# How much concrete do I need for a 20x20x4 slab?

Let’s talk about concrete ready mix – it’s like a blend of ingredients, including Portland cement, sand, gravel, and other approved stuff. For concreting the slab, you can use dry ready mix concrete. It’s easily available in different bag sizes and weights like, like 40lb, 60lb, 80lb, 90lb bags, and more.

But before you can order the bags, it’s best that you know how much concrete do I need for a 20x20x4 slab. You need to figure out the cubic yards of concrete you need. You can reckon the quantity by multiplying the length, width, and depth of the slab.

In our today’s blog, we’re going to calculate how much concrete we need for a 20×20 slab, which is 400 square feet, at 4 inches, 5 inches, or 6 inches thickness.

Usually, a standard slab is 4 inches thick in regular situations. However, for more load and traffic you can increase the thickness of the slab to 5 or 6 inches. But it’s all depends on the subgrade condition as well as the amount of load you want to impose. A thin slab may not work good as a driveway.

It’s also important to understand that a concrete in dry condition is about 133 pounds per cubic foot. That’s around 3600 pounds for every cubic yard. But in MKS system it is 2136 kilograms per cubic meter. This unit weight of concrete depends on its density, which can change based on how much aggregate, water, and air is in the mix.

In the United States, we like our simple measurements. So, 1 cubic yard is basically a volume that’s 3 feet long, 3 feet wide, and 3 feet deep (or 3’x3’x3′ = 27 cubic feet). So, 1 cubic yard equals 27 cubic feet.

## How much concrete do I need for a 20×20×4 slab

Let’s figure out how much concrete you need for a 20x20x4 slab. Here’s how you do it:

• Step 1: Calculate the volume in cubic feet. You’ve got a 20×20 slab, and it’s 4 inches thick, which is the same as 0.33 feet. So, to find the volume of the slab, multiply the length, width, and thickness, like this: 20′ x 20′ x 0.33′ = 132 cubic feet.
• Step 2: Convert cubic feet into cubic yards. To do this, you divide it by 27 because 1 cubic yard equals 27 cubic feet. So, the volume of concrete you need for your 20×20 slab, at 4 inches thick, in cubic yards is: 132 / 27 = 4.88 cubic yards.
• Step 3: Add 10% extra. It’s always good to consider a bit of extra concrete for any wastage during mixing and pouring. So, take 10% of 4.88 cubic yards, which is 0.488, and add it to the original amount: 4.88 + 0.488 = 5.368 cubic yards. That’s your final quantity of concrete needed.

## How much concrete do I need for a 20×20×4 slab

To create a 20x20x4 slab, here’s what you’ll typically require:

Concrete Volume: You’ll need about 4.88 cubic yards or 132 cubic feet of concrete.

Bags of Premixed Concrete: Alternatively, you could use approximately 294 bags of 60lb premixed concrete, 220 bags of 80lb, or 440 bags of 40lb.

For safer estimate you must include wastages, usually around 5 to 10% extra, to account for any unexpected factors during the pouring and mixing process.

This 4-inch thick slab of 20×20 is perfect for standard load conditions and is commonly used for constructing sidewalks, curbs, steps, ramps, walkways, and patios.

## How many yards of concrete do I need for a 20×20×4 slab

To build a 20x20x4 slab, here’s what you’d typically require:

• Concrete Volume: You’d need approximately 4.88 cubic yards or 132 cubic feet of concrete.
• Concrete Bags: Alternatively, you could use around 220 bags of 80-pound concrete, 294 bags of 60-pound, 352 bags of 50-pound, or 440 bags of 40-pound premixed concrete.

Here’s a breakdown of the math:

1. Calculate the square footage of the slab: 20×20 = 400 sq ft
2. Convert the 4-inch thickness to feet: 4 ÷ 12 = 0.33 feet
3. Determine the volume of the slab in cubic feet: 400 x 0.33 = 132 cubic feet
4. Find the volume of the slab in cubic yards: 132 ÷ 27 = 4.88 cubic yards

So, for a 20x20x4 slab, you’ll need approximately 4.88 cubic yards of premixed concrete. It’s always a good idea to have a bit more than needed, around 5-10%, for unexpected contingencies during the mixing and pouring process.

## How many cubic feet of concrete do I need for a 20×20×4 slab

To create a 20x20x4 slab, you’ll typically require around 132 cubic feet of premixed concrete. Here’s the math behind it:

1. Calculate the square footage of the slab: 20×20 = 400 sq ft
2. Convert the 4-inch thickness to feet: 4 ÷ 12 = 0.33 feet
3. Determine the volume of the slab in cubic feet: 400 x 0.33 = 132 cubic feet

That means, for a 20x20x4 slab, you’ll need around 132 cubic feet of premixed concrete. It’s always wise to have a little extra on hand, usually around 5-10%, to account for any unforeseen needs during the mixing and pouring process.

## How many bags of concrete do I need for a 20×20×4 slab

To determine how many bags of concrete you’ll require, divide the total cubic yards or cubic feet needed by the yields.

You’ll find different sizes of concrete bags like: 40lb, 60lb, 80lb, and 90lb. Here’s what you can expect from each:

• A 40lb bag of concrete yields approximately 0.011 cubic yards or 0.30 cubic feet.
• A 60lb bag of concrete yields about 0.017 cubic yards or 0.45 cubic feet.
• An 80lb bag of concrete produces roughly 0.022 cubic yards or 0.60 cubic feet.
• A 90lb bag of concrete results in about 0.025 cubic yards or 0.68 cubic feet.
1. Let’s find the square footage of the slab: 20×20 = 400 sq ft
2. Convert the 4-inch thickness to feet: 4 ÷ 12 = 0.33 feet
3. Determine the volume of the slab in cubic feet: 400 x 0.33 = 132 cubic feet
4. Number of 60lb bags needed: 132 ÷ 0.45 = 294 bags
5. Number of 80lb bags needed: 132 ÷ 0.60 = 220 bags

In case of a 20x20x4 slab, you should order 294 bags of premixed concrete if the weight is 60lb per bag or 220 bags of 80lb premixed concrete. However, I always recommend to have a bit extra on hand (usually 5-10%) to account for any unexpected needs during the mixing and pouring process. Saad Iqbal is a professional civil engineering and freelance write. He's passionate about structures, construction management, and home improvement topics. He's been working as a Senior Engineer in a consultant firm for over 8 years. Besides he loves writing informative and in-depth content focused on construction and home-related topics. You can catch him at his linkedin page or reach out via our contact us page.