Concrete Slab Calculator

A 12 by 20 foot slab poured 4 inches thick holds 80 cubic feet of concrete, or 2.96 cubic yards. Ordered with a 10% allowance that is 3.26 cubic yards, which you would call in as 3.5 yards of ready-mix, or 147 bags of 80 lb mix if you mix it yourself. Installed at US-average prices it comes to about $1,300, roughly $5.50 a square foot.

How the slab volume is calculated

A slab is a flat box: length times width gives the area, and the area times the thickness gives the volume. Measure thickness in inches and divide by 12 to work in feet.

Cubic feet = length (ft) × width (ft) × thickness (in) ÷ 12, and cubic yards = cubic feet ÷ 27

For the 12 × 20 ft example: 20 × 12 = 240 sq ft, × 4 ÷ 12 = 80 cu ft, ÷ 27 = 2.96 cu yd. Add 10%: 88 cu ft, 3.26 cu yd. At 0.60 cu ft per 80 lb bag (the yield on Quikrete's data sheet) that is 88 ÷ 0.60 = 146.7, so 147 bags. With 60 lb bags at 0.45 cu ft you need 196.

A round slab uses π × radius² for the area. A 10 ft circle 4 in thick is 26.2 cu ft, 1.07 cu yd with the 10% allowance.

Concrete slab calculator with square feet

If you already know the area, switch the shape to square feet and enter it directly. 240 sq ft at 4 in gives the same 80 cu ft. This is the quickest way to handle an L-shaped patio: add up its rectangles as square feet, or enter each rectangle as its own slab with "Add another slab" and let the calculator sum them.

Slab with footings: the thickened edge

Garage floors, shed slabs and many patios are poured monolithically, with the outer edge dug deeper so the slab carries its own footing. Turn on "Thickened edge" and enter how wide the deeper band is at the bottom and how deep it goes from the top of the slab. The calculator adds the band all the way around.

The 12 × 20 ft slab with a 12 in wide edge that goes 12 in deep gains a band of 60 square feet (240 sq ft minus the 18 × 10 ft inner area), 8 inches deeper than the slab. That is 40 more cubic feet: 120 cu ft in all, 4.89 cu yd with 10% extra, and 220 bags of 80 lb. The edge adds half again to the concrete, so it is worth drawing before you order. The size of the edge is a structural choice your building department or plans set; the concrete footing calculator checks separate footings against the IRC minimums.

Concrete slab calculator price

The price block under the result prices the slab you entered for your state. It uses these US-average figures, each with its source:

For the 12 × 20 ft example the range is $1,100 to $1,550, with $1,300 typical. About $175 of it is the short-load fee, charged because 3.5 yards is far below the 8 to 12 yards a mixer truck carries. Two neighbors pouring on the same day can share a truck and skip it. homewyse's September 2026 figure for a basic installed 4 in slab, $10.63 to $13.00 per sq ft, is higher than this build-up, so treat the range as a floor and get quotes. The concrete cost calculator adds pumping, finishes and a vapor barrier.

How thick should the slab be

The 2024 International Residential Code sets 3½ inches as the minimum for a concrete floor slab on ground (R506.1), over a 4 inch granular base (R506.2.2), with a 6-mil polyethylene vapor retarder under floors (R506.2.3). HomeAdvisor's June 2026 figures describe walkways at 4 inches, patios at 4 to 6 inches and driveways at 4 to 8 inches. Thicker costs more concrete but not much more labor.

Joints and the drawing

The drawing marks control joints in chalk blue. NRMCA recommends joints no farther apart than 24 times the slab thickness, which is 8 ft for a 4 inch slab; Quikrete's data sheet says every 6 to 8 ft. The calculator splits the slab into equal panels no longer than that, so the 12 × 20 ft slab shows panels about 6 ft 8 in by 6 ft.

Next steps

Work out the rebar for the slab with the same dimensions, compare bags of concrete in every size, or turn the total into a ready-mix order with trucks. The concrete calculator takes every other shape.

If the floor under the slab area needs leveling first, size the pour with the self-leveling concrete calculator.