How Much Concrete Do I Need?
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Volume (cubic feet) = length x width x thickness, all in feet, then divide by 27 for cubic yards. A 12 ft x 12 ft patio at 4 inches thick needs 48 cubic feet, or 1.78 cubic yards — round up and add 5-10% waste when ordering.
Concrete is ordered in cubic yards. Getting the number right matters more than with most materials: a short load means a cold joint in the middle of a pour, and excess concrete is paid for either way, since ready-mix trucks charge for what is batched, not what is used.
Key takeaways
- Slabs: volume = length x width x thickness in feet, then divide by 27 for cubic yards.
- Cylindrical post holes use volume = pi x radius squared x depth.
- Ready-mix is typically sold in quarter-yard increments with a minimum load charge, so round up.
- Add 5-10% waste for spillage and uneven excavation, more for a heavily formed or sloped pour.
- Mix strength (measured in psi) is specified by the job, not chosen freely — check local code or an engineer's spec.
Run the numbers with the Concrete Calculator.
Rectangular slabs
Volume = length x width x thickness, with every dimension in feet. A 12 x 12 ft patio at 4 inches thick is 12 x 12 x 0.333 = 48 cubic feet, or 48 / 27 = 1.78 cubic yards.
Thickness is the dimension most often left in inches by mistake. Converting it to feet first (divide by 12) keeps the rest of the formula in consistent units and avoids an error that is otherwise easy to miss until the truck arrives short.
Footings and strip foundations
Treat a footing as a long rectangular prism: total length of the run x width x depth. A footing run of 60 linear feet, 18 inches (1.5 ft) wide and 12 inches (1 ft) deep is 60 x 1.5 x 1 = 90 cubic feet, or 3.33 cubic yards. Calculate each run separately if widths or depths differ, then add the results.
Corners and intersections are easy to double-count or skip. Sketch the footing layout, label each straight run with its own length, and total the runs before applying width and depth, rather than trying to eyeball a combined perimeter.
Post holes (cylinders)
Volume = pi x radius squared x depth.
- For a 10-inch diameter hole 36 inches deep: radius = 5 in = 0.417 ft, depth = 3 ft.
- Volume per hole = 3.1416 x 0.417^2 x 3 = 3.1416 x 0.1739 x 3 ≈ 1.64 cubic feet.
- For 6 identical post holes: 1.64 x 6 = 9.84 cubic feet, or 9.84 / 27 ≈ 0.36 cubic yards.
- Most residential post-hole jobs are small enough that bagged concrete is more practical than a ready-mix delivery.
Second example: a stepped patio slab
Not every slab is a single flat rectangle. A patio that steps down from the house to a lower yard level needs each thickness zone calculated on its own before the volumes are combined.
- Upper section: 10 ft x 12 ft at a standard 4 in (0.333 ft) thickness: 10 x 12 x 0.333 = 40 cubic feet.
- Lower step section: 10 ft x 4 ft at a thicker 6 in (0.5 ft), since it also serves as a small retaining edge: 10 x 4 x 0.5 = 20 cubic feet.
- Total volume: 40 + 20 = 60 cubic feet, or 60 / 27 ≈ 2.22 cubic yards.
- Add 10% waste for the extra edge forming and the transition between thicknesses: 2.22 x 1.10 ≈ 2.44 cubic yards.
- Ready-mix suppliers commonly sell in quarter-yard increments, so this order rounds up to 2.5 cubic yards.
Bagged concrete yields
Approximate yield per bag (check the specific product label)
| Bag size | Approx. yield |
|---|---|
| 40 lb | 0.30 cubic ft |
| 60 lb | 0.45 cubic ft |
| 80 lb | 0.60 cubic ft |
Mix strength and when it matters
Concrete mix strength is expressed in psi (pounds per square inch) at 28 days of curing, commonly 2,500-3,000 psi for sidewalks and patios and 3,500-4,000 psi or higher for driveways, footings and anything exposed to freeze-thaw cycles or vehicle loads.
Mix strength is a structural decision, not a cosmetic one. Local building codes or an engineer's plan typically specify the required psi for footings and structural slabs; a decorative patio has more flexibility, but a driveway in a freeze-thaw climate should use a mix rated for that exposure, often with air-entrainment additives.
Ordering practicalities
- Add 5-10% waste for spillage, uneven subgrade and over-excavated edges.
- Ready-mix suppliers usually sell in quarter-yard increments with a minimum load; round up to the next increment.
- For the 9.84 cubic foot post-hole example, 9.84 / 0.60 ≈ 16.4, so 17 bags of 80 lb concrete would cover it with a small margin.
- For anything structural, follow the engineer's or building department's specified thickness and mix strength rather than a generic assumption.
- Confirm the delivery truck can access the pour site; a long wheelbarrow relay from street to backyard slows placement and risks the load starting to set before it is fully used.
Common mistakes
- Forgetting to convert thickness from inches to feet, which inflates or deflates the volume by a factor of 12.
- Ordering the exact calculated volume with no waste margin, risking a short pour.
- Assuming all bags of the same weight yield the same volume — yields differ by mix design.
- Not accounting for the volume displaced by rebar, forms or embedded posts on larger pours.
- Choosing a mix strength based on price alone instead of the exposure and load the slab will see.
- Underestimating how quickly a ready-mix load can begin to set on a hot day, leaving too little placement time once it is short by even a small margin.
Practical considerations
Curing time matters as much as the pour itself. Concrete reaches enough strength to walk on in roughly 24-48 hours but continues gaining strength for about 28 days, and full vehicle loading (driveways) should generally wait until the mix reaches its rated strength, per the product or supplier's guidance.
Weather affects both the pour and the cure. Pouring in temperatures below about 40°F or above roughly 90°F changes how the concrete sets, and many suppliers offer cold-weather or hot-weather mix adjustments; ask before ordering if the forecast is at either extreme.
Reinforcement and control joints
Rebar or wire mesh reinforcement does not meaningfully change the concrete volume calculation for typical residential slabs, since the steel occupies a small fraction of the pour, but it is a separate line-item cost worth budgeting alongside the concrete itself. Footings and structural slabs are far more likely to require reinforcement than a simple patio.
Control joints (saw-cut or tooled grooves) are placed to control where the slab cracks as it shrinks during curing rather than to prevent cracking entirely. As a rough guide, joints are commonly spaced at roughly 2 to 3 times the slab thickness in feet, so a 4-inch slab often gets joints every 8-10 feet in each direction.
Frequently asked questions
How many bags of concrete equal one cubic yard?
One cubic yard is 27 cubic feet. At roughly 0.60 cubic feet per 80 lb bag, that is about 45 bags; at 0.45 cubic feet per 60 lb bag, about 60 bags. Ready-mix delivery is usually far more economical for anything above roughly 1-2 cubic yards.
Why do ready-mix suppliers recommend a waste allowance?
5-10% above the calculated volume is a common allowance for spillage, form leakage and uneven excavation. Larger or more complex pours often order toward the higher end of that range.
Can I use the same formula for curved or irregular slabs?
Split the shape into simple rectangles and circular or triangular sections, calculate each one's volume separately, then add them together. This keeps the arithmetic simple and reduces the chance of an area error.
How thick should a concrete driveway slab be?
Residential driveways are commonly poured 4-6 inches thick, with the thicker end of that range used where heavier vehicles will park or where the subgrade is less stable. Check local code and soil conditions rather than assuming a single figure applies everywhere.
What psi concrete do I need for a patio versus a driveway?
A patio or sidewalk commonly uses a 2,500-3,000 psi mix, while a driveway typically calls for 3,500-4,000 psi or higher, especially in climates with freeze-thaw cycles. Confirm the required strength with local code or a contractor before ordering, since it is a structural specification, not a preference.
How long does concrete need to cure before it can be used?
Concrete is usually safe to walk on after about 24-48 hours and can typically support light traffic within about a week, but it continues gaining strength for roughly 28 days. Driveways generally should not carry full vehicle weight until the mix approaches its rated strength.
Sources and references
- American Concrete Institute — ACI 332 Residential Code Requirements — Residential slab thickness, reinforcement and placement standards.
- Portland Cement Association — Concrete basics — Mix proportions, yield per bag and curing guidance.
- NRMCA — Concrete in Practice series — Ready-mix ordering practice, waste allowance and truck minimums.