yard, and that job would be hopeless to mix from bags. One more inch of thickness would add 1.48 yd³ of concrete, and making the whole slab 6 inches thick would raise the order to 9.5 yd³, which shows how sensitive the total is to the thickness on the plans.

Reading the result: ordering without coming up short

Running out of concrete mid-pour is far more expensive than having a little left over, because a cold joint forms where the second load meets the first. That is why the waste allowance is included by default and why it is better to round the final figure up to the next quarter or half yard than down.

• The theoretical volume assumes a flat, exactly-thick slab. Subgrade that is dished, low spots and thickened edges all add concrete that the simple length × width × thickness misses.
• Small jobs and truck minimums. Most suppliers charge a short-load fee below a few cubic yards, so for a very small pour it can be cheaper to mix from bags; for anything over about a yard, ready-mix wins on both cost and consistency.
• Bags: about 45 bags of 80 lb mix make a cubic yard, and about 60 of the 60 lb size. A 4 in slab of 100 ft² is roughly 1.2 yd³.
• Rebar, wire mesh and vapor barriers do not change the volume, but forms that bow outward under load add some, which is another reason to keep the margin.

Notes & limitations

• Ready-mix trucks typically carry up to about 10 cubic yards — for anything beyond a small slab, check the plant's minimum order and whether your project needs more than one truck.
• This assumes a flat rectangular pour. An irregular shape (an L-shaped patio, a stepped footing) needs to be broken into simpler rectangular pieces and summed.

Common mistakes

• Entering the thickness in feet or leaving it in inches while thinking in feet. The calculator expects inches for thickness and feet for length and width.
• Ordering exactly the theoretical volume. There is always some loss, and being short is much costlier than a small surplus.
• Forgetting thickened edges, footings and steps, which are extra volume around and under the main slab.
• Mixing cubic feet and cubic yards. Dividing by 27 is the step that catches people out, and an order in cubic feet instead of yards would be twenty-seven times too large.
• Measuring to the inside of the forms, which underestimates the pour by the width of the form on each side.

Frequently asked questions