Rebar Calculator
Enter your slab dimensions, grid spacing, and edge clearance below to calculate how many rebar sticks you need. Rebar is placed in a grid pattern, so the math is about counting rows and columns, not measuring volume.
Common grid spacing: 18–24 in for a residential patio, 12–18 in for a driveway.
- Total bars needed
- 16
- Total linear feet
- 160.0 ft
- Sticks to order (20 ft)
- 8
| Direction | Bars needed |
|---|---|
| Running the slab's length | 8 |
| Running the slab's width | 8 |
Assumes a 18-inch grid with 3 inches of clearance from every edge. Sticks needed rounds up to the nearest whole 20-foot stick.
Step 1: Figure Out Your Usable Width
Rebar doesn’t run edge-to-edge. Building codes and standard practice call for a clearance gap (commonly 3 inches) between the rebar and the edge of the concrete, to keep it fully encased and protected from corrosion.
Usable width = Slab Width − (2 × Edge Clearance)
Example: A 10 ft × 10 ft slab (120 × 120 inches), 3-inch clearance, 18-inch grid spacing (a common spacing for residential slabs), using #4 rebar.
120 − (2 × 3) = 114 inches of usable width
Step 2: Count the Bars
Number of gaps = Usable Width ÷ Grid Spacing, rounded up
114 ÷ 18 = 6.33 → round up to 7 gaps → 8 bars running in that direction
Since this example slab is square, the same math applies in the perpendicular direction: 8 bars the other way too.
Step 3: Total Length
Total bars: 8 (one direction) + 8 (perpendicular direction) = 16 bars, each 10 feet long.
16 × 10 ft = 160 linear feet of rebar
Step 4: Convert to Sticks
Rebar is typically sold in 20-foot sticks. Since each of your 10-foot bars is half a stick, you can cut two bars from each stick with minimal waste:
16 bars ÷ 2 bars per stick = 8 sticks of 20-foot rebar
Choosing Grid Spacing and Bar Size
| Application | Slab Thickness | Typical Grid Spacing | Common Bar Size |
|---|---|---|---|
| Patio, walkway, shed pad | 4 in | 18–24 in | #3 or #4 |
| Residential driveway (cars) | 4 in | 16–18 in | #4 |
| Driveway with truck or RV traffic | 5–6 in | 12–16 in | #4 or #5 |
| Garage or workshop floor | 4–6 in | 12–18 in | #4 |
| Footings and grade beams | Per design | Per design | #4 or #5 |
| Structural / load-bearing slab | Per design | Per engineer’s design | Per engineer’s design |
Tighter spacing and larger bar sizes add cost, but also add crack resistance and load capacity. For anything beyond a basic patio slab, closer spacing is usually worth it, and for structural applications, spacing should come from an engineer’s design rather than a general planning estimate.
These figures are conventions for ordinary residential slabs on prepared subgrade. They are not a substitute for a design where loads, soil conditions, or frost depth are in play.
Bar Sizes
US rebar is numbered in eighths of an inch: a #4 bar is 4/8 of an inch, or 1/2 inch, in diameter. That single rule covers the whole range.
| Size | Diameter | Weight per ft | 20 ft Stick | Typical Lap (40d) |
|---|---|---|---|---|
| #3 | 3/8 in | 0.376 lb | 7.5 lb | 15 in |
| #4 | 1/2 in | 0.668 lb | 13.4 lb | 20 in |
| #5 | 5/8 in | 1.043 lb | 20.9 lb | 25 in |
| #6 | 3/4 in | 1.502 lb | 30.0 lb | 30 in |
| #7 | 7/8 in | 2.044 lb | 40.9 lb | 35 in |
| #8 | 1 in | 2.670 lb | 53.4 lb | 40 in |
The weight column matters for two reasons. Suppliers often price by weight rather than by the stick, so a quote in dollars per pound needs converting before you can compare it. And handling matters on site: a 20 ft #5 bar is 21 lb of awkward, springy steel, which is a two-person lift in practice even though one person can technically pick it up.
Laps: The Length the Grid Math Misses
The stick calculation assumes each bar runs the full dimension in one piece. Once a slab is longer than a stick, bars have to be spliced, and a splice is not a butt joint. The two bars overlap so the load transfers between them.
The common field rule for slabs on grade is a lap of 40 bar diameters, with a practical minimum of 12 inches. That is 20 inches for the #4 bar most residential slabs use.
Every lap consumes that length twice over in material terms: you lose the overlap from the total run. On a 40 ft slab using 20 ft sticks, each line of rebar needs one splice, adding 20 inches of overlap, so the run needs a little over 41 ft of steel, not 40. Across twenty parallel lines that is an extra 35 linear feet, close to two full sticks that a naive calculation drops.
Lap length is not universal. The actual requirement depends on concrete strength, bar coating, spacing, and whether the bar is in tension or compression, and ACI 318 sets it accordingly. Use 40d for planning quantities, and follow the drawings on anything engineered.
Where the Bar Sits in the Slab
Rebar placed on the ground and poured over does nothing. Concrete is strong in compression and weak in tension, and the steel is there to take the tension, which means it has to sit where the tension is.
For a slab on grade, that generally means the middle to upper third of the slab depth, held there by chairs or dobies at regular intervals. In a 4-inch slab the bar typically sits around 1.5 to 2 inches from the top.
Two habits to avoid:
- “Hooking” the bar up during the pour. Pulling the grid upward with a hook as concrete goes in is common and unreliable. It lifts unevenly, and any bar that drops back is doing nothing where it matters.
- Skipping the chairs on a small pour. The mesh or grid will sit on the ground under its own weight and the weight of anyone walking on it.
Edge clearance and depth cover both exist for the same reason: steel that is not fully encased in concrete corrodes, and corroding steel expands and cracks the slab from within. Minimum cover is set by ACI 318, the structural concrete building code, and concrete cast against and permanently exposed to earth is normally given 3 inches for this reason.
Frequently Asked Questions
- How much rebar do I need for a 10x10 slab?
- At an 18-inch grid with 3-inch clearance, a 10 ft × 10 ft slab needs about 160 linear feet of rebar, or 8 sticks of standard 20-foot rebar.
- What size rebar should I use for a patio?
- #3 or #4 rebar is standard for residential patios and walkways. #4 is the more common default choice since it offers meaningfully more strength for a modest cost difference over #3.
- Why does rebar need edge clearance instead of running to the edge?
- Concrete protects rebar from corrosion, but only if there's enough concrete cover on all sides. Rebar placed too close to the edge or surface can corrode faster and eventually cause the concrete around it to crack and spall.
- What rebar spacing should I use for a 4 inch slab?
- For a patio, walkway, or shed pad, 18–24 inches with #3 or #4 bar is conventional. A residential driveway carrying cars is usually tightened to 16–18 inches with #4. Anything taking truck or RV traffic normally goes to a 5–6 inch slab at 12–16 inch spacing. These are conventions for ordinary slabs on prepared subgrade, not a substitute for an engineered design.
- How much do rebar bars need to overlap?
- The common field rule for slabs on grade is 40 bar diameters, with a practical minimum of 12 inches, so 20 inches for #4 bar. Remember to add that overlap to your material total: every splice consumes extra steel that a straight length calculation misses. Actual lap requirements depend on concrete strength, coating, and whether the bar is in tension, so follow the drawings on engineered work.
- How deep should rebar sit in a concrete slab?
- Generally in the middle to upper third of the slab depth, around 1.5 to 2 inches from the top of a 4-inch slab, held there on chairs. Rebar lying on the subgrade does nothing, because the steel needs to be where the tension is. Pulling the grid up with a hook during the pour is unreliable and leaves sections sitting too low.
- Do I need rebar or would wire mesh work instead?
- For thin residential slabs (4 inches) with good soil conditions, welded wire mesh is sometimes used instead of rebar and is faster to install. For driveways, thicker slabs, or anything on expansive soil, rebar generally provides better crack control and load support.
Estimates on this page are for planning purposes. Grid spacing, bar size, and clearance requirements for structural or load-bearing applications should be specified by a licensed structural engineer.
Written by: The CubedCalc Team. We research and fact-check every calculator using manufacturer specs, building codes, and industry references. We are not licensed contractors or engineers; always verify structural or code-critical numbers with a qualified professional.