Rebar Calculator for a Slab
Find how many rebar bars a concrete slab needs, or how much wire mesh.
Measure your job
The IRC asks for 3 in of concrete over bars cast against the ground. So we stop the bars that far from each edge. We use the IRC's laps for footings and walls with Grade 60 bars (#4: 30 in). Grade 40 needs less (#4: 20 in). For #3 we use Texas DOT's 16 in.
Your answer
#4 rebar. A grid every 12 in, both ways.
We cut short pieces from whole bars. We don't join cut-off ends again, so you may have a few spare ends.
Or use wire mesh
The Wire Reinforcement Institute suggests 6×6 W1.4×W1.4 mesh for a 4 in slab up to 35 ft long. For 400 sq ft, that is about 84 lb of mesh.
This answer is a guide. Bars, spacing and laps for anything that holds weight must follow your plans. Check your local building rules or ask a builder or engineer.
What it costs
Prices are the middle of 3 to 5 US sellers per size (Oct 2026), before tax.
Quick answers
- Is rebar needed in a 4 in slab?
- The IRC does not require it. Of 4 towns we checked, 1 requires mesh in a sidewalk and 1 bans it. Check your local rules.
- Where the bars go
- Bars or mesh go in the middle to upper third of the slab.
- How far apart?
- We found no code or trade group that sets it. Two online guides (not codes) say #4 bars 12 in apart, or #3 bars 18–24 in.
- Wire mesh
- The Wire Reinforcement Institute (WRI) suggests 6×6 W1.4×W1.4 for a 4 in slab up to 35 ft long. It weighs 21 lb per 100 sq ft.
- Mesh roll price
- $189 for 5 × 150 ft, the middle of 5 sellers. That is about $0.25 a sq ft.
How to work it out
- Measure the slab length and width in feet.
- Pick a bar size and spacing. Your plans should give both.
- Change each side to inches, and take 3 in off both ends. Divide by the spacing, drop the part, and add 1.
- Each bar is as long as the slab, less the two edge gaps. Fit them into whole bars, with laps where needed.
20 × 20 ft slab, #4 bars every 12 in
(20 × 12 − 2 × 3) ÷ 12 = 19.5 → 20 bars each way
Each bar: 19.5 ft, so 1 per 20 ft bar
20 + 20 = 40 bars, 534 lb
20 × 20 = 400 places where bars cross
Do you need rebar in a 4 inch slab?
The US home building code is the IRC. The IRC does not require bars or mesh in a slab on the ground. It says where to put them if you use them.
Town rules for sidewalks differ:
- Sugar City, Idaho: not required in a 4 in sidewalk.
- Warren, Pennsylvania: 6×6 W2.9×W2.9 mesh required in a 4 in sidewalk.
- Sanford, North Carolina: the city engineer may ask for mesh.
- Palatine, Illinois: wire mesh is not allowed in the parkway (apron) or public walk area.
Bars and mesh don't stop cracks, says the National Ready Mixed Concrete Association (NRMCA). They can keep cracks narrow.
A driveway or a slab under a building holds weight. Check your local building rules or ask a builder or engineer.
Bars for common slab sizes
No code we read gives a bar spacing for house slabs. Two online guides (not codes) disagree, so this table shows both.
| Slab | #4 every 12 in | #3 every 18 in |
|---|---|---|
| 10 × 10 ft | 10 | 8 |
| 12 × 12 ft | 24 | 16 |
| 10 × 20 ft | 20 | 14 |
| 20 × 20 ft | 40 | 28 |
| 24 × 24 ft | 64 | 40 |
| 30 × 40 ft | 148 | 97 |
Bars start 3 in from each edge. The #4 lap (30 in) is borrowed from the IRC's wall and footing table. The #3 lap (16 in) is from a Texas highway spec.
#4 bars weigh 0.668 lb per foot. #3 bars weigh 0.376 lb, so they are lighter to carry.
Wire mesh instead of bars
Welded wire mesh is a grid of thin wires, sold in rolls or sheets. WRI picks the mesh by the slab's longest side. A bigger W number means heavier wire, as the weights show.
| Longest side | Mesh | Weight per 100 sq ft |
|---|---|---|
| Up to 35 ft | 6×6 W1.4×W1.4 | 21 lb |
| Up to 45 ft | 6×6 W2.0×W2.0 | 29 lb |
| Up to 60 ft | 6×6 W2.5×W2.5 | — |
| Up to 75 ft | 6×6 W2.9×W2.9 or W3.0×W3.0 | 42 lb |
| Up to 100 ft | 6×6 W4.0×W4.0 | 58 lb |
A 4 in slab on the ground, lightly reinforced, by its longest side. "6×6" means the wires are 6 in apart.
- Price: a 5 × 150 ft roll covers 750 sq ft. It costs $189, the middle of 5 sellers. Prices are before tax, but one store's includes tax.
- Laps: Texas DOT says to overlap at least 2 cross wires plus 2 in.
- Supports: hold the mesh up on supports 2–3 ft apart (WRI).
- Joints: cut the mesh, or stop it, at the control joints. Those are the planned cut lines in the slab.
For the 20 × 20 ft slab above, WRI suggests 6×6 W1.4×W1.4 mesh. That is about 84 lb, or 1 roll before overlaps.
Where bars and mesh sit
The IRC says to hold bars or mesh in the middle to upper third of the slab. Set them on supports so they stay in place.
Where salt is used to melt ice, keep at least 2 in of concrete over the steel, says NRMCA.
The calculator keeps bars 3 in from each edge. The IRC asks for that much cover where concrete is cast against the soil.
Questions people ask
Do you need rebar for a 4 inch slab?
The IRC does not require bars or mesh in a slab on the ground. It says where to put them if you use them. Town rules for sidewalks differ, so ask yours.
How much rebar do I need for a concrete slab?
A 20 × 20 ft slab with #4 bars 12 in apart takes 40 bars of 20 ft. Type your own size above.
What size rebar do I need for a concrete slab?
We found no code or trade group that says. Concrete Network, a how-to guide, suggests #4 bars 12 in apart for a driveway. MyConcreteCalc, another guide, gives #3 bars 18–24 in apart. Follow your plans.
How much rebar for a 30 × 40 slab?
With #4 bars 12 in apart, 148 bars of 20 ft. With #3 bars 18 in apart, 97 bars.
Does wire mesh stop concrete from cracking?
No. Mesh and bars can't stop a slab from cracking. They can keep cracks narrow.
How much does wire mesh cost?
A 5 × 150 ft roll is $189, the middle of 5 sellers (Oct 2026). Prices ran from $159 to $239.99, before tax except at one store.
How do you overlap wire mesh?
Texas DOT says to overlap at least 2 cross wires plus 2 in, on each sheet or roll.
Where our numbers come from
- Where bars go in a slab: US home building code (IRC R506.2.4)
- Footings, frost and footing rebar: US home building code (IRC R403)
- Rebar laps and wall weights: US home building code (IRC R608)
- Rebar and mesh laps: Texas DOT Item 440
- Rebar sizes and weights: Nucor
- Wire mesh by slab size: Wire Reinforcement Institute (TF 202)
- Mesh can't stop cracks: National Ready Mixed Concrete Association (CIP 4)
- Control joints and mesh: National Ready Mixed Concrete Association (CIP 6)
- Steps and drives in freeze–thaw areas: Virginia Ready-Mixed Concrete Association (ACI 332)
- Rebar spacing in a driveway: Concrete Network (a third-party guide)
- Rebar spacing guide: MyConcreteCalc (a third-party guide)
- Sidewalk rule, Sugar City, Idaho (no rebar needed)
- Sidewalk rule, Warren, Pennsylvania (mesh needed)
- Sidewalk rule, Sanford, North Carolina (engineer decides)
- How thick: a town building handout (Palatine, IL)
- Rebar prices: 3 to 5 US sellers per size (Oct 2026)
- Wire mesh price: the middle of 5 sellers (Oct 2026)
Made by BuildersCount. Last checked 3 Oct 2026.
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