Concrete Calculator Formula

Quick answer

Concrete Calculator Formula: rectangles: L×W×T. Cylinders: πr²h. Stairs: W×T×R×n(n+1)/2. Rings: π(R²−r²)×T. Then ÷27 for yards, ÷ bag yield for bags, × density for weight.

Concrete Calculator Formula: Rectangles

Slabs, footings, walls — three sides in feet. Keep units honest. Footing drawings that show 16×8 inches need conversion before multiplying by a length in feet.

Concrete Calculator Formula: Circles and tubes

Pads, columns, sonotubes, post holes. Rings subtract the inner circle. There is no hollow “core” inside a sonotube filled solid — do not subtract one.

Stairs, curb, monolithic

Solid stairs stack risers with the triangular series n(n+1)/2. Curb and gutter multiply section area by run length. Monolithic slabs add the edge beam below the floor thickness and count each corner once.

Waste and rounding are not the formula

Waste is a percentage you choose. Truck rounding is a plant rule (usually 0.25 yd). Bags always ceil. Those steps sit after the geometry.

Do it in Concrete Calculator

Pick the matching shape — the formula prints under the result. The volume calculator is the hub when you want every unit on one screen.

Worked formula sheet

Slab 10×12×0.333 ft = 40 ft³ = 1.48 yd³. Tube π×0.5²×4 = 3.14 ft³. Stairs 3×(11/12)×(7/12)×10 ≈ 16 ft³ for the four-step example on the stairs page. Curb section 0.583 ft² × 40 ft = 23.3 ft³. Monolithic = slab ft³ + edge ft³ with corners once.

After geometry: waste percent, then bag ceil or truck 0.25 round. Those are ordering rules layered on the formula — not part of the geometry itself.

Checking Concrete Calculator by hand

Every result screen includes the formula. Multiply it on a phone calculator. If your hand math disagrees, check units first — inches left as inches is the usual culprit — then check whether waste and rounding already applied.

Field notes to keep handy

Measure to the outside of the forms. Convert every side to the same unit before you multiply. Apply waste after you sum every piece of the job, then round once for the plant.

Bag counts always round up because a partial bag is still a whole bag at the store. Compare that total with a delivered price when short-load fees show up on the quote.

Normal-weight planning density here is 150 pounds per cubic foot unless you switch it. Lightweight mixes and specialty bags can differ — read the ticket or the label when they do.

Field notes to keep handy

Measure to the outside of the forms. Convert every side to the same unit before you multiply. Apply waste after you sum every piece of the job, then round once for the plant.

Bag counts always round up because a partial bag is still a whole bag at the store. Compare that total with a delivered price when short-load fees show up on the quote.

Normal-weight planning density here is 150 pounds per cubic foot unless you switch it. Lightweight mixes and specialty bags can differ — read the ticket or the label when they do.