Concrete needed
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Cubic feet
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80 lb bags
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Ready-mix order
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Your pour
SlabDiagram scales with your measurements.
Bags needed by size
| Bag size | Yield | Bags needed |
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Bag counts include your waste allowance and are rounded up to whole bags.
How to calculate concrete
Concrete is ordered by volume, in cubic yards (often just called "yards" or "yardage"). Whether you need a concrete slab calculator, a concrete volume calculator or a concrete mix calculator, the method is identical — this concrete yard calculator finds the volume of your pour and converts it to cubic yards:
- Measure length and width in feet and thickness in inches.
- Convert thickness to feet by dividing by 12 (a 4-inch slab is 0.333 ft).
- Multiply length x width x thickness for cubic feet.
- Divide cubic feet by 27 to get cubic yards.
- Add 5-10% waste, then convert to bags or a ready-mix order.
Example: a 10 ft x 10 ft slab at 4 inches is 10 x 10 x 0.333 = 33.3 cu ft = 1.23 cubic yards (about 1.35 yd with 10% waste). Need just the bag count? Use the dedicated concrete bag calculator, or the cubic yards calculator for any material.
Slabs, footings, walls, columns and steps
Each shape uses the same idea — area x depth — with one twist for round columns and steps:
- Slab / footing: length x width x thickness.
- Wall: length x height x thickness.
- Column / sonotube: π x radius² x height (radius = diameter ÷ 2). A 12-inch tube 4 ft tall is about 0.12 cu yd, so about 5-6 x 80 lb bags each.
- Steps: the solid under the treads, which grows with the square of the step count. Rise and run are entered in inches.
Concrete bags vs ready-mix
An 80 lb bag makes about 0.60 cu ft, so it takes roughly 45 bags per cubic yard; a 60 lb bag makes ~0.45 cu ft (60/yard) and a 40 lb bag ~0.30 cu ft (90/yard). Bags suit small jobs; beyond about a cubic yard, ready-mix is usually cheaper and faster. This tool shows both so you can choose.
Frequently asked questions
How do I calculate concrete for a slab?
Multiply length x width x thickness to get the volume, keeping every measurement in the same unit. For a slab, measure length and width in feet and thickness in inches, divide the thickness by 12 to convert to feet, then multiply the three together for cubic feet. Divide cubic feet by 27 to get cubic yards, which is how ready-mix concrete is ordered. This calculator does all of that for you and also adds a waste allowance.
How do you calculate concrete yardage?
Concrete "yardage" means cubic yards. Work out the volume in cubic feet (length x width x depth, all in feet) and divide by 27, because there are 27 cubic feet in one cubic yard. For example, a 10 ft x 10 ft slab at 4 inches thick is 10 x 10 x 0.333 = 33.3 cu ft, or about 1.23 cubic yards.
How do you calculate cubic yards of concrete?
To calculate cubic yards of concrete, multiply length x width x depth in feet to get cubic feet, then divide by 27 (27 cubic feet = 1 cubic yard). A fast shortcut for a slab is (length ft x width ft x thickness in inches) / 324, which gives cubic yards of concrete directly. Add 5-10% for waste before you order. (Working in metric? Multiply length x width x thickness in metres for cubic metres, or m3 — 1 cubic yard is about 0.765 m3.)
How much concrete do I need for a 10x10 slab?
A 10 ft x 10 ft slab poured 4 inches thick needs about 1.23 cubic yards (33.3 cubic feet) of concrete before waste — roughly 1.35 cubic yards with a 10% allowance. That is about 56 x 80 lb bags, 74 x 60 lb bags, or a small ready-mix order. Increase the thickness to 6 inches and you need about 1.85 cubic yards.
How much concrete for a 30x40 slab 4 inches thick?
A 30 ft x 40 ft slab at 4 inches thick is 30 x 40 x 0.333 = 400 cubic feet, or about 14.8 cubic yards before waste (order roughly 15-16 cubic yards with a 5-10% allowance). A pour this size is well beyond bag territory, so order ready-mix from a truck. At 6 inches thick it works out to about 22 cubic yards.
How much does a 20x20 concrete slab cost?
A 20 ft x 20 ft slab at 4 inches is 20 x 20 x 0.333 = 133 cubic feet, or about 4.9 cubic yards. Ready-mix concrete runs roughly $130-$170 per cubic yard in most of the US, so the material alone is about $650-$850; finished and installed (labor, forms, rebar, finishing) a slab this size typically costs several thousand dollars. This tool gives the exact quantity — prices vary by region, so confirm with local suppliers.
How many 80 lb bags of concrete are in a cubic yard?
An 80 lb bag of pre-mixed concrete yields about 0.60 cubic feet, so you need about 45 bags to make one cubic yard (27 / 0.60). A 60 lb bag yields ~0.45 cu ft (60 bags per yard) and a 40 lb bag ~0.30 cu ft (90 bags per yard). Above roughly 1 cubic yard it is usually cheaper and easier to order ready-mix.
Should I use bags or ready-mix concrete?
For small jobs — footings, post holes, a few fence posts, a small pad or steps — bags of pre-mixed concrete are convenient. Once you pass roughly 1 cubic yard (about 45 x 80 lb bags), ordering ready-mix from a truck is usually cheaper, faster and gives a more consistent pour. This calculator shows both so you can compare.
How much extra concrete should I order for waste?
Add 5% to 10% for waste, spillage, uneven subgrade and slab over-dig. Ten percent is a safe default for most DIY pours; use 5% for tightly formed work and up to 15% for rough ground or footings. Running short mid-pour causes a cold joint, so it is better to slightly over-order.
How thick should a concrete slab be?
Four inches is standard for patios, walkways and shed floors. Use 5-6 inches for driveways and areas that carry vehicles, and thicken the edges of a slab where it bears load. Always check your local building code, as requirements vary.
How many cubic yards fit in a concrete truck?
A fully loaded ready-mix truck typically holds about 10 cubic yards, though many deliver 8-9 to stay within weight limits. Suppliers usually sell in quarter-yard increments and may charge a "short load" fee below about 1 cubic yard.