Concrete Calculator

Enter your slab, footing, or column dimensions below to calculate exactly how much concrete you need, either in cubic yards for ready-mix delivery or in bags (40, 60, or 80 lb) if you’re mixing by hand. The calculator automatically adds a waste buffer so you don’t come up short mid-pour.

Slab / FootingRound Column / Post

Length (ft)

Width (ft)

Thickness (in)

Waste buffer (%)

Bag size

40 lb50 lb60 lb80 lb

Price per cubic yard ($ (optional))

Volume

33.3 cu ft

Ready-mix (incl. waste)

1.36 cu yd

80 lb bags needed

62

Estimated cost

—

Bags needed by bag size

Bag size	Bags needed
40 lb	123
50 lb	98
60 lb	82
80 lb	62

Includes a 10% waste buffer. Ready-mix concrete is typically sold in quarter- or half-yard increments. Round up when ordering.

How to Calculate How Much Concrete You Need

Whether you’re pouring a patio, a fence post footing, or a support column, the math behind every concrete project comes down to the same four steps.

Post footings are the case where the concrete volume depends on a count you have not made yet. Post spacing sets how many holes there are, and that follows from the fence layout rather than from the concrete: the fence calculator works out posts, rails and pickets from the run length first.

Step 1: Measure Your Project Dimensions

For a slab or footing, measure the length, width, and thickness in inches or feet. For a round column or post hole, measure the diameter (or radius) and the height or depth instead.

Most residential slabs (patios, walkways, and shed pads) are poured 4 inches thick. Driveways typically need 4 to 6 inches, and anything supporting a vehicle heavier than a standard car (RVs, trucks, equipment) should go to 6 inches or more.

Step 2: Calculate the Volume

For a rectangular slab or footing:

Volume (cubic feet) = Length (ft) × Width (ft) × Thickness (ft)

For a round column or post:

Volume (cubic feet) = π × Radius² × Height (ft)

Example: A 10 ft × 10 ft patio slab at 4 inches thick.

1. Convert thickness to feet: 4 ÷ 12 = 0.33 ft
2. Volume = 10 × 10 × 0.33 = 33.3 cubic feet

Step 3: Convert to Cubic Yards

Ready-mix concrete is sold by the cubic yard, not the cubic foot. There are 27 cubic feet in a cubic yard, so divide your total by 27.

33.3 ÷ 27 = 1.23 cubic yards

Step 4: Add a Waste Factor

Uneven subgrade, spillage, and over-excavation typically use up 5–10% more concrete than the clean geometry math predicts. Add 10% before you order or buy bags. It’s far cheaper than a second delivery trip.

1.23 × 1.10 = 1.36 cubic yards

Since most ready-mix suppliers sell in quarter- or half-yard increments (and charge a short-load fee for small orders), round up to 1.5 cubic yards for this example.

How Many Bags of Concrete Do I Need?

For smaller pours, bagged mix is usually more practical than ordering a truck. Each bag lists its dry weight, but what matters for this calculation is its yield, the volume it makes once mixed with water:

Bag Size	Yield (cubic feet)	Bags per Cubic Yard
40 lb	0.30	90
50 lb	0.375	72
60 lb	0.45	60
80 lb	0.60	45

Continuing the example above: 33.3 cubic feet, plus 10% waste = 36.6 cubic feet.

36.6 ÷ 0.60 (80 lb bag yield) = 61 bags → round up to 62 bags of 80 lb mix.

One thing bagged concrete is not for: filling the cores of a block wall. Core fill is a separate product with a much higher slump, and reaching for a bag of concrete mix instead is a common and expensive substitution. The mortar, grout and core fill guide covers why the three products are not interchangeable.

Bagged Concrete vs. Ready-Mix Delivery

As a rule of thumb: under about 1 cubic yard, bagged concrete is usually cheaper and easier to manage yourself. Once a project crosses 1.5–2 cubic yards, ready-mix delivery is typically the better deal. Bagged concrete works out to roughly double the cost per cubic yard once you account for the number of bags needed, and hand-mixing 60+ bags is a full day of labor most people would rather avoid.

In 2026, ready-mix concrete costs approximately $120–$180 per cubic yard delivered (national average around $150), depending on your region, the PSI strength you need, and delivery distance. Standard 3,000 PSI mix covers most residential slabs; garage floors and heavy-load areas typically call for 4,000 PSI or higher.

What You Have to Specify When You Order Ready-Mix

Bagged concrete comes as one product. Ready-mix does not: the plant needs you to state what you want, and the defaults you get by saying nothing are not always right for the job.

Strength. Quoted in PSI at 28 days. Residential patios, walkways and shed pads normally use 3,000 PSI. Driveways and garage floors are usually specified at 3,500 to 4,000, and anything exposed to de-icing salts wants the higher figure with air entrainment. Strength is cheap relative to the cost of replacing a failed slab, so this is a poor place to economise.

Slump. How far the mix settles under its own weight, a proxy for how wet it is. Four inches is a typical slab slump. Asking for a wetter mix makes placing easier and the finished concrete weaker, because the extra water raises the water-to-cement ratio. If the crew wants it wetter on site, that is what a plasticiser is for, not a hose.

Air entrainment. Deliberate microscopic bubbles that give freezing water somewhere to expand into. Anything exposed to freeze-thaw needs it. It is not optional in cold climates and it cannot be added usefully after the truck arrives.

Three charges catch people out on a first order. Short-load fees apply below roughly 3 or 4 cubic yards and can make a slightly larger order cheaper than a small one. Standby time is billed once the truck waits beyond an allowance, commonly 5 to 7 minutes per cubic yard, so the site needs to be ready before it arrives. And the washout has to happen somewhere: agree where before the truck is on site, because concrete rinse water is caustic and cannot go into a drain or a planting bed.