How do I calculate how much concrete I need?

Multiply length by width in feet, multiply by the thickness in inches divided by 12, and you get cubic feet. Divide by 27 for cubic yards. Then add 5% to 10% for waste. A 10 by 10 foot slab that is 4 inches thick is 33.3 cubic feet, or 1.23 cubic yards, and about 1.36 cubic yards with 10% extra.

How many bags of concrete do I need for a 10 by 10 slab?

For a 10 by 10 foot slab 4 inches thick you need about 56 bags of 80 pounds without any extra, and 62 bags with a 10% allowance. That is 5,500 pounds of concrete, so most people use a ready-mix truck for a job this size.

How much does a bag of concrete yield?

An 80-pound bag yields about 0.6 cubic feet, a 60-pound bag about 0.45 cubic feet and a 40-pound bag about 0.3 cubic feet. It takes roughly 45 bags of 80 pounds to make one cubic yard.

Is it cheaper to buy bags or order ready-mix?

For small jobs of a few bags, bags are cheaper and easier. Past about half a cubic yard to one cubic yard, ready-mix usually costs less per cubic yard and saves hours of mixing, but suppliers often charge a short-load fee for small orders. Call your local plant for its minimum.

How much extra concrete should I order?

Ten percent is a safe default for a patio or driveway. Use 5% for a perfectly formed, level pour and 15% when the ground is uneven, you are digging footings in soil or the forms may move. Running short is worse than having a little left over.

How thick should a concrete slab be?

A residential slab on the ground is normally at least 3.5 inches thick under the International Residential Code. Patios and walkways are commonly 4 inches, driveways for cars and light trucks 4 to 5 inches, and garage floors or areas with heavy loads 5 to 6 inches. Your local code and soil conditions decide the final design.

How much does a cubic yard of concrete weigh?

Normal-weight concrete weighs about 150 pounds per cubic foot, which is about 4,050 pounds, or just over 2 tons, per cubic yard.

🏠 Home & Garden

Concrete Calculator · Cubic Yards, Bags and Cost for a Slab

Ordering the right amount of concrete matters because you cannot go back for more once the truck leaves or the mix starts to set. Too little leaves a cold joint or an unfinished corner. Too much is money poured on the ground. This calculator turns the size of your project into cubic yards for a ready-mix delivery and into the number of 40-, 60- and 80-pound bags for a small job you mix yourself.

Enter the length and width in feet and the thickness in inches. The calculator multiplies them, converts the result to cubic feet and cubic yards, and adds an allowance for spillage, an uneven subgrade and over-excavation, 10% by default. It then shows how many bags of each size you need, the approximate weight of the pour and an estimate of the cost both ways, so you can see where bags stop being cheaper than a truck.

It works for any rectangular pour: a patio, a walkway, a shed or garage pad, a driveway or a footing. For a footing or a wall, enter its width in feet (a 16-inch footing is 1.33 feet) and its depth in inches. The prices are examples you should replace with a quote from your local plant and the price on the bag at your store. Bag counts are planning figures: brands vary slightly, and a structural pour should follow your plans and local code.

Detailed Breakdown

11 metrics

PRIMARY RESULT

1.36 cubic yards

Concrete Needed (With Extra)

Volume Without Extra

1.23 cubic yards (33.33 cubic feet)

Cubic Feet With Extra

36.67 cubic feet

Cubic Meters With Extra

1.04 cubic meters

80-lb Bags Needed

62 bags

60-lb Bags Needed

82 bags

40-lb Bags Needed

123 bags

Approximate Weight

5,500 lb (2.75 tons)

Surface Area

100 square feet

Ready-Mix Cost (Concrete Only)

$224.07

Cost in 80-lb Bags

$465.00

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exploreReal-World Scenarios

When to Use This Calculator

Everyday financial situations where this tool gives you fast, accurate clarity:

🏠

Plan a Patio or Walkway

Get the cubic yards or bags for a 4-inch slab before you buy.

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Order for a Driveway or Garage

See the volume for a 5- or 6-inch pour and the weight the truck must bring.

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Size a Footing or Pad

Enter the width and depth of a footing or a shed pad and add a waste allowance.

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Compare Bags and a Truck

Check the cost of mixing bags yourself against a ready-mix delivery.

Calculation Formula

Standard Method

Volume (cubic feet) = Length (ft) × Width (ft) × Thickness (in) ÷ 12 Cubic yards = Cubic feet ÷ 27 With extra = Volume × (1 + Extra %) Bags = Cubic feet with extra ÷ Yield per bag, rounded up Yield per bag: 80 lb ≈ 0.60 cu ft, 60 lb ≈ 0.45 cu ft, 40 lb ≈ 0.30 cu ft Weight ≈ Cubic feet × 150 lb (normal-weight concrete) Cubic meters = Cubic feet ÷ 35.31 Ready-mix cost = Cubic yards with extra × Price per cubic yard Bag cost = 80-lb bags × Price per bag One cubic yard covers about 81 square feet at 4 inches thick, 65 square feet at 5 inches and 54 square feet at 6 inches. The bag yields are the rounded values printed on packaged dry mix. They differ a little by brand, and the price inputs are examples, not quotes.

Benchmark Example:

A 10 ft by 10 ft patio poured 4 inches thick, with 10% extra: • Volume: 10 × 10 × 4 ÷ 12 = 33.33 cubic feet = 1.23 cubic yards • With 10% extra: 36.67 cubic feet = 1.36 cubic yards (1.04 cubic meters) • 80-lb bags: 36.67 ÷ 0.60 = 61.1, so 62 bags • 60-lb bags: 82 bags, and 40-lb bags: 123 bags • Weight: 36.67 × 150 = 5,500 lb, about 2.75 tons • Ready-mix at $165 a cubic yard: 1.36 × $165 = $224.07 • 80-lb bags at $7.50: 62 × $7.50 = $465.00 The bags would cost about twice as much as the truck, but 62 bags is also 4,960 pounds to haul, mix and pour before the concrete sets. Most people mix by hand or with a small mixer only up to roughly 1 cubic yard, and a ready-mix supplier may add a short-load fee for small orders, so ask for the minimum before you decide.

Frequently Asked Questions

7 answers

Applicable Laws & Regulatory Standards

International Code Council, American Concrete Institute & ASTM International

Official Legal Reference

International Residential Code (IRC) §R506 — Concrete floors (on ground)

Sets the minimum thickness of 3.5 inches for a residential concrete slab on the ground, which is why the calculator warns when the thickness is below that value.

ASTM C94/C94M — Standard Specification for Ready-Mixed Concrete

Governs how ready-mixed concrete is batched, delivered and sold by the cubic yard, the unit this calculator uses for a truck order.

ASTM C387/C387M — Packaged, Dry, Combined Materials for Mortar and Concrete

Covers the bagged dry mix sold at home improvement stores, whose printed yield per bag is the basis for the bag counts shown here.

ACI 318 — Building Code Requirements for Structural Concrete

The reference for structural concrete design. A structural pour, such as a footing, a foundation or a driveway for heavy loads, should follow your plans and local code, not a quantity estimate alone.

Legal citations and statutes are provided for consumer transparency, analytical validation, and educational reference under U.S. consumer financial regulations. This tool is not a substitute for formal legal, accounting, or tax counsel.

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