Concrete Calculator

Cubic yards, bags of mix, and cost for slabs, footings, and pads. Updated for 2026.

Last reviewed July 2026 · Free · Nothing you enter is stored

How this calculator works

Volume is length × width × thickness converted to cubic yards (divide cubic feet by 27), plus a 10% waste allowance for uneven subgrade and spillage — the standard contractor practice. Bag counts use manufacturer yields: 0.60 cu ft per 80 lb bag, 0.45 cu ft per 60 lb bag. Ready-mix pricing uses the current national average of about $135 per cubic yard delivered (short-load fees can apply under 4 yards).

The waste allowance

This calculator adds 10% to the raw volume. That is not padding for its own sake — it covers subgrade that is never perfectly level, forms that bow slightly under load, spillage at the chute, and the simple fact that a slab poured a quarter inch deep across a 200 square foot patio swallows an extra third of a yard without anyone noticing. Running short mid-pour is the expensive failure: a cold joint where the first batch has begun to set is a permanent weakness, and a second short-load delivery costs more than the concrete in it.

Bags or ready-mix, and where the line sits

The honest answer is that bags stop making sense far sooner than most people expect. A cubic yard is 27 cubic feet, and an 80 lb bag of standard mix yields about 0.60 cubic feet. That is 45 bags to the yard — roughly 3,600 pounds of material to haul, and each one wants water measured and mixed to a consistent slump.

For a fence post or a mower pad, bags are obviously right. For anything approaching a cubic yard, the calculation changes: you are moving nearly two tons by hand, mixing for hours, and fighting the clock because each batch begins setting while you mix the next. Ready-mix arrives in one load, at a consistent strength, and is placed in under an hour.

The catch is the short-load fee. Mixer trucks carry nine to eleven yards, and plants charge a surcharge below roughly three to five yards to cover sending a truck out half empty. That fee is dealer pricing rather than any published standard, so it varies by plant and region — worth two phone calls before you decide.

Thickness, and why the slab under a car is different

Thickness is where a slab is won or lost, and it is also where people economise first because the cost scales directly with it. A four inch slab uses a third more concrete than a three inch one over the same footprint.

Patios, walkways and shed pads are conventionally poured at four inches. Driveways carrying passenger vehicles are usually four to six, with the thicker end where trucks park or where the subgrade is soft clay. The reason is not the weight of the vehicle spread over the slab — it is the concentrated load under each tyre, and the bending that load induces across an unsupported span.

What sits underneath matters as much as the thickness. A well compacted gravel base drains water away and supports the slab evenly; a slab poured on soft or uneven fill will crack no matter how thick it is, because concrete is strong in compression and weak in tension, and an unsupported span puts the underside into tension.

Reinforcement, and the mistake almost everyone makes

Welded wire mesh only works if it sits near the middle of the slab depth. Laid on the ground and poured over — which is what happens on most residential jobs — it ends up at the bottom, where it does essentially nothing, because the tensile stress it is meant to carry is in the upper portion of a slab spanning a soft spot.

Mesh has to be pulled up into the pour as it goes, or supported on chairs beforehand. Rebar on chairs is easier to place correctly and stays where you put it, which is a large part of why it has become the default for residential slabs even though mesh is cheaper on paper.

Neither prevents cracking. Concrete shrinks as it cures and it will crack; reinforcement holds the crack closed and keeps the two sides in the same plane. Control joints, cut or tooled at a spacing of roughly two to three times the slab thickness in feet, decide where those cracks happen.

Curing is not drying

Concrete does not dry out to gain strength — it hydrates, a chemical reaction that needs water to continue. A slab that dries too fast stops gaining strength early and stays weaker than the mix it was made from.

It reaches roughly 70% of design strength in the first week and continues gaining for 28 days, which is the age at which strength is conventionally specified. Keeping the surface damp for the first several days, with plastic sheeting or a curing compound, matters more to the finished result than almost anything else you can do after the pour.

This is also why sealing or heavily loading a fresh slab is a mistake. Wait out the first week before parking on it, and give it the full month before anything heavy.

Bag yields and coverage

Yields are the manufacturer's published figures for standard concrete mix. Coverage assumes a four inch slab.

Bag yields from Quikrete's published data sheet for standard concrete mix. Coverage figures are derived from those yields at four inches thick and exclude the waste allowance.

What goes wrong on residential pours

Mesh left on the ground. Reinforcement at the bottom of a slab does almost nothing. It has to be lifted into the pour or set on chairs first.

Ordering exactly what the maths says. Subgrade is never flat and forms bow. Running out mid-pour creates a cold joint that becomes a permanent line of weakness.

Skipping the base. A thick slab on poor fill cracks; a properly supported thinner slab often does not. Compacted gravel underneath is not optional.

Sealing or loading too early. Concrete keeps gaining strength for 28 days. The first week is when curing conditions matter most, and when damage is easiest to cause.

No control joints. The slab will crack. Joints decide where. Cut them at roughly two to three times the slab thickness in feet.

Frequently asked questions

Why your state changes the price (but not the quantity)

How much concrete you need is physics — it doesn't care where you live. What it costs does. Labor rates, delivery distance, fuel, and local demand move installed prices by roughly 45 points from end to end: Arkansas, Mississippi, North Carolina sit near the bottom of the range, while Alaska, Hawaii, New York sit at the top. The Price this for selector at the top of the calculator applies a published construction cost index for your state to the 2026 national averages, so the dollar figures move and the material list stays put.

Treat the result as a budgeting range, not a quote. Two metros inside the same state can differ by more than the statewide index suggests, and a single busy contractor can outweigh both. See the full cost index for all 50 states and where the price data comes from.