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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. Driveways, garage slabs, and anything carrying real loads usually get rebar or mesh on the plans. Structural slabs are designed under ACI 318; slabs on ground follow the plans, local standards, and ACI 360 guidance. (Need the concrete volume first? See how to calculate concrete for a slab.)
A common residential spec is #4 rebar at 18” on center both ways. Your plans or local standard control.
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.
Rebar stops short of the slab edge. For structural concrete, ACI 318-19 Table 20.5.1.3.1 calls for 3 inches of cover where concrete is cast against and permanently exposed to earth, and 1-1/2 inches for #5 and smaller bars exposed to weather. Residential slab specs commonly hold bars 2 to 3 inches back from the edges.
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 usually comes in 20-foot sticks. Any run longer than that needs a lap splice.
Lap length depends on bar size, grade, concrete strength, cover, and spacing, so the number belongs on your plans. ACI 318-19 Section 25.5 has the design method. For a residential reference point, the IRC’s table for concrete walls (Table R608.5.4(1), Grade 60) uses:
| Bar Size | Lap Splice, Grade 60 |
|---|---|
| #4 | 30” |
| #5 | 38” |
| #6 | 45” |
That’s about 60 bar diameters. For a 30-foot run of #4, splice a 20-foot stick and a 12-foot piece with a 30-inch lap: 20 + 12 - 2.5 = 29.5 feet. Close, but short. You’d need a 12’-6” piece. The same lap splice rules apply to concrete footing reinforcement, where continuous #4 bars run the full perimeter.
Each splice adds 30 inches of extra #4. For 21 bars running the 30-foot direction, that’s 21 x 2.5 feet = 52.5 extra linear feet.
Support and Placement
Chairs or dobies hold the bars at the height your plans call for. In slabs on ground, reinforcement for crack control is commonly placed at mid-depth or in the upper half, so a 4-inch slab often gets 2-inch chairs. Space chairs close enough that the mat doesn’t sag when crews walk on it; follow the spec or the chair maker’s spacing.
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
For structural slabs designed under ACI 318, Section 24.4 sets a minimum shrinkage and temperature reinforcement ratio of 0.0018 for Grade 60 bars. (Slabs on ground aren’t covered by that section, but the math is a useful reference.)
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.
ACI 318 also caps that steel’s spacing at the lesser of 5 times the slab thickness or 18 inches (24.4.3.3). For a 4-inch slab, 18 inches governs. That’s where the common #4 at 18” OC comes from.
Wire Mesh vs Rebar
Welded wire reinforcement (usually 6x6 W1.4/W1.4 or 6x6 W2.9/W2.9) is cheaper and faster to place than a rebar mat for lightly loaded slabs.
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 52.5 feet of laps = 922.5 linear feet.
Weight: 922.5 x 0.668 = 616 pounds
That’s 47 twenty-foot sticks. At $14 a stick (Trussly’s default price; your yard will differ), about $660 before tie wire, chairs, and delivery.
Faster Takeoffs
Trussly runs rebar quantities for any slab size, bar size, and spacing, including lap splices and total weight. The material list prices the bars at your Prices tab rate. 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.