Concrete Slab Calculator

Q: Should I order extra concrete beyond what the calculator says?

Yes. Most contractors order 5% to 10% more than the calculated amount. This covers uneven ground, spillage, and small errors in your measurements. Running short during a pour can create weak spots in the slab, so it is better to have a little extra.

Q: What is the minimum thickness for a concrete slab?

For most residential projects, the minimum thickness is 4 inches . This works well for patios, sidewalks, and shed floors. If vehicles will drive or park on the slab, use at least 5 to 6 inches.

Q: How much does a cubic yard of concrete weigh?

A cubic yard of wet, mixed concrete weighs about 3,900 to 4,050 pounds (roughly 2 tons). Keep this in mind when planning delivery and making sure the ground can support the load during the pour.

Q: What is the difference between cubic feet and cubic yards?

There are 27 cubic feet in 1 cubic yard . Cubic feet is a smaller unit, while cubic yards is the standard unit used when ordering ready-mix concrete from a supplier. To convert cubic feet to cubic yards, divide by 27.

Q: How much does a concrete slab cost?

Ready-mix concrete typically costs between $120 and $200 per cubic yard , depending on your location and the mix design. For a small DIY project using bagged concrete, expect to pay around $5 to $7 per 80-pound bag. Labor, forms, and base preparation add to the total cost.

Q: Do I need gravel under my concrete slab?

Yes, in most cases. A layer of 4 to 6 inches of compacted gravel under the slab helps with drainage and prevents the soil from shifting. This gives the slab a stable, even base and reduces the chance of cracking over time.

Q: How long does concrete take to dry and cure?

Concrete is usually hard enough to walk on in 24 to 48 hours . However, it takes about 28 days to reach its full strength. You should keep the surface moist for at least the first 7 days to help it cure properly.

Introduction

The Concrete Slab Calculator helps you figure out how much concrete you need for your project. Whether you are pouring a patio, a driveway, or a garage floor, this tool takes the guesswork out of ordering concrete. Just enter the length, width, and thickness of your slab, and the calculator will tell you how many cubic yards or cubic feet of concrete to buy. Ordering the right amount of concrete saves you money and keeps your project on track. Too little concrete means extra trips and delays, while too much means wasted material. Use this calculator before your next pour to get it right the first time.

How to use our Concrete Slab Calculator

Enter the size of your slab below to find out how much concrete you need. The calculator will give you the total volume of concrete in cubic yards and cubic feet.

Length: Type in the length of your concrete slab in feet. Measure from one end to the other along the longest side.

Width: Type in the width of your concrete slab in feet. This is the shorter side measured from edge to edge.

Thickness: Enter the thickness (or depth) of your slab in inches. Most residential slabs are 4 to 6 inches thick. Driveways and garage floors often need at least 4 inches, while patios may use 4 inches as well.

Quantity: If you are pouring more than one slab of the same size, enter the number of slabs here. The calculator will multiply the volume for you so you get a total for all slabs at once.

A concrete slab is a flat, horizontal surface made of poured concrete. It is one of the most common parts of any building project. Slabs are used for floors, driveways, patios, sidewalks, and garage foundations. They are strong, long-lasting, and can support heavy loads when built the right way.

How Concrete Slabs Work

A concrete slab sits on a prepared base of compacted soil or gravel. The typical residential slab is 4 inches thick, but thicker slabs of 5 or 6 inches are used for areas that carry heavier weight, like garages or driveways. The concrete is poured into forms, spread evenly, and then smoothed out before it hardens. If you need to determine how much material to remove before pouring, our Excavation Calculator can help you plan the dig.

Why Calculating Concrete Matters

Concrete is sold by the cubic yard or cubic meter. If you order too little, you'll have to stop your pour and wait for more — which creates a weak joint in the slab. If you order too much, you waste money. A concrete slab calculator helps you figure out exactly how much concrete you need based on the length, width, and thickness of your project. For general concrete volume estimates beyond slabs, you can also use our Concrete Calculator. If you need to convert your measurements into cubic yards, our Cubic Yard Calculator is a handy companion tool.

Tips for a Good Concrete Slab

Add 5–10% extra to your calculated amount to account for spillage, uneven ground, and waste.
Use wire mesh or rebar inside the slab to add strength and prevent cracking. Our Rebar Calculator can help you determine how much reinforcement you need.
Compact the base before pouring. A loose base leads to settling and cracks over time. Many contractors use a layer of gravel or sand as a sub-base for proper drainage and stability.
Control joints should be cut into the slab every 8 to 12 feet to guide where cracks form.
Cure the concrete by keeping it moist for at least 7 days. This helps it reach its full strength.

Common Slab Thicknesses

4 inches — sidewalks, patios, and shed floors
5 inches — residential driveways
6 inches or more — garages, heavy equipment pads, and commercial floors

Getting the right amount of concrete before you start your project saves time, money, and headaches. Use the calculator above to quickly find out how many cubic yards of concrete your slab will need. If your project also involves building block walls or foundations, check out our Concrete Block Calculator. For measuring the area of your slab before calculating volume, our Square Footage Calculator can help you get precise dimensions.

Frequently asked questions

How do I calculate concrete for a slab in cubic yards?

Multiply the length (in feet) by the width (in feet) by the thickness (in feet). Then divide the result by 27. That gives you cubic yards. For example, a slab that is 20 feet long, 10 feet wide, and 4 inches thick (0.33 feet) equals 20 × 10 × 0.33 = 66 cubic feet. Divide 66 by 27 to get about 2.44 cubic yards.

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

It depends on the bag size. An 80-pound bag of premix concrete makes about 0.6 cubic feet. A 60-pound bag makes about 0.45 cubic feet. Divide your total cubic feet by the yield per bag to find how many bags you need. For example, if you need 66 cubic feet and use 80-pound bags, you would need about 110 bags.

Should I order extra concrete beyond what the calculator says?

Yes. Most contractors order 5% to 10% more than the calculated amount. This covers uneven ground, spillage, and small errors in your measurements. Running short during a pour can create weak spots in the slab, so it is better to have a little extra.

What is the minimum thickness for a concrete slab?

For most residential projects, the minimum thickness is 4 inches. This works well for patios, sidewalks, and shed floors. If vehicles will drive or park on the slab, use at least 5 to 6 inches.

How much does a cubic yard of concrete weigh?

A cubic yard of wet, mixed concrete weighs about 3,900 to 4,050 pounds (roughly 2 tons). Keep this in mind when planning delivery and making sure the ground can support the load during the pour.

Can I use this calculator for a round or circular slab?

This calculator is designed for rectangular slabs. For a circular slab, you need to use the formula: π × radius² × thickness. Measure the diameter of your circle, divide by 2 to get the radius, then plug it into that formula to find the volume.

What is the difference between cubic feet and cubic yards?

There are 27 cubic feet in 1 cubic yard. Cubic feet is a smaller unit, while cubic yards is the standard unit used when ordering ready-mix concrete from a supplier. To convert cubic feet to cubic yards, divide by 27.

How much does a concrete slab cost?

Ready-mix concrete typically costs between $120 and $200 per cubic yard, depending on your location and the mix design. For a small DIY project using bagged concrete, expect to pay around $5 to $7 per 80-pound bag. Labor, forms, and base preparation add to the total cost.

Do I need gravel under my concrete slab?

Yes, in most cases. A layer of 4 to 6 inches of compacted gravel under the slab helps with drainage and prevents the soil from shifting. This gives the slab a stable, even base and reduces the chance of cracking over time.

How long does concrete take to dry and cure?

Concrete is usually hard enough to walk on in 24 to 48 hours. However, it takes about 28 days to reach its full strength. You should keep the surface moist for at least the first 7 days to help it cure properly.

What happens if my slab is too thin?

A slab that is too thin will crack and break under weight. It will not hold up to foot traffic, vehicles, or freeze-thaw cycles. Always pour at least 4 inches for light-use areas and 5 to 6 inches for driveways and garages.

How do I convert inches to feet for the thickness?

Divide the number of inches by 12. For example, 4 inches ÷ 12 = 0.33 feet, 5 inches ÷ 12 = 0.42 feet, and 6 inches ÷ 12 = 0.5 feet. The calculator handles this conversion for you when you enter thickness in inches.