What Is a Rebar Calculator?
A rebar calculator determines how many reinforcing bars and how much total bar length you need to build a reinforcement grid in a concrete slab, footing, or driveway. Rebar adds tensile strength to concrete, which is strong in compression but weak in tension. Enter the slab dimensions and your desired bar spacing, and the calculator returns the number of bars in each direction and the total linear footage to order.
How to Use the Rebar Calculator
- Enter the slab length and width.
- Enter the bar spacing — the center-to-center distance between bars (for example, 12 inches).
- Set the edge clearance if needed.
- Calculate — see the bar count in each direction and the total length.
How the Grid Is Calculated
Bars run in two directions to form a grid. The number of bars along one axis is the slab dimension divided by the spacing, plus one for the starting bar:
Bars = (Length ÷ Spacing) + 1, rounded up, in each direction.
Total length is the sum of all bars times their span. For a 20 ft × 10 ft slab at 12-inch spacing, you get roughly 11 bars one way and 21 the other.
Common Rebar Spacing
| Application | Typical Spacing |
|---|---|
| Residential slab / patio | 12–18 inches |
| Driveway | 12 inches |
| Footings | Per engineer / code |
| Heavy-load slab | 6–12 inches |
Tips for Rebar Placement
- Maintain proper concrete cover (usually 2–3 inches) so rebar is protected from corrosion.
- Support bars on chairs so they sit in the middle of the slab, not on the ground.
- Overlap bar splices by the recommended length (often 40× the bar diameter).
- Tie intersections to keep the grid in place during the pour.
Note: For structural work, follow local building codes and engineering specifications.
Frequently Asked Questions
How much rebar do I need for a slab?
Divide each slab dimension by your bar spacing and add one, for each direction, then sum the bar lengths. The calculator gives the bar count and total linear footage automatically.
What spacing should I use for rebar?
Residential slabs and patios commonly use 12–18 inch spacing, while driveways and heavier loads often use 12 inches or less. Structural elements follow engineering specs.
How much should rebar overlap at splices?
A common rule is to overlap splices by about 40 times the bar diameter, but always follow local code and project specifications.
How deep should rebar sit in concrete?
Rebar should sit roughly in the middle of the slab with adequate cover (typically 2–3 inches) using chairs or supports, so it is protected and positioned to resist tension.
Is this rebar calculator free?
Yes — it is completely free, requires no signup, and gives bar counts and total length.