Concrete calculator: slabs, footings, and columns

What estimating concrete volume actually involves

Estimating concrete volume is the step between a dimensioned drawing and a delivery ticket. The drawings give you slab outlines, footing runs, and column schedules in feet and inches. The supplier sells in cubic yards. The work is converting one into the other, consistently, with every quantity traceable back to a sheet.

It is worth separating two questions that get merged on site. How much concrete is there is a geometry question, answered by the dimensions and nothing else. How much concrete to order is a procurement question, answered by the geometry plus a waste allowance plus the supplier's load rules. This calculator answers the first exactly and the second honestly, which is why it shows the bare volume and the ordered volume as separate numbers.

Estimators also distinguish volume from cost. This page and the calculator above stay on quantity. If you need the priced side, the concrete slab cost calculator carries the cost per cubic yard, waste, and pump day.

The concrete estimating formula

Every shape on a concrete takeoff reduces to the same chain:

volume (cy) = length (ft) x width (ft) x thickness (ft) / 27

The 27 is the only constant worth memorising: a cubic yard is 3 ft x 3 ft x 3 ft, so 27 cubic feet. Everything else is converting the drawing's units into feet before you multiply.

Inches to feet is where most arithmetic errors enter, because slab thicknesses are almost always called out in inches while plan dimensions are in feet. Divide the inches by 12 and carry the decimal rather than rounding early:

• 4 in = 0.333 ft
• 5 in = 0.417 ft
• 6 in = 0.5 ft
• 8 in = 0.667 ft
• 10 in = 0.833 ft
• 12 in = 1.0 ft

Rounding 0.333 to 0.33 looks harmless and is not: on a 5,000 sf slab at 4 in it moves the answer by about 0.6 cy, which is most of a short-load fee.

Estimating concrete volume for slabs, footings, and columns

Concrete is ordered in cubic yards, drawings are dimensioned in feet and inches, and the calculator bridges the two. Every shape reduces to the same chain: convert dimensions to feet, multiply to cubic feet, divide by 27 for cubic yards, then add a waste factor before ordering.

Slab: length (ft) x width (ft) x thickness (ft). A 6 in slab is 0.5 ft thick. A 30 x 50 slab at 6 in is 30 x 50 x 0.5 = 750 cf, and 750 / 27 = 27.8 cy.

Continuous footing: total run (lf) x width (ft) x depth (ft). A 120 lf footing at 16 in wide and 12 in deep is 120 x 1.333 x 1.0 = 160 cf = 5.9 cy.

Round column: pi x radius (ft) squared x height (ft) x count. Four 18 in diameter columns at 10 ft are 3.1416 x 0.75 x 0.75 x 10 x 4 = 70.7 cf = 2.6 cy.

Note the radius, not the diameter. An 18 in column is 1.5 ft across and therefore 0.75 ft in radius. Squaring the diameter instead is the single most common column error, and it overstates the pour by a factor of four.

After this calculator establishes cubic yards for the pour, carry that quantity into the concrete slab cost calculator to apply the delivered rate and price the ready-mix.

Estimating concrete yardage from a drawing set

Working from real documents rather than a single rectangle changes the job. A slab on grade is rarely one clean shape, and the yardage you order has to account for what the plan actually shows.

Take the slab off in pieces. Break an irregular footprint into rectangles, compute each separately, and total them. Trying to average an L-shaped or notched slab into one rectangle is where quantities drift. Keep each piece on its own line with its own sheet reference so a reviewer can follow it.

Read the thickness from the section, not the plan. Plans give the outline; the structural sections and the slab schedule give the thickness, and it often changes across one pour. A warehouse floor may be 6 in over most of its area and 8 in in the dock apron and equipment pads. Those are separate lines.

Add the thickenings. Turndown edges, interior thickened slabs under partitions, and equipment pads sit below the slab plane and do not appear in the length x width x thickness figure. On a slab with a 12 in x 12 in turndown around a 200 lf perimeter that is 200 x 1 x 1 = 200 cf, or 7.4 cy of concrete the plan area alone never showed.

Deduct only what is worth deducting. Openings, pits, and blockouts reduce the pour, but small penetrations are usually left in as part of the waste allowance. A rule that works: deduct anything larger than about 10 sf, ignore anything smaller, and say which convention you used in your notes.

Footings by run, not by count. Continuous footings are taken off as total linear feet at each width and depth combination, not as individual segments. Group the runs by size first, then multiply once per group.

Why the waste factor matters

Field conditions consume concrete that the geometry does not show: uneven subgrade, form deflection, spillage, and pump line loss. Most estimators carry 5 to 10 percent on slabs and footings, more on irregular pours. Running short mid-pour forces a second truck dispatch and risks a cold joint, so the waste factor is cheap insurance. The calculator applies your percentage to the ordered volume, and bag counts round up to whole bags.

Truck loads and bag math

Ready-mix trucks carry about 10 cy, so a 30 cy order is three trucks, and suppliers commonly charge short-load fees below their minimum. For small pours, premixed bags substitute: a 60 lb bag yields about 0.45 cf and an 80 lb bag about 0.60 cf. A 10 cf repair therefore needs 10 / 0.60 = 17 bags of 80 lb mix, rounded up.

Common mistakes when estimating concrete

These are the errors that show up repeatedly on reviewed takeoffs, in rough order of how much money they move.

Rounding the thickness early. Covered above, and it is the most frequent. Carry the decimal to three places until the final cubic yard figure.

Using the diameter as the radius on columns. Overstates a round pour four times over.

Missing the turndowns and thickenings. The plan area is the floor, not the pour. Anything that drops below the slab plane is additional volume.

Treating the whole slab as one thickness. Dock aprons, equipment pads, and vault slabs are commonly thicker than the field. One averaged number hides both an overage and a shortfall.

Forgetting that the supplier sells in increments. A 7.7 cy requirement is an 8 cy order. Ordering the exact computed figure is how a pour ends 0.3 cy short with the finishers already on site.

Carrying no waste at all on a clean-looking job. Even a flat, well-formed slab on prepared subgrade loses concrete to spillage and form deflection. Zero waste is not a tight estimate, it is a missing line.

Confusing volume with cost. The cubic yards are the same whoever pours them; the dollars are not. Keep the quantity takeoff and the pricing as separate steps so each can be checked on its own.

From volume to a priced line

Volume is only the quantity half of the estimate. The priced line multiplies the cy by an all-in unit cost covering the mix design, placement labor, pump or crane time, and finishing. That is exactly the step Ruh automates: it reads the slab and foundation dimensions off your drawings, computes the volumes with your waste factors, and prices them against your own price book, with your estimator reviewing the result. See the takeoff guide for the full quantity workflow, or the cy glossary page for the unit itself.