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12x20 Concrete Slab

How much concrete do you need for a 12x20 slab? Uncover the amount, cost, bags needed and more.
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    Amount Needed

    How much concrete for a 12x20 slab?

    Short answer: you'll need 80 cubic feet or ~2.96 cubic yards of concrete.

    Here’s how you can calculate the volume of concrete required for a 12x20 slab:

    1. Determine the thickness of the slab. Let's say the thickness is 4 inches (which is a common thickness for residential slabs).
    2. Convert the thickness to feet. 4 inches is 1/3 of a foot (since 1 foot = 12 inches), so it becomes 1/3 feet, or 0.33 feet.
    3. Calculate the volume. Use the following formula: length (in feet) x width (in feet) x thickness (in feet).
    4. Multiply the volume by the concrete mix ratio. For typical residential applications, a mix ratio of 1:2:4 (1 part cement, 2 parts sand, 4 parts gravel) is common.

    So, for a 12x20 slab with a thickness of 4 inches:

    1. Thickness in feet = 4 inches / 12 = 1/3 feet
    2. Volume = 12 feet x 20 feet x 0.33 feet = ~80 cubic feet
    3. Assuming a mix ratio of 1:2:4, the total volume of concrete needed will be higher, as this ratio includes not just cement but also sand and gravel. So let's say the total volume needed is 1.5 times the volume of concrete (to account for air space between aggregate particles and the voids they fill).

    With those numbers in mind, the total volume of concrete needed would be 80 cubic feet or ~2.96 cubic yards of concrete.

    Cost

    12x20 concrete slab cost

    The cost of a 12x20 concrete slab can vary based on several factors. Factors such as the location, local labor and material prices, site preparation requirements, and the specific design of the slab can influence the overall cost. To get a more accurate estimate, it's recommended to obtain quotes from local contractors who can take these factors into account and provide a detailed cost breakdown for your project.

    Additionally, considering any optional features or finishes, such as stamped patterns or decorative coatings, will also impact the final cost of the concrete slab.

    Average Cost by Thickness

    Slab Thickness Average Price per Square Foot
    2 inches $5.00
    4 inches $5.00
    6 inches $6.00
    8 inches $8.00

    Cost of 12x20 concrete slab 4 inches thick

    For a 4 inch slab, you can expect to pay around $665 plus additional material and labor costs (if needed).

    Slab Thickness Average Price
    4 inches $665

    Cost of 12x20 concrete slab 6 inches thick

    For a 6 inch slab, you can expect to pay around $1,197 plus additional material and labor costs (if needed).

    Slab Thickness Average Price
    6 inches $1,197

    Average cost for 12x20 Slab

    Based on the average price for each thickness, here's how much you can expect to pay given the thickness:

    Slab Thickness Average Price for 12x20 Slab
    2 inches $333
    4 inches $665
    6 inches $1,197
    8 inches $2,128
    Thickness

    How thick should a 12x20 concrete slab be?

    Short answer: A 4-inch thickness for a 12x20 slab is most common, but it depends.

    The thickness of a concrete slab depends on a bunch of different factors, including the intended use of the slab, the soil conditions, and the expected loads it will need to bear. For a 12x20 slab, here are some considerations for thickness:

    • Intended Use: If the slab is for light-duty residential use, such as a patio or walkway, a thickness of 4 inches is commonly used. For heavier applications, such as a small driveway or garage floor, a thicker slab may be required.
    • Soil Conditions: The soil's load-bearing capacity beneath the slab affects the required thickness. If the soil is stable and well-compacted, a thinner slab will work fine. However, if the soil is less stable or prone to settling, a thicker slab may be necessary to distribute the load more evenly.
    • Expected Loads: Consider the weight and frequency of loads the slab will bear. For example, if you plan to park a heavy truck on it, you’ll need a thicker slab as opposed to a walkway for people.
    • Reinforcement: Adding reinforcement, such as rebar or wire mesh, can increase the strength and durability of the slab, allowing for a thinner thickness while still supporting the necessary loads.

    With that said, while a 4-inch thickness is common for a slab of this size, the specific thickness for a 12x20 slab should be determined based on factors like intended use, soil conditions, expected loads, and whether reinforcement will be used. 

    These are simply guidelines — consulting with a structural engineer or local building codes can provide additional guidance on the appropriate thickness for your specific project.

    Number of Bags

    How many bags of concrete do I need for a 12x20 slab?

    Short answer: If using a 80 lb. bag, you'll need about 134 bags. This amount will vary if you use bags that are smaller or larger.

    To determine the number of bags of concrete needed for a 12x20 slab, you'll need to calculate the volume of concrete required and then divide by the volume of concrete each bag can produce.

    Assuming a slab thickness of 4 inches (which is 1/3 of a foot), we've already calculated the volume of concrete needed for the slab as approximately 2.96 cubic yards.

    Now, you need to convert cubic yards to cubic feet because concrete bags are usually sold in cubic feet.

    1 cubic yard = 27 cubic feet

    So, to convert 2.96 cubic yards to cubic feet:

    • Volume in cubic feet = 2.96 cubic yards × 27
    • Volume in cubic feet ≈ 79.92 cubic feet

    Now, we can determine how many bags of concrete you’ll need. Typically, an 80 lb. bag of concrete mix produces 0.6 cubic feet of concrete. So, to find out how many bags you need:

    • Number of bags = Volume in cubic feet / Volume per bag in cubic feet
    • Number of bags = 79.92 cubic feet / 0.6 cubic feet per bag
    • ​Number of bags ≈ 134 bags

    So, you would need approximately 134 bags of concrete for a 12x20 slab with a thickness of 4 inches if using 80 lb. bags. This is a pretty large amount, so double-checking your calculations and possibly considering ordering concrete by the cubic yard from a supplier might be more practical and cost-effective.

    Number of bags needed

    Bag Size Bags Needed
    40 lbs. 266
    50 lbs. 213
    60 lbs. 178
    80 lbs. 134
    YardS

    How many yards of concrete for a 12x20 slab?

    Short answer: ~2.96 yards for a slab that’s 4 inches thick.

    To determine how much concrete is needed for a 12x20 slab, you'll need to calculate the volume of concrete required based on the dimensions of the slab and its desired thickness.

    Assuming the slab will be 4 inches thick:

    1. Convert the thickness to feet: 4 inches is equal to 1/3 of a foot (since 1 foot = 12 inches).
    2. Calculate the volume of concrete in cubic feet: Use the formula: Length (in feet) x Width (in feet) x Thickness (in feet).
    3. Convert the volume to cubic yards: Divide the volume in cubic feet by 27 (since 1 cubic yard is equal to 27 cubic feet).

    Here's how to do it:

    1. Thickness in feet = 4 inches / 12 = ~0.33 feet
    2. Volume = 12 feet x 20 feet = 240 cubic feet
    3. Number of cubic yards = (240 cubic feet / 27) * 0.33 feet = approximately 2.96 cubic yards

    So, you would need roughly 2.96 cubic yards of concrete for a 12x20 slab with a thickness of 4 inches. However, it's always a good idea to order a little more concrete than calculated to account for any miscalculations or spillage during pouring.

    Rebar REquirements

    How much rebar do I need for a 12x20 slab?

    Short answer: For one direction, you'll need about 96 feet of #4 rebar. For both directions, you'll need double that amount, so about 192 feet.

    To figure out how much rebar you’ll need for a 12x20 concrete slab, you'll want to consider factors such as the intended use of the slab, the soil conditions, and any local building codes or regulations. If you aren’t familiar, Rebar (short for reinforcement bar) is typically used to reinforce concrete and improve its strength and durability.

    For a 12x20 slab, you can follow these general guidelines:

    • Spacing: Rebar is typically spaced evenly throughout the slab. Common spacing for residential applications is 18 inches apart in both directions, creating a grid pattern.
    • Size of Rebar: Rebar comes in various sizes, commonly ranging from #3 to #11. The size you choose depends on the load requirements of your slab. For a 12x20 slab, #3 or #4 rebar may be sufficient for light-duty applications.
    • Calculating Rebar Length: To calculate the length of rebar needed, you'll multiply the perimeter of the slab by the spacing between the rebars. Since you're spacing them at equal intervals, you only need to calculate the length of one side.
      • Perimeter = 2 * (Length + Width)
      • Length of rebar needed = Perimeter * Spacing

    For example, if you're using #4 rebar spaced 18 inches apart:

    • Perimeter = 2 * (12 feet + 20 feet) = 64 feet
    • Length of rebar needed = 64 feet * (1.5 feet / 1 rebar) = 96 feet

    So, for one direction (either length or width), you would need approximately 96 feet of #4 rebar. For both directions, you'll need double that amount, so a total of about 192 feet.

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        A diagram of slab and footing measurements
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