Rebar Calculator: Spacing, Quantity, and Weight for Concrete Reinforcement
Calculate rebar quantity, spacing, and total weight for concrete slabs, walls, and footings. Get bar count, linear feet, and material weight for any reinforced concrete project.
Reinforcing steel turns plain concrete from a material that's strong in compression but brittle in tension into a composite that can handle structural loads. Getting the rebar spacing and size right isn't just about structural adequacy — it also affects how much you spend on material, which is more significant than people expect on large slabs.
The CalcHub Rebar Calculator calculates the number of bars, total linear feet, and total weight for any slab, footing, or wall using your specified grid spacing.
Rebar Designation and Size
Rebar is labeled by number, where the number roughly equals the bar diameter in eighths of an inch:
| Bar # | Diameter | Weight (lb/ft) | Common Use |
|---|---|---|---|
| #3 | ⅜" | 0.376 | Flatwork, light-duty slabs, masonry |
| #4 | ½" | 0.668 | Residential slabs, driveways, footings |
| #5 | ⅝" | 1.043 | Structural slabs, footings, beams |
| #6 | ¾" | 1.502 | Heavy slabs, grade beams |
| #7 | ⅞" | 2.044 | Large structural elements |
| #8 | 1" | 2.670 | Columns, heavy structural |
Calculating Bars for a Slab Grid
Rebar is placed in a grid (two layers of parallel bars perpendicular to each other).
Number of bars in one direction = (Slab dimension − 2 × edge cover) ÷ Spacing + 1Then repeat for the other direction and multiply by 2 for a two-way grid.
Example: 20 ft × 24 ft Driveway Slab, #4 Rebar at 18" Spacing
North-South direction (20 ft):- Usable span: 20 ft − 2 × (0.25 ft edge cover) = 19.5 ft
- Bars: 19.5 × 12 ÷ 18 + 1 = 14 bars
- Length each: 24 ft
- Total: 14 × 24 = 336 LF
- Bars: (20 ft − 0.5 ft) × 12 ÷ 18 + 1 = 14 bars (of 24-ft length)
- Wait — flip dimensions: bars span the 20-ft direction
- 24 ft span: (24 − 0.5) × 12 ÷ 18 + 1 = 17 bars
- Length each: 20 ft
- Total: 17 × 20 = 340 LF
Footing Rebar
Footings typically use 2–4 horizontal bars running the length of the footing, plus stirrups (hoops) at spacing intervals.
| Footing Width | Typical Longitudinal Bars | Typical Stirrups |
|---|---|---|
| 12" wide | 2 bars | #3 at 24" o.c. |
| 16" wide | 3 bars | #3 at 18" o.c. |
| 24" wide | 4 bars | #4 at 18" o.c. |
| 36" wide | 4–6 bars | #4 at 12" o.c. |
Cover Requirements
Rebar needs concrete cover (distance from bar surface to concrete edge) for fire protection and corrosion resistance:
| Location | Minimum Cover |
|---|---|
| Slab (interior, not exposed) | ¾" |
| Slab (exterior, exposed to weather) | 1½" |
| Beams and columns | 1½" |
| Footings (cast against earth) | 3" |
| Foundation wall (interior face) | ¾" |
Do I need rebar in a residential driveway slab?
It's not always required by code, but it significantly improves crack resistance and longevity, especially in areas with freeze-thaw cycles, clay soil, or vehicle loads. Many contractors use wire mesh for light-duty slabs as a less expensive alternative — both provide crack reinforcement.
Can I use wire mesh instead of rebar?
Wire mesh (WWF — welded wire fabric) provides similar crack control for flatwork like driveways and patios at lower cost. It doesn't add as much structural strength as rebar for structural elements. They can also be combined: rebar for strength, mesh for crack control in large slabs.
How do I join rebar lengths?
Rebar is overlapped (lapped splice) by a minimum of 24–40 bar diameters (varies by code and load). For #4 rebar (½" diameter), minimum lap is 12–20 inches depending on the structural situation.
Related Tools
- Concrete Calculator — the concrete volume the rebar goes into
- Beam Load Calculator — structural loads that determine rebar size
- Lumber Calculator — formwork lumber for your slab