How Much Rebar Do I Need for a Concrete Slab?

Count the bars in each direction, multiply by how long they are, then add your laps. That last part is where most people come up short. A 24 x 30 garage slab with #4 bar at 18 inches takes about 1,014 feet of steel laid out, and closer to 1,095 once you account for the overlaps. Order for the first number and you are making a second trip.

Rebar is not usually the expensive part of a pour, but it is one of the easiest things to get wrong on a bid. Guess low and you are running to the supply house on pour morning. Guess high and you have a pile of steel leaning against the shop wall that you paid for and did not use.

Start with bar size

Rebar is sized by diameter, in eighths of an inch. A 3 bar is 3/8 of an inch. A 4 bar is 1/2 inch. A 5 bar is 5/8. The number is just how many eighths across it is, which is one of the few things in construction that makes immediate sense.

For most residential flatwork you are looking at #3 or #4. Sidewalks and patios can often get by with #3, or with wire mesh instead. Driveways, garage slabs, and anything that will see a truck usually get #4. Footings and structural work go up from there, and at that point the drawing tells you what to use, not a blog.

Then spacing

Spacing is called out on center, meaning from the middle of one bar to the middle of the next. Common residential spacing runs 18 to 24 inches on center each way for a slab, tightening to 12 inches where the loads are heavier.

Tighter spacing means more steel, more tying, and more time. It also means better crack control. If you have a plan or an engineer, follow it. If you do not, 24 inches each way with #4 is a common starting point for a residential slab, and your redimix plant or supply house will usually tell you what they see most around your area.

The math

For a grid running both directions, you count the bars in each direction separately.

Then multiply the count by the length of those bars, and do the same for the other direction. Add them together.

A worked example

Say a 24 by 30 garage slab, #4 bar, 18 inches on center both ways. A garage holds vehicles, so this one gets tightened up from the 24 inch starting point. Eighteen inches is 1.5 feet.

Bars running the long way, spaced across the 24 foot width:

• 24 ÷ 1.5 = 16, plus 1 = 17 bars
• 17 bars × 30 feet = 510 feet

Bars running the short way, spaced along the 30 foot length:

• 30 ÷ 1.5 = 20, plus 1 = 21 bars
• 21 bars × 24 feet = 504 feet

That is 1,014 feet of bar in the mat. You are not done.

Now add the laps

Rebar commonly comes in 20 foot sticks. Your bars are 30 feet and 24 feet, so every single one of them has to be spliced. That is 38 splices on this slab, and each splice needs overlap.

The overlap is not a guess. A Class B tension lap under ACI 318 is 1.3 times the development length, with a 12 inch minimum. For a #4 Grade 60 bar in 4,000 psi concrete, that works out to somewhere around 25 inches.

For our slab, 38 splices at roughly 25 inches each is about 79 feet of steel that exists only to overlap other steel.

1,014 + 79 = about 1,093 feet.

At 20 feet a stick, that is 55 sticks. If you had ordered off the 1,014 number you would have bought 51 and come up four sticks short with the mat half tied.

A small note on the other side of it. You do not run bar all the way to the edge of the slab, you hold it back a couple inches for cover, so the real number lands a hair under this. Ordering slightly long is the right direction to be wrong.

What about mesh?

Welded wire mesh is faster to place and cheaper to buy. It also does less. Testing has put mesh at roughly forty percent of the performance of traditional rebar in slab tests, and mesh alone tends to be the most brittle option.

That does not make it wrong. It makes it right for lightly loaded work, which is a lot of what gets poured. Sidewalks and residential patios are usually fine with mesh. A driveway that will hold a loaded truck is a place for bar.

The other thing nobody mentions is that mesh has to stay up in the pour. Mesh that ends up laying in the bottom of the slab because somebody walked it down is doing almost nothing at all. Chairs cost very little compared to a slab that cracks in three years.

The short version

Count the bars in each direction, remember the plus one, multiply by the lengths, then add the laps. The laps are the part that gets skipped, and on a slab with a lot of splices they are worth real money.

If you bid enough of these, the arithmetic is not hard, it is just tedious, and tedious at nine at night after a full day is where mistakes come from.