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Rebar Calculator: How Much Rebar for a Slab
When Slabs Need Rebar
Not every slab gets rebar. A 4-inch sidewalk on well-compacted soil might just get wire mesh or nothing at all. But driveways, garage slabs, structural floors, and anything supporting loads should have rebar per ACI 318 or your engineer’s specs. (Need the concrete volume first? See how to calculate concrete for a slab.)
The most common residential slab reinforcement is #4 rebar at 18” on center both ways. That’s the default you’ll see on a lot of residential plans. Commercial work and structural slabs often call for #5 at 12” OC or heavier.
Counting Bars
The formula for each direction is:
(Slab dimension / spacing) + 1 = number of bars
For a 20 x 30 foot slab at 18” OC:
Running the 30-foot direction (bars parallel to the 20’ side):
30 feet = 360 inches
360 / 18 + 1 = 21 bars, each 20 feet long
Running the 20-foot direction (bars parallel to the 30’ side):
20 feet = 240 inches
240 / 18 + 1 = 14.33, round up to 15 bars, each 30 feet long
Total bars: 21 + 15 = 36 bars.
But rebar doesn’t extend to the edge of the slab. ACI 318-19 Section 20.5.1 requires a minimum of 1.5 inches of concrete cover for slabs cast against soil and 0.75 inches for slabs not exposed to weather. Most residential specs call for 2-3 inches of cover from the slab edge.
With 3 inches of edge clearance on each side, your 20-foot bars are actually 19’6” and your 30-foot bars are 29’6”. For ordering purposes, you still buy 20s and 30s and cut them down on site.
Bar Sizes and Weight
| Bar Size | Diameter | Weight per ft | Common Use |
|---|---|---|---|
| #3 | 3/8” | 0.376 lb | Light slabs, temperature steel |
| #4 | 1/2” | 0.668 lb | Residential slabs, driveways |
| #5 | 5/8” | 1.043 lb | Commercial slabs, walls |
| #6 | 3/4” | 1.502 lb | Foundations, heavy structural |
For our 20 x 30 slab with #4 at 18” OC:
21 bars x 20 ft = 420 linear feet (long direction)
15 bars x 30 ft = 450 linear feet (short direction)
Total: 870 linear feet of #4 rebar
Weight: 870 x 0.668 = 581 pounds
That’s about a quarter ton of steel. Not something you’re carrying by hand from the truck to the back of the lot.
Lap Splices
Rebar comes in 20-foot and 60-foot lengths (20s are standard at most suppliers). If your slab is wider than 20 feet in any direction, you’ll have splices.
ACI 318-19 Section 25.5 specifies lap splice lengths based on bar size and concrete strength. For standard 3,000 PSI concrete:
| Bar Size | Minimum Lap Splice |
|---|---|
| #3 | 12” |
| #4 | 18” |
| #5 | 23” |
| #6 | 27” |
For #4 rebar, that’s 18 inches of overlap at each splice. On our 30-foot bars, you’d splice one 20-foot bar with one 12-foot bar, overlapping 18 inches in the middle. The actual material used is 20 + 12 = 32 feet, minus the 18” overlap gives you 30.5 feet of effective length.
Each splice adds 18 inches of extra rebar. For 21 bars running the 30-foot direction, that’s 21 x 1.5 feet = 31.5 extra linear feet of #4.
Support and Placement
Rebar chairs (aka dobies or high chairs) hold the rebar at the right height in the slab. For a 4-inch slab with rebar centered, you want 2-inch chairs. Most specs call for rebar at the bottom third of the slab, so 1.5-inch chairs on a 4-inch slab.
You need about 1 chair per 4 square feet for a standard mat. A 20 x 30 slab needs roughly 150 chairs.
Pull the rebar up before the pour. Rebar sitting on the ground does nothing. I’ve watched crews lay out a perfect grid of #4 and then let the concrete push it all to the bottom of the slab. Might as well not have rebar at all.
Temperature and Shrinkage Steel
Even if your structural engineer didn’t spec reinforcement for loads, ACI 318-19 Section 24.4 requires temperature and shrinkage reinforcement in slabs. The minimum ratio is 0.0018 for Grade 60 rebar.
For a 4-inch slab (48 square inches per linear foot of width):
0.0018 x 12” x 4” = 0.0864 sq inches of steel per foot of width
One #4 bar has a cross-sectional area of 0.20 sq inches. So:
12 / (0.20 / 0.0864) = one #4 bar every 27.8 inches, roughly 28” OC.
That’s less reinforcement than the typical 18” OC spec, which tells you that most residential slab specs already exceed the minimum temperature steel requirements. Your engineer’s 18” OC spec covers both structural and temperature needs.
Wire Mesh vs Rebar
Welded wire mesh (WWF, usually 6x6 W1.4/W1.4 or 6x6 W2.1/W2.1) is the cheaper option for slabs that don’t carry heavy loads. It’s lighter, faster to install, and costs about 60% less per square foot than a rebar mat.
The trade-off is strength. Mesh works for lightly loaded interior slabs, patios, and sidewalks. For driveways, garages, or anything with point loads (heavy equipment, vehicle lifts, column pads), rebar is the call.
Mesh also has a placement problem. It ships in rolls or flat sheets and is hard to keep at the right height during a pour. Rebar on proper chairs stays where you put it.
Ordering Rebar
Rebar suppliers sell by the ton or by the linear foot. Know your total linear footage and weight when you call.
For our example slab (870 linear feet of #4 plus 31.5 feet for splices = 901.5 linear feet):
Weight: 901.5 x 0.668 = 602 pounds = 0.30 tons
At roughly $0.75-$1.00 per linear foot for #4 delivered, that’s $675-$900 in rebar for a 600 sq ft slab.
Add tie wire (about 1 roll per 200 ties, and you’ll tie every intersection) and rebar chairs, and your total reinforcement cost is $800-$1,100.
Faster Takeoffs
SiteCalc runs rebar quantity calculations for any slab dimensions, bar size, and spacing. It accounts for lap splices, edge clearance, chair count, and total weight. You get a material list and cost estimate without doing the long-form math. Use guided projects to chain a concrete slab calculator with rebar into a single material list. Useful when you’ve got four slabs on one project and the GC wants a bid by Friday.