How a rebar grid is laid out
A slab grid has bars running both ways at the same spacing, tied where they cross. Bars stop short of the slab edges so they stay inside the concrete. That gap is the edge clearance. ACI 318 calls for 3 in of cover where concrete is cast against and permanently exposed to earth, and 3 in is the default here.
Bar sizes in the US are named in eighths of an inch: #3 is ⅜ in, #4 is ½ in, #5 is ⅝ in. Your plans or engineer set the size and spacing. The 18 in default is only a starting value for the calculator, not a design recommendation.
The math
- Clear length = slab length − 2 × edge clearance. Clear width is worked out the same way.
- Bars running lengthwise = floor(clear width ÷ spacing) + 1. Bars running widthwise use the clear length.
- Any run longer than one stock bar gets an extra lap length for each splice.
- Total length × (1 + waste) ÷ stock length = bars to buy.
With the defaults (a 20 × 12 ft slab, 18 in spacing, 3 in clearance, 20 ft stock, 12 in lap), you get 8 bars one way and 14 the other, 332.9 ft of bar and 17 stock bars.
Rebar sizes and weights
Suppliers sometimes sell by weight. Standard nominal weights per ASTM A615:
| Bar | Diameter | Weight per ft | Weight of a 20 ft bar |
|---|---|---|---|
| #3 | 0.375 in | 0.376 lb | 7.5 lb |
| #4 | 0.500 in | 0.668 lb | 13.4 lb |
| #5 | 0.625 in | 1.043 lb | 20.9 lb |
| #6 | 0.750 in | 1.502 lb | 30.0 lb |
| Bar | Diameter | Weight per metre | Weight of a 6 m bar |
|---|---|---|---|
| 10 mm | 10 mm | 0.617 kg | 3.7 kg |
| 12 mm | 12 mm | 0.889 kg | 5.3 kg |
| 16 mm | 16 mm | 1.580 kg | 9.5 kg |
| 20 mm | 20 mm | 2.469 kg | 14.8 kg |
Multiply the total length from the calculator by the weight per foot to get the order weight.Multiply the total length from the calculator by the weight per metre to get the order weight.
Spacing vs. bar count
Tighter spacing increases the steel quickly. For the same 20 × 12 ft slab:Tighter spacing increases the steel quickly. For a 6 × 3.6 m slab with 6 m bars:
| Spacing | Total length | 20 ft bars | Ties |
|---|---|---|---|
| 12 in | 464 ft | 24 | 240 |
| 16 in | 348 ft | 18 | 135 |
| 18 in | 317 ft | 16 | 112 |
| 24 in | 232 ft | 12 | 60 |
| Spacing | Total length | 6 m bars | Ties |
|---|---|---|---|
| 20 cm | 208.8 m | 35 | 540 |
| 30 cm | 139.2 m | 24 | 240 |
| 40 cm | 104.4 m | 18 | 135 |
| 45 cm | 95.1 m | 16 | 112 |
Worked examples
Two-car garage slab, 24 × 24 ft, #4 at 18 inTwo-car garage slab, 7.32 × 7.32 m, #4 at 45.7 cm
Slab length 24 ft · Slab width 24 ft · Bar spacing 18 in · Edge clearance 3 in · Stock bar length 20 ft · Lap splice length 12 in · Waste / overage 5 %Slab length 7.32 m · Slab width 7.32 m · Bar spacing 45.7 cm · Edge clearance 7.6 cm · Stock bar length 6.1 m · Lap splice length 30.5 cm · Waste / overage 5 %
- Total rebar length: 823.2 ftTotal rebar length: 250.9 m
- Stock bars needed (20 ft): 42Stock bars needed (6.1 m): 42
- Grid intersections (ties): 256
The 24 ft runs are longer than 20 ft stock, so each needs one lap splice.The 7.32 m runs are longer than 6.1 m stock, so each needs one lap splice.
Patio, 12 × 16 ft at 24 inPatio, 3.66 × 4.88 m at 61 cm
Slab length 16 ft · Slab width 12 ft · Bar spacing 24 in · Edge clearance 3 in · Stock bar length 20 ft · Lap splice length 12 in · Waste / overage 5 %Slab length 4.88 m · Slab width 3.66 m · Bar spacing 61 cm · Edge clearance 7.6 cm · Stock bar length 6.1 m · Lap splice length 30.5 cm · Waste / overage 5 %
- Total rebar length: 194.3 ftTotal rebar length: 59.2 m
- Stock bars needed (20 ft): 10Stock bars needed (6.1 m): 10
Placement tips
- Support bars on chairs or concrete blocks (dobies) so they end up inside the slab, not on the ground. Bar that sits on the base does little.
- Lap splices are usually specified as a multiple of bar diameter. The 12 in default is a placeholder, so use your plan's value.
- Tie wire: one tie at each intersection is a typical count for budgeting. Many crews tie every other one in the field (rule of thumb).
- Cut and bend bars wearing eye protection, and cap exposed bar ends to prevent impalement injuries.
- Welded wire mesh is an alternative in some light-duty slabs. Whether it's acceptable depends on the design.
Rebar in footings and thickened edges
Strip footings usually get continuous bars running lengthwise rather than a grid, often two bars. Some designs add cross bars or vertical dowels tying into the wall. To estimate continuous footing bars, multiply the footing length by the number of bars, then add lap splices at each joint and extra at the corners. Your plans give the bar count, size and laps. For example, two bars along a 100 ft footing with 20 ft stock and a 12 in lap need about 11 stock bars before waste. That's 200 ft of bar plus a 1 ft lap at each of the joints.
Rebar, wire mesh or fibers?
Rebar, welded wire reinforcement and synthetic fibers do different jobs. Rebar and mesh mainly hold cracks tight and, in structural elements, carry tension loads. Fibers mainly reduce early plastic-shrinkage cracking. None of them stops a slab from cracking if joints are missing or the base is poor. Which you need, if any, depends on the design, the loads and local code. Many residential flatwork slabs rely mostly on a good base and properly spaced control joints. When plans call for reinforcement, use what they specify.
Common mistakes
- Bar lying on the base. Bar that ends up at the bottom of the slab does little good. Support it with chairs or dobies.
- Too little edge cover. Bars too close to the edge or the soil corrode, and the concrete over them spalls.
- Forgetting laps. On runs longer than one stock bar, the overlaps add up. The calculator adds them for you.
- Buying exact lengths. Allow a few percent for offcuts, bends and mistakes.
Checked against
Each case below is re-run against the calculator by our automated test suite.
| Case | Our result | Checked against | Result |
|---|---|---|---|
| 20 × 12 ft slab, bars 18 in apart, 3 in edge: bars running lengthwise | 8 | Hand calculation | Match |
| Case | Our result | Checked against | Result |
|---|---|---|---|
| 6.1 × 3.66 m slab, bars 45.7 cm apart, 7.6 cm edge: bars running lengthwise | 8 | Hand calculation | Match |
Frequently asked questions
How much rebar do I need for a 20 × 20 slab?
At 18 in spacing with 3 in clearance, 14 bars each way, 546 ft of bar in total (28 stock 20 ft bars) before waste. Change the inputs above for your spacing.
What size rebar for a driveway?
It depends on the design and local practice. #3 and #4 are common in residential flatwork, but follow your plans.
How far from the edge should rebar stop?
ACI 318 requires 3 in of cover for concrete cast against earth.
How many ties do I need?
One per intersection at most. The calculator reports the intersection count.
Sources & assumptions
The results are planning estimates, and the cost figures only use prices you enter. Product yields, densities and coverage rates vary, so check the label or ask your supplier, and follow local codes. Figures marked "approximate" or "rule of thumb" are general practice, not exact standards.
- ACI 318-19 Table 20.6.1.3.1 (specified concrete cover)
- ASTM A615 (deformed carbon-steel bars), standard nominal weights
- CRSI: Concrete Reinforcing Steel Institute
See how we calculate and test our formulas and the changelog. Last updated 2026-10-08.
Reviewed by the MaterialWhiz Editorial Team. This estimate uses the assumptions shown in the calculator. Review the result against your supplier's product coverage and site conditions. Last reviewed: October 10, 2026. See how we calculate · Report a correction.