Calculate rebar for a concrete slab grid: bar counts each way, total linear feet, steel weight and 20-ft stock bars to buy, from slab size and spacing.
Rebar Slab Grid Formula
The calculator sizes the grid one direction at a time. The number of bars running in one direction depends on the slab dimension perpendicular to those bars, the edge clearance, and the on-center spacing:
The total length of rebar in the grid is the bar count in each direction multiplied by the bar length in that direction:
The steel weight follows from the total length and the unit weight of the bar size:
In spacing mode, the calculator rearranges the first formula to spread a known number of bars evenly across the clear run:
Variables:
- N is the number of bars running in one direction
- D is the slab dimension perpendicular to those bars (use W for bars running the length, L for bars running the width)
- L and W are the slab length and width
- C is clear concrete cover to the steel surface; db is nominal bar diameter
- S is maximum center-to-center spacing in quantity mode, or actual even spacing in spacing mode
- T is the total length of rebar in the grid
- NL and NW are the bar counts running the length and the width
- w is the unit weight of the bar size (pounds per foot or kilograms per meter)
In quantity mode, the calculator includes both outer bar centerlines while keeping actual spacing at or below the maximum entered. It uses plan-specified clear cover and nominal diameter, adds only the entered lap and extra-material allowances, and reports nominal weight. Stock pieces based on aggregate length are a lower bound, not a feasible cutting plan. Intersections, all-intersection and half-intersection tie scenarios, and an editable support area-density estimate are also shown. In spacing mode, entered whole bar counts produce an even center-to-center spacing. This quantity tool does not check structural design compliance or assume a universal 18-inch limit.
Common Slab Grids and Rebar Weights
The first table records illustrative slab setups, not design recommendations. Required reinforcement depends on the structure, support conditions, loading, exposure and governing design. Use approved project plans.
| Application | Typical slab thickness | Common grid |
|---|---|---|
| Sidewalk or patio | 4 in | #3 bars at 18 in on center |
| Driveway | 4 to 5 in | #4 bars at 12 to 18 in on center |
| Garage or shop slab | 5 to 6 in | #4 bars at 12 to 16 in on center |
| Heavy equipment pad | 6 in or more | #5 bars at 12 in on center |
The second table covers what most rebar calculators leave out: the accessories that hold the grid together and in position. Ties fasten the bars at intersections, and chairs hold the grid at the correct height in the slab so it does not sit on the subgrade.
| Accessory | How much you need | Notes |
|---|---|---|
| Tie wire (16 gauge) | About 1 lb per 100 to 150 ties | Required perimeter and interior tying patterns come from placing drawings; half-intersection counts are arithmetic scenarios |
| Bar chairs or dobies | Area-density estimate: ceil(slab area / assumed support spacing squared) | Use plan-specified support layout, size and height; reinforcement is not universally placed at slab mid-depth |
| Lap splices | Use the lap length specified by the project designer | Lap lengths and locations depend on bar, material and design conditions; a 40-diameter rule is not universal |
Rebar Slab Grid Example
Example 1: A 20 ft by 12 ft slab uses #4 bars (0.500 in nominal diameter), maximum spacing of 18 in and clear edge cover of 3 in. Centerline spans are 233.5 in and 137.5 in. Bars running the length: ceil(137.5/18) + 1 = 9, each cut to 19.5 ft. Bars running the width: ceil(233.5/18) + 1 = 14, each cut to 11.5 ft. Straight length = 9 × 19.5 + 14 × 11.5 = 336.5 ft; nominal mass = 336.5 × 0.668 = 224.782 lb, before laps or extra material. There are 9 × 14 = 126 intersections. Although ceil(336.5/20) = 17 is the aggregate stock-length lower bound, the required cuts cannot be assumed to fit into 17 stock pieces. A separate-stock-per-run estimate is 23 pieces; a cutting plan may reuse offcuts where feasible.
Example 2: Nine #4 bars run each way in a 10 ft by 10 ft slab with 3 in clear edge cover. The centerline span is 120 - 6 - 0.5 = 113.5 in. Even center-to-center spacing = 113.5/(9 - 1) = 14.1875 in in both directions. This is a geometric result; compare it with the project’s specified maximum spacing.
Rebar Slab Grid FAQ
What spacing should I use for rebar in a slab? Use the reinforcement spacing specified by the governing design. Requirements differ between structural slab types, reinforcement purposes and slabs on ground. This calculator performs takeoff geometry and does not decide which ACI provisions or local requirements apply.
Does the grid need to stop short of the slab edge? Use the specified concrete cover, measured to the reinforcement surface. Three inches is a familiar ACI requirement for concrete cast against and permanently in contact with ground, not a universal slab edge rule. The tool subtracts twice the clear cover from straight cut lengths and also subtracts a nominal diameter from transverse centerline spans.
How is rebar different from welded wire mesh for a slab? Both reinforcing bars and welded wire reinforcement can serve different structural or crack-control roles when designed and placed for the application. Product form alone does not determine suitability, required amount or placement height. Follow the project design when choosing reinforcement.
