Concrete Calculator

Free Online Concrete Calculator

This concrete calculator works out exactly how much concrete you need for a slab, footing, wall, round column, or even a set of staircase steps. Pick the shape that matches your project, type in the dimensions, and the calculator instantly shows the volume in cubic yards, cubic feet, and cubic meters, along with the number of premix bags or an estimated ready-mix delivery cost.

Whether you're pouring a small fence post footing with a few bags of premix concrete, or ordering a full truckload of ready-mix for a driveway or foundation, getting the volume right the first time keeps your project on schedule and your budget on track. Under-order and the pour stops halfway through with a visible cold joint; over-order and you're paying for leftover concrete you can't use.

Why Concrete Volume Matters

Concrete is priced and delivered by volume, not by weight or area, so an accurate cubic yard figure is the single most important number when planning a pour. Ready-mix trucks usually have a minimum order and charge extra fees for a second short-load delivery, and bagged premix runs out fast on anything bigger than a small DIY job. A wrong estimate in either direction costs real money — either in wasted material or in an extra delivery charge.

This calculator also matters because concrete work is unforgiving once it's poured. Unlike materials such as mulch or gravel where you can always add a bit more later, a concrete pour needs to be finished in one continuous session to avoid weak joints, so knowing the volume in advance is essential.

Concrete Volume Formulas

The formula depends on the shape you're pouring, but every one of them reduces to a version of the same idea — area × depth. Here's how each shape is calculated:

• Slab or Footing (rectangular): Volume = Length × Width × Thickness
• Wall: Volume = Length × Height × Thickness
• Round Column or Round Footing: Volume = π × Radius² × Height (radius is half the diameter)
• Staircase: Volume = sum of each stacked step's block = (1 + 2 + 3 + ... + number of steps) × Run × Rise × Width
• Cubic Yards = Cubic Feet ÷ 27
• Cubic Meters = Cubic Feet ÷ 35.3147

How to Use This Concrete Calculator

First, choose the shape of what you're pouring — a flat slab or footing, a vertical wall, a round column or footing, or a set of steps. Enter the dimensions the calculator asks for, choosing feet, inches, or yards depending on how you measured. Add a waste allowance to cover spillage, over-excavation, and uneven subgrade — 10% is a common default for slabs and footings poured directly against soil, while a well-formed wall pour can often use a smaller allowance. If you're buying ready-mix concrete, enter your supplier's price per cubic yard to see the total delivered cost. If you're using bagged premix instead, enter the price per bag for whichever bag size you're buying, and the calculator shows the total bag cost too. The step-by-step panel walks through exactly how the final volume was calculated.

Worked Example: Concrete Slab

Say you're pouring a 12 by 12 foot patio slab, 4 inches thick. First, convert the thickness to feet: 4 ÷ 12 = 0.333 ft. Multiply the three dimensions: 12 × 12 × 0.333 = 48 cubic feet. Divide by 27 to get cubic yards: 48 ÷ 27 = 1.78 cubic yards. Adding a 10% waste allowance brings that to about 1.95 cubic yards, which rounds up to a recommended order of 2 cubic yards. At a ready-mix price of $150 per cubic yard, that's roughly $300 delivered. If you were buying 80 lb premix bags instead, you'd need about 89 bags.

Ready-Mix vs. Bagged Concrete — Which Should You Use?

Ready-mix delivered by truck is almost always cheaper per cubic yard and far less physical work, but most suppliers have a minimum order (often 1 cubic yard) and charge a short-load fee below that. It makes sense for anything from a modest patio slab upward, and it's the only practical option for large pours like driveways, foundations, and walls.

Bagged premix concrete is more expensive per cubic yard but doesn't require a minimum order or truck access to the site, which makes it the better choice for small jobs — fence post footings, small stepping stones, repairs, or anywhere a truck simply can't reach. As a rough guide, once a job needs more than about 1 cubic yard of concrete, it's worth pricing out a ready-mix delivery instead of buying dozens of bags.

There's also a middle option worth knowing about on larger DIY jobs: some hardware and building supply stores rent small towable batch mixers or offer volumetric mixing trucks that measure out cement, sand, aggregate, and water on site and mix only what you need, which can work out cheaper than bags without requiring a full ready-mix minimum order.

How Many Bags of Concrete Do I Need?

Bag yield depends on the bag size. These are the standard published yields used by most premix manufacturers:

• 40 lb bag — yields about 0.011 cubic yards of concrete.
• 60 lb bag — yields about 0.017 cubic yards of concrete.
• 80 lb bag — yields about 0.022 cubic yards of concrete (the most common size for larger DIY jobs, since it needs the fewest bags per cubic yard).
• Roughly 45 bags of 60 lb concrete, or 60 bags of 40 lb concrete, make up 1 cubic yard.

Why Add a Waste Allowance?

The raw volume from a formula assumes perfectly smooth, perfectly level forms with zero material loss, which rarely happens on an actual job site. Ground isn't perfectly flat, excavated footings are rarely exact, some concrete spills or gets left in the truck chute, and formwork can bow slightly under the weight of wet concrete. A waste allowance of 5% to 10% covers these real-world losses.

A smaller allowance works for a well-formed, above-grade pour like a wall or a precisely built form. A larger allowance is safer for footings and slabs poured directly against excavated soil, where the exact depth is harder to control.

Common Mistakes When Calculating Concrete

A handful of errors come up again and again when people estimate concrete by hand:

• Forgetting to convert units — mixing feet and inches without converting one to match the other first.
• Using diameter instead of radius (or vice versa) in the cylinder formula for round columns and footings.
• Skipping the waste allowance entirely and ordering the exact bare-minimum volume calculated from the plan.
• Not rounding up to the supplier's minimum order or delivery increment, which can mean paying a short-load fee unexpectedly.
• Underestimating a staircase, since each step adds its own full block of concrete rather than a single flat slab volume.

Where This Calculator Is Used

Concrete volume calculations come up across almost every type of construction project:

• Homeowners and DIYers use it for patios, sidewalks, fence post footings, and small repair pours.
• Contractors use it to quote and order ready-mix concrete for driveways, foundations, and slabs on grade.
• Structural crews use it for columns, footings, and retaining wall bases.
• Landscapers use it for steps, curbing, and hardscape footings under pavers and walls.
• Estimators use it to cross-check material take-offs before finalizing a project budget.