How to Estimate Concrete for a Slab | Reddit & LinkedIn

The math is straightforward once thickness is in feet. A 4-inch slab is 0.33 feet thick. Multiply length by width by thickness to get cubic feet, then divide by 27 to convert to cubic yards, which is how ready-mix is ordered.

For example, a 20 by 30 foot slab at 4 inches is 20 times 30 times 0.33, or about 200 cubic feet. Divided by 27, that is roughly 7.4 cubic yards before waste. Getting the volume right is the base of the whole estimate, so double-check the thickness conversion, which is where most errors start.

The calculated volume is the perfect-world number. On a real pour you lose concrete to spillage, over-excavated low spots, uneven subgrade, and the simple fact that you cannot run a truck empty. A short pour mid-job is far more expensive than a little extra, so you build in a buffer.

A 5 to 10 percent waste factor covers most slabs, with the higher end for rough or sloping subgrade. After adding it, round up to the increment your supplier delivers, since ready-mix is ordered in set quantities, not exact yards.

The concrete itself is only one line. A complete slab estimate includes the base: gravel or crushed rock and compaction. It includes a vapor barrier where required, reinforcement such as rebar or welded wire mesh, and the forms and stakes to hold the pour.

It also includes the work after the pour: finishing, control-joint cutting or tooling, and curing. If the pour needs a pump truck for access, that is its own line. Leaving any of these out is how a slab bid comes in low and the margin disappears.

Estimate the crew hours for site and form prep, the pour, finishing, and joint cutting, then multiply by your burdened labor rate. Access, slab size, and the finish the client wants all change the hours, so a broom-finish garage slab and a polished interior slab are not the same labor line even at the same volume.

Once labor and materials are in, apply your markup on the full job to cover overhead and profit. Pricing only the concrete and a thin labor number is the classic way a slab job stays busy and still loses money.

ContractShield drafts the estimate from your inputs: enter the dimensions and snap photos of the site, and the AI builds the material and labor lines from your own rate tables in about 25 minutes instead of an afternoon of takeoff.

Every line shows where its price came from, and the AI uses your numbers rather than inventing them. You review the volume, waste, and labor, apply your markup, and send a quote the client can accept on their phone.