Construction

Rebar Calculator

Laying out a rebar grid for a concrete slab — driveway, garage floor, patio, foundation — requires bar count in two directions plus calculating splice overlap for any bar runs longer than your stock length (typically 20 ft). This calculator takes your slab length, width, target spacing (12", 16", 18", or 24" on-center), bar size (#3 through #6), and stock length, then returns total linear feet of rebar, count of bars in each direction, pieces to buy at your stock length, and tie wire intersection count. With 40 × bar-diameter lap splice allowance for code compliance.

Enter the slab dimensions.

Rebar sizing for slabs

Number designation = bar diameter in eighths of an inch:

#3 (3/8"): light-duty applications, light residential
#4 (1/2"): standard for residential slabs, garage floors, driveways
#5 (5/8"): heavy residential, light commercial, thicker slabs (6"+)
#6 (3/4"): heavy commercial, foundations under load-bearing walls
#7-11: structural beams, columns, large foundations

For residential slabs the workhorse is #4 rebar at 16" oc grid. Driveways take vehicle loads, so #4 or #5 at 12-16" oc. Garage floors with shop equipment: #5 at 12" oc.

Lap splice math (ACI Code)

When a single bar exceeds your stock length, you splice by overlapping two bars and tying them together. ACI 318 requires lap splices of 30-50 bar diameters for tension; 40d is a safe default for slabs-on-grade.

#3 (0.375"): 40d = 15" lap
#4 (0.5"): 40d = 20" lap (1.67 ft)
#5 (0.625"): 40d = 25" lap (2.08 ft)
#6 (0.75"): 40d = 30" lap (2.5 ft)

Worked example: 40-ft long slab needs 40-ft of rebar in that direction. Stock is 20 ft — so you need 2 bars overlapping. Total rebar per row: 20 + 20 + 20/12 (1.67 ft lap) = 41.67 ft instead of 40 ft. Account for laps in your order.

How to use this calculator

Slab length and width in feet.
Spacing: 16" oc for residential standard; 12" for heavy loads; 18-24" for light applications.
Bar size: #4 default for residential; #5 for driveways with truck loads.
Stock length: 20 ft is the most common from supply houses.
Lap splice: 40 × bar diameter (default), per ACI code.
Output: total linear feet needed, bar count each direction, pieces to buy, tie wire intersections.

Common scenarios

20x20 garage floor, #4 at 16" oc, 20-ft stock. 16 bars each direction at 20 ft = 320 ft + 16 ft (lap allowance) = 336 LF. Pieces: 17 sticks. Tie intersections: 256. ~$200 in rebar at $1/ft for #4.

40x12 driveway, #4 at 16" oc, 20-ft stock. Bars along length: 10 bars at 40 ft each = 400 ft (with laps ~417 ft). Bars along width: 31 bars at 12 ft = 372 ft. Total: 789 LF. Pieces: 40 sticks. ~$475-550 in #4 rebar.

30x30 monolithic foundation slab, #5 at 12" oc. 31 bars each direction at 30 ft = 1,860 ft + laps. Pieces: ~95 sticks of 20-ft #5. ~$1,200 in #5 rebar.

FAQ

What's the difference between #3, #4, #5? +

Bar diameter in eighths of an inch. #4 = 4/8 = 1/2". #5 = 5/8". Bigger numbers = thicker bars, more tensile strength. For residential slabs, #4 at 16" oc is the standard — plenty of strength, manageable cost, easy to handle.

Why 40 × bar diameter for lap splice? +

ACI Code requires 30-50 bar diameters for tension lap splices depending on bar size, concrete strength, and conditions. 40d is the safe default for slabs-on-grade. The lap creates a continuous tensile path through the splice.

Do I need rebar in a residential slab? +

Concrete will crack — that's not optional. Rebar holds the cracks tight and prevents them from spreading or shifting. For driveways, garage floors, and any slab over 4" thick, rebar is highly recommended. Fiber mesh added to concrete is an alternative for thin sidewalks; not equivalent strength for vehicle loads.

How thick should the slab be? +

4" for sidewalks. 5" for residential driveways (light vehicles). 6" for heavy vehicles / RV pads. 8" for industrial floors. Concrete cover over rebar: 2" from the bottom of the slab in soil-contact applications.

How do I position rebar in the slab? +

Rebar should sit in the middle third of the slab thickness. For 4-5" slab: place rebar 1.5-2" up from the bottom. Use rebar chairs (plastic or wire) to hold position before pouring. Don't lay rebar on the ground and "pull it up" during pouring — doesn't work reliably.

What's the difference between rebar and welded wire mesh? +

Rebar: 1/2"+ steel bars laid in grid pattern, manually tied. Welded wire mesh (WWF): pre-welded grid sheets of thinner wire. Rebar is stronger and more flexible (you can place exactly where needed). WWF is faster to install but less strong per material cost.

What's the cost of rebar in 2025? +

~$0.80-1.20 per linear foot for #4 black (uncoated) rebar in 20-ft sticks. ~$1.20-1.80 per LF for #5. Epoxy-coated bars cost 50-100% more, used in coastal/salt environments. Prices fluctuate with steel commodity prices.

Do I need epoxy-coated rebar? +

For exterior slabs in salt/freeze regions or coastal areas where chloride exposure is constant: yes. For interior slabs and most residential applications: no — black rebar is fine. The 50-100% premium for epoxy is overkill unless you're in a high-corrosion environment.

How do I tie the rebar grid? +

16-gauge black annealed tie wire, twisted with a tie wire twister tool (or pliers and patience). You don't need to tie every intersection — every other one in a pattern, or 50-60% of intersections, is sufficient to hold the grid in place during the pour. Pros use battery-powered tie tools (MAX RB401, etc.) for production speed.

Can I use fiber-reinforced concrete instead of rebar? +

Synthetic or steel macro-fibers added to the concrete mix can replace rebar for thin slabs and sidewalks where the only concern is shrinkage cracking. For slabs that carry real vehicle loads (driveways, garage floors), rebar is still the standard — fibers don't provide the structural tensile reinforcement at the right depth. Many engineers spec both: fibers for crack control + rebar for structure.

What's the cover requirement over rebar? +

For slabs on grade with soil contact (no vapor barrier or thin slab): 3 inches of cover from the soil side. With vapor barrier on prepared subgrade: 1.5-2 inches typical. Top cover (toward the air): 1.5 inches minimum, 2 inches preferred for cracking and corrosion resistance. ACI 318 has full tables; for residential slabs on grade, 2 inches up from the bottom on a 5-inch slab is the workhorse position.