Concrete Volume Calculator

Concrete basics — volume, yards, and bags

Concrete is sold by the cubic yard when ordered from a ready-mix supplier. One cubic yard equals 27 cubic feet (3 ft × 3 ft × 3 ft). When you see a concrete truck with a capacity listed as "8 yards," it can carry 8 cubic yards — 216 cubic feet — of mixed concrete.

The unit conversion that trips most DIYers up: thickness is almost always given in inches (a 4-inch slab, a 6-inch slab), but the cubic foot formula requires all measurements in the same unit. Convert thickness to feet by dividing by 12. A 4-inch slab = 4 ÷ 12 = 0.333 feet thick. Then: Volume (cu ft) = Length × Width × Thickness (in feet). Divide by 27 to get cubic yards.

For columns: The formula is different — a cylinder. Volume = π × r² × height, where r is the radius in feet (diameter ÷ 2 ÷ 12 if given in inches). Our calculator handles this automatically for the column shape option.

Always add a waste factor: Ordering exactly the calculated volume is risky. Concrete left in the truck is wasted, but running short mid-pour means the second batch may set slightly differently, creating a cold joint — a visible seam and potential weak point. Our calculator applies a 10% waste buffer automatically. For complex pours with many penetrations or irregular forms, consider 15%.

Bag concrete: Pre-mixed bagged concrete (Quikrete, Sakrete, and similar brands) is sold in 40 lb, 60 lb, and 80 lb bags. The yield per bag is approximately: 40 lb = 0.30 cu ft, 60 lb = 0.45 cu ft, 80 lb = 0.60 cu ft. Our calculator shows both 60 lb and 80 lb bag counts. Most DIYers prefer 60 lb bags for back-friendliness; the 80 lb bags are heavier but mean fewer trips to the hardware store.

DIY vs hire a contractor — when each makes sense

The decision between DIY concrete and hiring a contractor is one of the most consequential choices in a home improvement project. Concrete is unforgiving — once it begins to set, your window to fix mistakes is short. Poor mixing, inadequate reinforcement, or improper curing can produce a slab that cracks within the first winter.

Good candidates for DIY concrete: Small decorative projects (stepping stones, fence post holes, small pads), repairs and patches to existing concrete, setting posts for fences or mailboxes, and backyard projects where cosmetic perfection is not critical. Volume under 0.5–1 cubic yards with bagged concrete is manageable for a capable DIYer with help.

Always hire a professional for: Foundation work, structural footings, garage slabs larger than 2 car bays, driveways in freeze-thaw climates (require proper sub-base prep, control joint placement, and mix design), any pour exceeding 2–3 cubic yards (managing a ready-mix truck alone is nearly impossible), and any project requiring engineered specifications or building permits. Improper structural concrete is not just cosmetically bad — it can be unsafe and may void permits or cause title issues when selling.

The hidden cost of DIY: "Free labour" is not free. A 2-cubic-yard slab requires moving and mixing approximately 110 bags of 60 lb concrete — the bags alone weigh over 3 tons. Renting a concrete mixer, buying forming lumber, purchasing rebar or wire mesh, and renting a plate compactor for sub-base preparation all add up. Get a contractor quote before deciding — the gap between DIY material cost and professional all-in cost is often smaller than expected.

Concrete mix ratios and PSI strength

Concrete strength is measured in PSI (pounds per square inch) — the compressive strength of a cured sample. Different applications require different strengths, and specifying the wrong mix is a common amateur mistake.

2,500–3,000 PSI: General residential use — walkways, patios in mild climates, small pads, and interior applications. This is the minimum acceptable for any exterior residential use in most building codes.

4,000 PSI: Recommended for residential driveways, slabs that will see vehicle traffic, and any exterior application in a freeze-thaw climate. The lower water-cement ratio in this mix reduces permeability — critical for resisting salt and freeze-thaw damage. This is the default residential mix specified by most concrete contractors.

5,000+ PSI: Commercial and structural applications, high-traffic areas, thin slabs that need extra strength, and any engineered structural element. Quikrete 5000 is available in bags for DIY applications where extra strength is needed.

Additives that affect mix performance: Fiber reinforcement (polypropylene or steel fibers added to the mix) reduces plastic shrinkage cracking and is increasingly standard in residential pours. Accelerators speed setting time — useful in cold weather. Retarders slow setting — helpful in hot weather or for large pours. Air entrainment (required by most codes in freeze-thaw climates) creates microscopic air bubbles that absorb expansion pressure as water freezes in the slab. Always confirm the mix specification with your ready-mix supplier before ordering.

Curing time and how to do it right

Concrete doesn't dry — it cures through a chemical reaction called hydration. Water is not evaporating out; it's reacting with cement to form the crystalline matrix that gives concrete its strength. This distinction matters because the most common DIY mistake — letting new concrete dry out too quickly — actually weakens it.

The 28-day rule: Concrete reaches its rated design strength at 28 days of proper curing. At 7 days it has approximately 70% of final strength; at 14 days, about 85%. These numbers assume proper moisture maintenance during curing. Concrete that dries out in the first week may reach only 40–50% of its potential strength.

How to cure properly: The simplest method for DIY projects is to spray the surface with a curing compound immediately after finishing — this forms a membrane that retains moisture. Alternatively, cover with wet burlap or plastic sheeting, keeping it moist for 7 days. In hot, sunny, or windy conditions, misting the surface during the first few hours can prevent premature surface drying and crazing.

Cold weather concreting: Below 40°F (4°C), hydration slows dramatically; below freezing, fresh concrete can be permanently damaged. If the pour will be exposed to freezing temperatures in the first 7 days, you need protection — heated enclosures, insulating blankets, or accelerated mix designs. Most concrete contractors will not pour when temperatures are expected to drop below freezing within 24–48 hours without protection measures in place.

Traffic timing: Walk on cured slabs after 24–48 hours (longer in cold weather). Light vehicle traffic after 7 days. Full vehicle loads after 28 days. Heavy equipment should wait for full strength and, for slabs not engineered for heavy loads, may not be appropriate regardless of age.

Full cost breakdown for a concrete project

Our calculator provides the concrete volume and estimated ready-mix cost. But concrete projects involve several other cost categories that are easy to underestimate:

Sub-base preparation: Any slab poured on undisturbed native soil without proper sub-base preparation risks uneven settlement and cracking. Standard practice is 4–6 inches of compacted gravel (road base or 3/4" crushed stone). For a 10×20 ft driveway section, that's approximately 2.5–3.5 cubic yards of gravel — add $100–$250 for materials, plus compactor rental.

Forming: Lumber for forms (2×4 or 2×6 depending on slab thickness), stakes, and screws. For a 10×20 ft slab: approximately 60 linear feet of forming lumber, $40–$80 in materials. Forms are usually stripped after 24–48 hours and can be reused.

Reinforcement: Wire mesh or rebar. For residential slabs, 6×6 W1.4×W1.4 wire mesh is the most common — approximately $0.15–$0.25/sq ft. Rebar (typically #4 at 12" on center) is $0.20–$0.35/sq ft plus chair/support costs. Structural footings and engineered applications specify exact rebar size and spacing.

Labour: Professional concrete finishing labour runs $2–$6/sq ft on top of the material cost, depending on complexity, finish type, and region. A basic broom-finished driveway is at the low end; decorative stamping, exposed aggregate, or polished finishes are at the high end. Structural work (footings, walls) is typically priced by the project scope.

Permits: Many jurisdictions require a permit for concrete work on structural elements (foundations, retaining walls, garage slabs) — check with your local building department before starting. Permit fees vary widely, $50–$500+ depending on project value and jurisdiction.

Frequently asked questions