Concrete Slab Calculator: How Much Concrete Do I Need?

Use this calculator to estimate the amount of concrete and number of bags you’ll need for a concrete foundation. Enter the length and width in feet. Next, enter the desired thickness (depth) in inches of your concrete slab foundation.

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How to Measure Concrete

Manually calculate your concrete slab volume in cubic yards (yd3) by following the formula below: 

Step 1: Measure the square footage

Get the surface area by multiplying the length and width of your garage footing or foundation.

For example, a 25’x25’ two-car garage will be 625 sq. ft. 

Step 2: Determine the slab thickness 

A concrete slab is usually at least four to six inches thick, unless otherwise determined by your local building codes. Next, convert the inches into feet by dividing the number by twelve. This will be the depth of your foundation.

Example: For a slab four inches thick, calculate 4 ÷ 12 = 0.333 ft.

Step 3: Multiply the square footage with the slab thickness

This results in the volume of your concrete slab in cubic feet. 

Example: For a 625-sq.-ft. slab that is 0.333 feet thick, calculate 625 x 0.333 = 208.13 ft3.

Step 4: Convert the cubic feet to cubic yards

You’ll need to do this since concrete purchases are measured in cubic yards (yd3). To get this number, divide the square footage above by twenty-seven. 

Example: 208.13 ft3 ÷ 27 = 7.71 yd3

Step 5: Determine the number of bags needed

Once you have your volume above, you can approximate the number of premixed bags you’ll need using the formulas below:

• 90 40-lb bags make 1 yard
• 60 60-lb bags make 1 yard
• 45 80-lb bags make 1 yard

For example, to approximate the number of 40-pound bags needed for a 7.71-yd3 concrete slab, you will multiply 90 x 7.71, equaling about 694 bags needed. 

For more Concrete Slab Formworkinformation, please contact us. We will provide professional answers.

Quick Tip: It’s better to overestimate the amount you need so you don’t come up short.

Frequently Asked Questions

The first thing to do is decide if you want volumetric or ready-mixed concrete. Here at EasyMix we always recommend volumetric concrete as it offers more benefits to customers, no matter whether you need a small amount for a DIY project or a large batch for a commercial development.

Volumetric concrete is mixed onsite in a specialist volumetric mixing vehicle, and is delivered immediately.

Why Choose Volumetric Concrete?

Getting the right quantity

Volumetric concrete is mixed on site, so if you discover you need more or less concrete when your operator arrives, they can mix exactly what you ask for. This means you save money as you don’t end up paying for concrete you don’t need, and if you need an extra load, you don’t have to place another order!

Getting the right type

Having an idea of the type of concrete you need when you call up to place your order is essential, but if on the day of delivery you need to alter the slump, grade or even include an extra admixture to suit changing conditions or requirements, your operative can provide that.

No mess

You might be tempted to mix your own concrete if you just have a small DIY project to complete, but the mess can be difficult to tidy up. Our mixer vehicles can deliver exactly the amount you need, even if it’s just a few square metres, and we always clean up after ourselves.

No waste

Volumetric concrete is much more cost-effective than ready mixed concrete. As we only mix up the exact amount you need, there is no leftover concrete – which means no part-load charges and no waste disposal charges for the excess!

Types of Volumetric Concrete

Next, you need to determine what type of concrete you need. As we’ve already explained, we can alter this when we arrive on site, but having a good idea of the concrete you want will ensure we have all the right ingredients (such as the right size aggregate and any admixtures you might need) when we arrive on site.

Do you want screed?

Screed is a thinner layer of concrete that is usually laid over a concrete base to create a level, smooth and hard-wearing final floor covering. It is usually of a higher quality than structural concrete so it uses finer aggregates. If you need to finish a concrete floor, then you need to order screed, not concrete.

Do you want concrete?

If you need concrete for any other project, such as building a base, foundations or walls, then you will need to choose a grade of concrete. The standard grades on offer range from C7/8 to C40 and each offers a different strength and suits different purposes.

The information supplied below is for guidance only, so if you are at all unsure what grade you need, we recommend consulting a builder or specialist.

• C7/8: Cavity filling, kerbing, domestic foundations & haunching.
• C10: Foundations for steps, trench fill, floor blinding & drainage works.
• C15: Foundations for small walls, sheds & conservatories. Paving for steps and paths.
• C20: Foundations for large walls, garages, houses & extensions. Paving for patios. Reinforced bases & oversites for conservatories, garages, sheds, greenhouses.
• C25: Foundations and reinforced bases for houses & extensions. Trench fill, kerbing & patios.
• C30: Paving external kennels and reinforced hard standings. Reinforced bases for workshops and unreinforced bases for houses & extensions.
• C35: Reinforced bases for commercial buildings and agricultural light storage areas.
• C40: Foundations for septic tanks, paving HGV parks and agricultural yards.

Do you want admixtures?

Depending on your application and requirements, you may benefit from adding additional qualities to your concrete to ensure it meets your requirements. These could include:

For more information, please visit Concrete Steel Formwork.

• Fibres for enhanced strength
• Plasticisers for more free-flowing concrete and better workability
• Retarding agents to reduce the rate of curing, allowing you longer to work with it
• Accelerating chemicals to increase the rate of curing (if your project is on a tight deadline)
• Corrosion inhibitors to reduce the corrosion of reinforced steel rebars within the concrete

Placing your order