How to Estimate a Concrete Slab | Reddit & LinkedIn

Volume in cubic yards is length times width times thickness, all in feet, divided by 27. A 20-by-20 slab at four inches is 20 times 20 times 0.33, divided by 27, which is about 4.9 cubic yards. Add 10 percent for waste and order 5.5 yards. Ordering short and paying a short-load fee, or ordering a second truck, is how a tight slab job loses its margin.

A complete slab estimate has concrete delivery, gravel base, vapor barrier where code requires it, rebar or wire mesh, forms, labor to set forms and place and finish, and any pump or short-load fee. Break each out so the client sees where the money goes. A single lump sum invites price shopping. Itemized numbers signal that you have actually measured the job.

A typical 20-by-20 four-inch slab runs about $3,600 to $7,200 installed in 2026, or roughly $9 to $18 per square foot depending on thickness, reinforcement, finish, and access. Thicker slabs for driveways or equipment pads, decorative finishes, and hard-access sites that need a pump push toward the high end. Price per job, not per square foot alone.

Labor to set forms, place, screed, float, edge, and finish is usually the largest single line on a slab. Finishing is skilled work and time-sensitive, since concrete does not wait. Crew size, weather, and finish type all move the number. Bill labor at your loaded rate, and never let a low material cost tempt you into underbilling the crew hours that actually make the slab flat and durable.

ContractShield lets you save a slab template with your per-yard concrete cost, rebar and base rates, and finishing labor, so a new estimate is a quick edit from field measurements. Snap a site photo, enter dimensions, and send an itemized, signable quote the same day. You run the pour and invoice on milestones for a flat 2% per job (1% each side), capped at $250, no per-lead fees.

What is under the slab is as important as the slab itself, and it is where estimates go wrong. Soft or organic soil needs over-excavation and imported structural fill. Poor drainage needs a thicker gravel base or a drain. Expansive clay may need a thicker slab or more reinforcement. You cannot see most of this from the driveway, so add a subgrade contingency line and note in the quote that unforeseen soil conditions may require a change order. Naming the risk up front is far better than eating it later.

A basic broom finish is the default, but many clients will pay for more once they see the option. Offer finish tiers as separate line items: broom finish, a smooth trowel finish, exposed aggregate, or stamped and colored decorative concrete. Decorative finishes add both material and skilled labor, sometimes doubling the per-foot price, so never bury them in the base number. Presenting them as clear upgrades lets the client trade up on their own terms and protects you from doing premium finish work at a basic-finish price.