What Size Rebar Should You Use for a Concrete Slab?

What Size Rebar Should You Use for a Concrete Slab?

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If you are planning to work with concrete, one thing you may very well need to do is to reinforce it. While there are things like wire mesh, the most common method for doing so is with rebar, which is short for “reinforcement bar.”

With that said, rebar comes in many sizes, and each size is designed for a slightly different purpose. Today we want to talk about the most common rebar sizes and what size you should use for reinforcing concrete slabs.

Common Rebar Sizes

Before we talk about what size of rebar you should use for a concrete slab or for a driveway, it would probably help if you knew what the different sizes are.

First off, remember that rebar is measured differently in the United States than in Europe. While the United States uses the imperial measurement system, Europe and most of the rest of the world uses the metric system.

When using the imperial measurement system, the most common rebar sizes are #4 and #5 rebar. These two types of rebar are used in light commercial and heavier commercial construction, as well as for residential concrete work.

The below table summarizes the most commonly used rebar sizes, as well as some important information about each.

Imperial Bar SizeMetric Bar SizeWeight (lbs/foot)Weight (kg/meter)Nominal Diameter (Inches2)Nominal Diameter (mm2)

Keep in mind that the most common sizes range from #3 to #9, with anything larger than that usually not being used for many types of residential or commercial construction. As you can tell, for instance, #18 rebar is absolutely massive.

Why is it Important to Choose the Right Rebar Size?

Ok, so the whole point of rebar is of course to reinforce concrete, and therefore, the size of rebar you use will determine how much weight and pressure the finished product can handle.

For example, common #3 type rebar features a minimum yield strength of 6,600 lbs, whereas #4 type rebar has a total yield strength of 11,780 lbs, or about 78% greater than #3. What is interesting to note is that #3 rebar is only 1/8 of an inch thinner than #4 rebar, yet has nearly 80% higher minimum yield strength.

Therefore, simply put, choosing the right size of rebar is important because what size you use will determine the final yield strength of the concrete. The thicker the rebar is, the more weight it is going to be able to handle, and therefore, larger applications will require larger rebar.

For example, if you are supposed to use #5 rebar for a project, but you only use #3, that load-bearing wall or concrete driveway meant for several vehicles is not going to last. If you don’t use the right size of rebar, the concrete slab will quite literally crumble underneath as it will not be able to bear all of the weight being put on it. Choosing the wrong rebar size can literally cause walls to fall.

Keep in mind that using rebar that is too small for an application is a much bigger issue than using rebar that is too large. Although using rebar that is too large is not ideal, particularly due to the high cost of it, it’s not nearly as damaging to the finished project as if you were to use rebar that is too small.

What Size Rebar Should You Use for a 4-inch and 5-inch Concrete Slab?

This question is actually not quite as simple as it seems. The reason why is because not all four- or five-inch concrete slabs are used for the same purpose.

For instance, a patio designed to hold a few humans won’t need to be reinforced as well as a driveway for several vehicles, which in turn does not need to be as heavily reinforced as the foundations for walls or buildings, or walls themselves.

So, whether you are constructing a four- or five-inch slab, what size of rebar you will need is determined by the purpose or application of the slab.

If the slab in question is just for a patio or a similar purpose, #3 rebar with a diameter of 3/8-inch will do just fine.

However, if the concrete slab is meant to be the footing of a small structure, #4 rebar with a diameter of 1/2-inch should be used. If the concrete slab in question is meant to be the footing of a house, then #5 rebar with a diameter of 5/8-inch should be used.

The above are just general guidelines, and there are several factors that will affect the right rebar size, though. As such, make sure to always check with your local construction company first – especially if you are going to be making a slab that will be bearing significant loads.

What Size Rebar Should You Use for a Concrete Driveway?

What size rebar you use for a concrete driveway also depends on how heavy your vehicles are. In most cases, #3 rebar is fine for concrete driveways. Although, if you have several vehicles, especially heavier ones like pickup trucks, then you may want to consider #4 rebar.

On a side note, if you don’t want to invest in expensive rebar, and your driveway is only meant to hold a single vehicle or two small vehicles, you could look into using metal wire mesh as a cheaper alternative, although wire mesh is not as strong as rebar.


The bottom line here is that if you are building a concrete slab, #3, #4, or #5 rebar is what you will need. Generally speaking, unless you are building something massive, you won’t ever have any use for rebar sizes larger than #5.

Now that you know which type to use, one other thing you will need to know before starting is how much rebar you need.