Rebar Grid Calculator
Enter slab size, bar size and spacing to get bar counts, total length with laps, weight, and whether the spacing meets the ACI 0.0018 shrinkage minimum — with a spacing chart.
Last updated: 2026-09-22
How the calculation works
- Bar counts use ceil(dimension in inches ÷ spacing) + 1 — bars run full length with a lap at each splice.
- Weight uses ASTM A615 bar weights: #4 = 0.668 lb/ft, #5 = 1.043 lb/ft, #8 = 2.670 lb/ft.
- The ACI check compares your spacing's steel area against the shrinkage/temperature minimum 0.0018 × b × d.
- The chart plots steel area vs spacing for the selected bar with the ACI minimum as a reference line — where the curve crosses the line is the tightest spacing that still passes.
Formula
bars_per_dir = ceil(dim_in / spacing) + 1 total_len = bars_len × (L + lap) + bars_wid × (W + lap) weight = total_len × lb_per_ft(bar) As per ft = (12 / spacing) × bar_area — compare ACI min 0.0018 × 12 × d
| Symbol | Meaning | Unit |
|---|---|---|
As | Steel area per foot of slab width | in²/ft |
d | Effective depth (thickness − 1.5 in cover) | in |
0.0018 | ACI shrinkage/temperature minimum ratio | — |
Worked example
Interpreting the result
The weight drives the purchase: rebar is priced per hundredweight (cwt), so 430 lb is 4.3 cwt — at $40–60/cwt, $170–260 of steel. The ACI check is a floor, not a design: structural slabs on grade carrying loads need much more, computed from the actual load pattern. Suspended slabs are a different world entirely — the spacing there comes from flexural design, and this calculator's minimum check does not apply.
Assumptions
- Two-way grid at uniform spacing; one-way (single direction) layouts need halving.
- Full-length bars with lap splices — not welded wire mesh (different product, different math).
- 1.5 in cover assumed for effective depth in the ACI check.
Limitations
- No accounting for chairs, supports or extra bars at corners and openings.
- Not a flexural design tool — carrying load through a suspended slab needs structural design.
- Lap length for seismic or high-strength applications may exceed the default.
Lap splices: why bars overlap
Two rebar bars never butt end to end — force transfers between them through the concrete surrounding the overlap, a lap splice. The 18 in default here is standard practice for slabs on grade; the theoretical requirement is a multiple of bar diameter (Class B splice ≈ 30× the bar diameter, so 18 in for #4, 24 in for #6). Under-lapping is a hidden defect: the concrete looks fine, the bars are in place, but tension cannot cross the gap — the slab behaves as if unreinforced along that line. When bars are scarce, masons overlap scrap pieces; the lap measurement, not the bar count, is what an inspector checks.
Frequently asked questions
How much rebar do I need per cubic yard of concrete?
Slabs on grade typically run 80–120 lb of steel per yd³ (light reinforcement). The weight this calculator reports, divided by the concrete volume, should land in that range for a typical slab.
What spacing should rebar be in a 4 inch slab?
18–24 in each way with #3 or #4 bar is the residential norm for a driveway or patio slab. Stepping down to 12 in with #4 roughly doubles the steel and is where heavier-use slabs (RV pads, small shop floors) start.
Rebar or wire mesh for a driveway?
Both work; they do different jobs. Mesh (WWR) controls shrinkage cracking; rebar holds cracks that do form closed and adds load capacity. For a residential driveway, 6×6 W1.4 mesh is common; rebar at 18 in with #4 is the stronger choice for vehicle turns and soft subgrade.