Summary

This concrete calculator tells you how many cubic yards and bags of concrete a slab, footing or round column needs. Enter the size, pick a waste buffer, and it returns the order quantity plus a rough material cost for bagged mix and ready-mix. Prices are editable estimates. A 10 by 10 foot slab at 4 inches needs about 1.5 cubic yards.

Free tool

Concrete Calculator: How Much Do You Need?

Enter your slab, footing or column size. Get cubic yards, bags and a rough material cost in seconds.

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Worker screeding a freshly poured concrete slab in a backyard

Concrete calculator

Fill in the size of the pour. Results update as you type. Prices are editable estimates, so swap in the numbers from your local supplier.

Enter your dimensions to see how much concrete you need.

How it works

What goes into your number

Volume first

For a slab or footing the calculator multiplies length by width by thickness, with thickness converted from inches to feet. For a round column it uses pi times the radius squared times the depth. Then it multiplies by the number of identical pieces.

Extra on top

Real pours need more than the math says. The tool adds 5%, 10% or 15% for spills, bowed forms and a subgrade that is not perfectly flat. Then it rounds your order up to the next quarter yard, the way most plants take orders.

Bags and cost

Cubic feet divided by the yield of your bag size gives the bag count, rounded up. Ready-mix cost is your order in yards times the price per yard. Both prices are editable, because they change a lot by region.

Before you order

Measure twice, then add the waste

Most concrete mistakes happen before the truck shows up. Measure the inside of the forms, not the outside of the lumber. Check the depth in several spots, because a base that dips an inch across a 10 by 20 foot slab adds about two-thirds of a cubic yard. If the forms hold rebar or mesh, that steel barely displaces any concrete, so do not subtract for it. Here's what it should cost for a small job: a 10 by 10 slab at 4 inches needs about 62 bags of 80 lb mix, which at 7 dollars a bag is around 430 dollars. The same pour as ready-mix is about 1.5 yards, near 250 dollars before any short-load fee. That gap is why the bag-versus-truck tip in the result matters.

  • Measure the inside of the forms
  • Check depth at several points
  • Round up, never down
  • Ask the plant about short-load fees
Tape measure over rebar mesh inside wooden forms before a concrete pour
Quick guide

Three steps to a safe order

  1. 1

    Pick your shape and size

    Choose rectangle or round, then enter feet for length and width, inches for thickness or depth. Use the pieces field for repeats like four post footings.

  2. 2

    Compare bags and truck

    Check both cost lines. Under one cubic yard, bags usually win. Over two, the truck wins on time, labor and a consistent mix.

  3. 3

    Get a real quote

    Red flag: if your contractor quotes a flat price and will not show the yards of concrete in it, ask. A fair quote lists materials, labor and base work separately, and you can check the concrete line against this number.

Questions

Concrete calculator FAQ

Is this concrete calculator free?
Yes. It runs in your browser, there is no signup, and nothing you type is sent anywhere. Change any number and the answer updates right away.
How many cubic yards of concrete do I need for a 10x10 slab?
A 10 by 10 foot slab that is 4 inches thick holds about 1.23 cubic yards. Add 10% for spills and uneven ground and you are at roughly 1.36, so you would order 1.5 cubic yards from a ready-mix plant.
How many 80 lb bags make a cubic yard?
About 45 bags. One cubic yard is 27 cubic feet, and a standard 80 lb bag yields roughly 0.60 cubic feet. Smaller bags yield less: a 60 lb bag gives about 0.45 and a 40 lb bag gives about 0.30.
How thick should a concrete slab be?
Four inches is the usual thickness for patios, walkways and garage floors that only see cars. Driveways that take heavier trucks, or slabs on soft soil, are often poured at 5 to 6 inches. Check your local code and ask your contractor what the base requires.
Why add 10% extra?
Subgrades are never perfectly flat, forms bow a little, and some mix stays in the truck chute or wheelbarrow. Running short in the middle of a pour leaves a cold joint, which is a weak seam. Ten percent is a normal buffer. Use 5% on tight, level forms and 15% on rough ground.
What does ready-mix concrete cost per cubic yard?
Plan on roughly 150 to 200 dollars per cubic yard in most of the US for a standard 3,000 to 4,000 psi mix, and more in high-cost metros. Delivery, short-load fees on small orders, and pumping are billed on top. Call two local plants and ask for their all-in price.
Does this include labor, forms, rebar or the base?
No. The cost lines cover concrete only. Labor, forming, gravel base, rebar or wire mesh, finishing, sealing and permits are separate. A contractor quote for a slab is often two to four times the material cost, so compare it against your own number before you sign.
How accurate are the bag yields?
They are the yields printed on common bagged mixes, rounded. Brands differ slightly, so check the label on the bag you buy. The calculator rounds bags up so you do not come up one short.

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