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Rebar Calculator

Last Updated Jul 11,

Use the rebar calculator to estimate the amount of material required for a concrete pour or paving project.

Table of contents

What is Rebar?

Rebar beams are reinforced steel rods cut to various dimensions and inserted into concrete slabs or blocks. The beams are cut with a rigid pattern to help improve adhesion to the concrete and sized according to the build. Rebar provides structural support, improving the strength and reducing the tensile stress of the end product. 

Concrete is capable of withstanding some serious compression stress, but doesn&#;t hold up quite as well against tension. Because of this, contractors incorporate rebar into almost every concrete structure, from foundations to swimming pools to driveways. With proper installation and spacing, rebar can help to reduce the risk of failure.

How to Calculate Rebar Needs

Ensuring that you have the correct quantity and gauge of rebar is essential for the strength and durability of your finished product. The information below will explain the process of measuring the required surface area and depth of the pour, as well as how to determine the appropriate rebar gauge and the number of pieces necessary for your project. 

1. Measure surface area and depth.

Start by measuring the length and width of the area where the concrete will be poured. For rectangular areas, multiply the length by the width. For irregular shapes, break the area down into smaller, more manageable geometric shapes, compute their individual areas, and sum them up. Always ensure to use consistent units (e.g., feet or meters) and double-check your measurements for accuracy.

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The depth or thickness of the pour is crucial, as it will influence the amount of concrete, and consequently, the amount of rebar required for structural integrity.

2. Determine the rebar gauge.

The gauge, or diameter, of the rebar is selected based on the purpose and the load-bearing requirements of the structure: For standard residential concrete slabs, #3 (3/8 inch diameter) to #5 (5/8 inch diameter) rebar is typically used.

For driveways and patios, #4 (1/2 inch diameter) rebar is a common choice. Industrial or commercial projects might necessitate thicker rebar gauges, such as #6 (3/4 inch diameter) and above. Always consult with an engineer or local building codes to ensure the correct rebar gauge is chosen for your specific project.

3. Estimate the number of rebar pieces.

Once you've ascertained the surface area and chosen the rebar gauge, the final step is to calculate the number of rebar pieces needed, which depends on the spacing between them. 

Rebar is commonly spaced at intervals of 18 to 24 inches, center-to-center, both ways in a grid pattern, and fastened together with wire where they meet. However, this can vary based on specific project needs.

To estimate the number of rebars for the length, divide the length of the pour by the chosen spacing. Similarly, for the width, divide the width of the pour by the spacing. Multiply the two results to get the total number of rebars required for the grid.

Remember, accurate measurements and consultations with professionals will ensure the structural integrity and longevity of your project.

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Im going to be pouring a 42 by 60 x4 in 5 bag mix floor, i see most people on here space rebar in 12" squares in there buildings . now my buddy say they place rebar in 5 ft squares but i dont see any one else doing this . he works for a big concrete company also . I was thinking of using mesh and rebar around where the lift would be ,but he say no just use rebar 5ft squares is all you need because if it cracks the rebar with hold it from shifting up . Iam in cental wi and will be doing infloor heat an 2in insulation under if that matters. im thinking this is incorrect doing 5ft sq's

The American Concrete Institute has calculations for this, but there are a lot of variables. Going off what other people do, may not be applicable to your situation. Some of the variables to consider:
- How much insurance you want to have against cracking outside of control or expansion joints.
- Grade of rebar/reinforcing
- Cross sectional area of rebar/reinforcing per foot of slab
- Strength of concrete
- Depth of concrete
- Types and spacing of expansion/control joints

What is your plan for expansion/control joints? How thick will your slab be? What strength of concrete are you going to go with?


I just poured a 5" psi slab, on 2" foam, with radiant tubing and had to have 7.75" squares of #4 rebar on one slab section and 9.5" squares on another. (I called for larger bars and stronger bars initially, but my contractor showed up only with #4 60KSI bar - so I had to whip out my computer and re-run the calculations. Larger, stronger bars would have made for wider spacing, but they were the ones who had to work around it and tie up all that extra rebar.)

The 7.75" was for a slab where I have no control (saw cut) joints. It's a lot of steel, but is what is called for to make the cracks (which will happen) hairline. Those will be filled in/covered by an epoxy coating later.

The 9.5" floor is what I needed for resistance cracking due to bending in my garage slab. I designed this originally for added piece of mind with my lifts and was REALLY glad I did as we went to pour the slab. I found that the 2" 60 psi foam board was so stiff that it bridged imperfections in the compacted base. So, the top of my 2" foam was anywhere from 0" to 1/2" higher than it should have been. Hopefully, this reinforcement will keep things together as the slab settles and can account for any small voids that may be left under the foam.

The 9.5" slab does have saw cut control joints, but the rebar also enabled me to stretch them out around my lift to 13.5' at the widest point.

Spacing of control/expansion joints is a game of probabilities. Cracking will happen and the trick is to use the joints as a channel to pick where they occur. The variables above impact the likelihood of cracking occurring outside of those control/expansion joints. ACI also gives tables for balancing all of these variables, but the tighter you can make them, the higher the likelihood that cracking will be limited to the joints.

If you are just doing a simple slab with normal (10' saw cut control joints) you may not need any reinforcing, at all. The reinforcing will just help steer the cracking that does happen to those control joints and will help keep any cracking that does happen narrower.

Spacing rebar at 5' seems a bit wide but it may not be as crazy as it sounds. Reinforcing in a slab is based on the cross sectional area of the reinforcing relative to the slab section. I haven't done the math but the cross sectional area of a #4 bar (1/2" dia) at 5' might not be that different from the wire in a 6 x 6 mesh.

It's around twice the steel as 10 ga. mesh, which I found surprising.

Rebar is superior to mesh in that it doesn't sag as much between supports. A #4 bar is rigid enough to span supports that are 4' apart. Mesh would need to be supported every couple feet to keep it from ending up on the ground where it will do no good. The extra supports will coat more than the extra cost for the rebar.

Let me give you a couple additional things to consider;

• Make sure you install a well compacted, granular sub base under your slab
• Install a vapor barrier with taped seams
• Make sure your rebar is properly supported in the middle of your slab
• Keep the mix water to a minimum by limiting slump and/or using a mid range water reducer
• If a random crack will ruin your day, cut some control joints 1/4 the depth of the slab.
• Properly cure your slab for at least seven days

There's obviously more to it but if you hit these items, you'll be well on your way to a good slab. Good luck with your project.

I agree. I did a small, non-critical, section of my slab with mesh on chairs to save some money and it was a total disaster. (It just got trampled and ended up in the bottom of the slab, anyway.) Totally worthless. Fortunately, I planned for this and have enough saw-cut control joints to deal with any cracking. If I was to do it all over again, i would have sprung for the rebar and just spaced it widely. In regards to your spacing, I'm not an engineer and won't pretend to be. That being said, I've been in construction for a long time building bridge structures and heavy commercial structures (prisons, event centers, banks, etc).

I've never seen rebar spacing on 5' centers. My own homes I've built I've never seen spaced on that large of centers. Based on what I've seen in my professional and personal life, I would not be comfortable with it even if the math worked out. For the small expense, I'd rather run #4's on 12" or 18" centers with additional reinforcing in your perimeter footings, as well as any intermediate grade beams.

the building is a hybrd steel web frame with wood purlins and gurts . iIwas told i could drill 4 ft concrete piers/footings ? then put up the building then pour the inside slab after that. not sure on this part yet or just do a monolithc slap

In bridge construction, we commonly used drilled shafts. We drill down to rock socket and set in about 5', place our rebar cage and then pour concrete. Our application is probably much different than yours. Typical diameters are 48" to 72", some up to 108"! A shallow depth would be 10' and long depths are 125'+. Obviously when you drill, you are going through existing earth so engineers typically provide us soil boring logs so we have an idea the hardness of soil (and/or rock) we may encounter when drilling. The softer it is, the faster it goes and the cheaper the price. We have specialty drill rigs we send to drill the holes and then use cranes to set rebar cages in place. Finally, concrete is pumped into place.

That's probably more info than you need or want. My point is I like drilled shaft construction because I think it provides some strength advantage. However, depending on specifics of your shafts and if you can do this with rented equipment or need to bring in truly special drilling equipment, this could become an expensive option.

Alternatively, some states use concrete or steel H-piles in combination with spread footings in lieu of drilled shafts.