Rebar Calculator

What is rebar?

Rebar is a term for beams or rods, most often made of steel, that are used to reinforce concrete during the construction process. Concrete contractors and other industry professionals typically embed rebar within concrete slabs or blocks in a mesh or grid pattern to enhance the material’s tensile strength and improve durability. 

On its own, concrete is highly resistant to damage from compression but weak against tension. Reinforcing it with rebar prevents cracks and other structural issues, vastly extending the life of the finished product. 

The rebar itself is usually textured with ridges or ribs to improve adhesion.

How do I calculate how much rebar I need?

To correctly calculate the amount of rebar you’ll need for a project, you need to accurately measure the area where the concrete will be poured, choose the appropriate size of rebar, and determine the spacing.

1. Measure the length and width of the area

The first step in figuring out how much rebar to use is measuring the length and width of the space into which you’ll be pouring concrete, then calculating the total area.

If the area you’re working with is rectangular, you’ll simply need to multiply the length by width.

If the area is an irregular shape, you’ll need to separate it into smaller sections, determine the area of each section, and add each area together for the total.

2. Choose the appropriate size of rebar

The size of rebar you should use depends on the specifications of your project. Large structures that will be under a significant amount of tension and need to be reinforced with considerable strength typically require a larger size of rebar. You can get specific recommendations from the project engineer, from the rebar manufacturer, or by checking applicable building codes. 

There are a few general rules, however. Smaller projects like residential driveways will often use #3 rebar. Structural, load-bearing elements like walls and columns may use #4 rebar, while foundations may use #5 rebar. Industrial projects will often require larger sizes, including #6 rebar and beyond.

Keep reading to learn more about rebar sizing.

3. Determine the rebar spacing

The final piece of the puzzle you need to determine the correct amount of rebar for your project is the spacing of the rebar. 

Similar to rebar size, while the recommended spacing of rebar will vary depending on what you’re building, rebar will typically be laid in a grid pattern and there are some general best practices. For example, when pouring a concrete slab, each piece of the grid is most commonly spaced 18 to 24 inches apart from center to center. You should check with the project engineer or manufacturer for the appropriate rebar spacing based on your project requirements.

Once you’ve determined the correct spacing, divide the length of the area where concrete will be poured by the spacing amount. Then, repeat the process for the width.

To get the total number of rebar beams needed, multiply these two numbers together.

Rebar sizes

In the U.S., rebar is labeled with a number related to its diameter:

• #3 rebar = 3/8 in. diameter

• #4 rebar = 4/8 in. diameter (or ½ in.)

• #5 rebar = 5/8 in. diameter

• #8 rebar: 8/8 in. diameter (or 1 in.)

These are not the only sizes of rebar available, just an example.

If you’re using metric measurements, rebar is typically labeled by diameter rounded to the nearest millimeter:

• No. 6 rebar = 6.35 mm. diameter

• No. 10 rebar = 9.525 mm. diameter

• No. 19 rebar = 19.05 mm. diameter

• No. 25 rebar: 25.4 mm. diameter

Rebar FAQs

Why do you put rebar in concrete?

Rebar is used to reinforce concrete and protect it against damage caused by tension, extending its lifespan. Concrete that has not been reinforced with rebar is more prone to cracks and other damage.

How far apart should rebar be spaced?

Rebar spacing varies greatly depending on the project requirements, and pieces of rebar may be spaced closer together or further apart depending on a number of factors, including manufacturer specifications and building codes.

For concrete slabs, rebar is commonly spaced 18 to 24 inches apart from center to center in a grid or mesh pattern. 

For building foundations, rebar is often required to be spaced no more than 18 inches apart.

However, it’s best practice to review spacing guidelines with the project engineer or other experts before placing any rebar.

What is rebar made of?

Rebar is most often made of carbon steel that has been textured with ridges to improve the rebar’s adhesion to concrete. It can also be made of galvanized steel, stainless steel, and very rarely other materials.

Is rebar harder than steel?

Most rebar is made from a type of carbon steel that is designed for durability and flexibility to reinforce the tensile strength of concrete.

There are other high-carbon versions of steel that can be considered to be harder than most rebar.

How long does rebar last?

When properly installed, concrete reinforced with rebar can last up to 100 years, especially in good condition.

Factors like construction errors and exposure to elements may affect the lifespan of both rebar and concrete.

Track rebar use (and other materials) with Raken

Raken’s construction material tracking app helps busy contractors better manage all their most important resources.

Accurately log the rebar and other materials you use on phone or tablet, then view real-time production insights to measure performance against estimates at a glance. You’ll always know how much material you need, when to order more, and whether or not your projects are on schedule.  

All our tools are designed for ease-of-use, so your teams will actually use them. Get better data from the field and share it with the office faster.