Concrete Calculator
Estimate concrete volume and bags needed for slabs and footings.
Concrete needed
1.36 cu yd
includes 10% overage
Exact volume
1.23 cu yd
80 lb bags
56
60 lb bags
75
Volume with a safety margin
Volume = length × width × depth, with depth converted to feet and the result divided by 27 for cubic yards. Pros order about 10% extra: the subgrade is never perfectly flat, forms bow slightly, and some concrete is always lost to the chute and the wheelbarrow. Running short mid-pour leaves a cold joint — a permanent weak seam — so the surplus is cheap insurance.
Worked example: a 10 × 10 ft patio slab
At 4 inches thick that is 10 × 10 × 0.333 = 33.3 cubic feet, or 1.23 cubic yards exactly. With 10% overage you order 1.36 cubic yards. In bags that is 56 of the 80 lb size or 75 of the 60 lb size — roughly 4,500 lb of material to mix by hand, which is the point where most people switch to ready-mix.
Bags vs. ready-mix
An 80 lb bag yields about 0.6 cubic feet and a 60 lb bag about 0.45. Below roughly half a cubic yard — post holes, a small pad, a few steps — bags win on convenience and leftover waste. Above about one cubic yard, ready-mix wins on labor, consistency, and cost per yard, though most suppliers add a short-load fee under 3–4 yards and set a minimum delivery.
Thickness, reinforcement, and base
Four inches suits patios, walkways, and shed floors; driveways and anything carrying vehicles want 5–6 inches, often with thickened edges. Below the slab, 4 inches of compacted gravel controls drainage and frost movement — and it is not part of your concrete volume. Rebar and welded wire don't change the volume either, though fiber mesh and air entrainment can nudge the yield.
Ordering and pouring day
Order in cubic yards, not bags, and tell the supplier the slab thickness and whether it faces freeze-thaw so they can set the mix and air content. Line up more hands than you think you need: a yard of concrete weighs about 4,000 lb and stays workable for well under an hour in warm weather. Dampen the subgrade, never the mix — adding water on site to buy working time permanently weakens the slab.
Frequently asked questions
- How many 80 lb bags of concrete are in a cubic yard?
- About 45. A cubic yard is 27 cubic feet and an 80 lb bag yields roughly 0.6 cubic feet, so 27 ÷ 0.6 ≈ 45 bags — around 3,600 lb of material. For 60 lb bags it is closer to 60 bags per cubic yard.
- How thick should a concrete slab be?
- Four inches is standard for patios, sidewalks, and shed floors. Use 5–6 inches for driveways or anywhere a vehicle parks, and check local code, since frost depth and soil type often require a thicker slab or a footing around the perimeter.
- How much extra concrete should I order?
- Ten percent is the common allowance, and what this calculator adds. Go to 15% if the subgrade is uneven, the forms are irregular, or you are pouring over gravel that may settle. You can't pause a pour to fetch more, so err high.
- Do I need gravel under a concrete slab?
- Almost always. Four inches of compacted crushed gravel gives you drainage, a stable base, and protection against frost heave. It sits below the slab, so don't include it when calculating concrete volume.
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