How to Estimate a Concrete Job

Concrete is unforgiving in a specific way. Order too little and the pour stops with a cold joint you'll be explaining for the life of the slab. Order too much and you've paid for material sitting in a truck. Neither problem gives you time to think, because the clock starts the moment the drum turns.

Cubic yards, and the arithmetic worth doing twice

Volume is length times width times thickness, all in the same unit, then converted. The conversion people get wrong is that a cubic yard is 27 cubic feet, not 9. A slab 20 feet by 30 feet at 4 inches thick is 20 times 30 times 0.333, which is about 200 cubic feet, which is roughly 7.4 cubic yards.

Then add waste. Five to ten percent is typical for a flat slab on well-prepared subgrade. Go higher when the subgrade is uneven, because low spots swallow concrete invisibly and the slab that measured 4 inches averages 4 and a half by the time it's screeded. On footings dug in soft or crumbly soil, overdig makes actual volume run well above calculated volume, and ten percent is optimistic.

Round up to something you can actually order. Suppliers deliver in fractions of a yard but charge short-load fees below a full truck, so the economics of ordering 6.2 yards versus 7 are worth checking against your supplier's minimum before you price it.

The concrete is often the smaller half

Material is the visible cost and frequently not the dominant one. Forming, excavation, base preparation, reinforcement, placing, finishing, and stripping are all labor, and on a residential flatwork job labor plus prep commonly exceeds the mud itself.

• Excavation and haul-off, which depends on soil and whether a machine can get to it
• Base material, typically compacted gravel, plus the compaction labor
• Forming: lumber or forms, stakes, bracing, and the hours to set and strip them
• Reinforcement: rebar, mesh, or fiber, plus chairs and tie wire
• Vapor barrier where the slab is interior or will be covered
• Placing, screeding, floating, and finishing crew hours
• Control joints, cut or tooled, and curing compound or covering

Forming cost scales with perimeter and complexity, not with area. A round patio, a curved walkway, or a slab with multiple steps takes far more forming labor per yard than a rectangle, and that's the adjustment most commonly missing from a per-square-foot rate.

Thickness and reinforcement follow the load

Four inches is standard for a residential sidewalk or patio. Driveways generally want more, and anything that will see a loaded truck, an RV, or a dumpster wants more again along with heavier reinforcement. Getting this wrong isn't a margin problem, it's a cracked slab and a warranty conversation.

Reinforcement choice moves both cost and labor. Fiber mixed into the concrete costs less to place than mesh or rebar because there's nothing to tie or lift, but it does a different job structurally. If the customer is comparing your bid against one that specified rebar on chairs and yours says fiber, that difference needs to be visible in your estimate or you look cheap for the wrong reason.

Finish type is a real price variable

A broom finish and a stamped, colored, sealed finish are the same volume of concrete and nothing like the same job. Stamping is time-sensitive skilled labor performed in a narrow window, usually with a larger crew standing by, plus release agent, color hardener, and sealer. Exposed aggregate, polished, and salt finishes all sit at their own points on that scale.

Price finishes as their own line rather than folding them into a square foot rate, because customers frequently change their mind about the finish after seeing the number, and a separated line makes that a simple conversation rather than a full requote.

Access decides more than people expect

Whether the truck can back up to the pour is close to a binary cost event. Direct chute access is the cheap case. Beyond chute reach, you're into wheelbarrows and a much larger crew, a pump truck with its own day rate and minimum, or a conveyor. A backyard patio behind a narrow gate can cost more to place than a driveway twice its size.

Look at the route on the walkthrough and photograph it. Gate widths, slopes, soft lawn the truck would rut, overhead wires, and the distance from where the truck can legally park to where the concrete needs to land. This is the single most commonly underpriced part of residential concrete.

Weather is a scheduling cost, not a footnote

Concrete cares about temperature. Hot weather accelerates set and can require retarders, earlier starts, or evaporation control. Cold weather means accelerators, blankets, or heated enclosures, and below certain temperatures it means not pouring at all. Both show up as either added cost or lost days, and a quote written in May for a job poured in December should say which it assumes.

Getting the site into the estimate

Almost every expensive surprise in concrete traces back to something visible on site and absent from the estimate. The slope nobody noted. The gate the truck couldn't reach. The soft ground where the footing overdug. None of those are pricing mistakes exactly, they're recording mistakes that became pricing mistakes.

Photograph the access route, the grade, the existing surface, and anything that will have to be cut, removed, or worked around. An estimate built from that record, rather than from a measurement written on a scrap of paper, prices the pour you're actually going to do.