How do you calculate rebar for a concrete slab?
For a grid at 18-inch spacing, divide each slab dimension by 1.5 feet, round up, and multiply by the other dimension. A 20×20 ft slab needs 14 bars each way: 14 × 20 × 2 = 560 linear feet, plus 10% for laps and cuts — about 620 feet, or 31 twenty-foot #4 bars.
Worked example: 20×20 ft slab, #4 rebar at 18 in on center
| Step | Math | Result |
|---|---|---|
| Bars across 20 ft | 20 ÷ 1.5 = 13.3 | Round up to 14 |
| Linear feet (both ways) | 14 × 20 × 2 | 560 ft |
| Add 10% laps/waste | 560 × 1.10 | 616 ft |
| 20-ft bars to buy | 616 ÷ 20 | 31 bars |
How to read this table
- 20-ft bars to buy sits at the top of the table (616 ÷ 20) — 31 bars. If your situation looks like this row, plan against the upper half of the range rather than the midpoint.
- Linear feet (both ways) anchors the bottom (14 × 20 × 2) — 560 ft. Treat this as the conservative case you should still be profitable at.
- With 4 reference points in the "worked example: 20×20 ft slab, #4 rebar at 18 in on center" table, the fastest way to use this page is to find the closest row, take its math, then stress-test it ±30% before you build a plan on it.
Context
Placement matters more than quantity: rebar does its work in the lower-middle third of the slab — chairs or dobies holding it 2 inches up in a 4–6 inch slab — because rebar lying on the ground adds almost nothing. Wire mesh is the cheaper alternative for patios and walkways; rebar is worth it for driveways and garages. Lap splices need 24–40 bar diameters (12–16 inches for #4), which is where the 10% allowance goes.
What moves this number
Regional pricing
Ready-mix, bag, and labor prices vary 30–50% between US metros. Sun Belt markets with many plants run well below Northeast and remote-area pricing.
Waste and site conditions
Uneven subgrade, spillage, and form bowing consume 5–15% extra material. Orders placed at exact volume routinely come up short mid-pour.
Local code requirements
Frost depth, footing dimensions, and reinforcement rules are set by county. The same project can require twice the concrete in a northern climate versus a southern one.
Methodology
Grid count = ⌈dimension ÷ spacing⌉ + edge bar per direction. Spacing (12–18 in on center for #4 in residential slabs) and lap lengths follow ACI 318 development-length basics simplified for flatwork.
Assumptions and caveats
- Volumes are theoretical; order quantities include a waste allowance but site conditions vary.
- Prices reflect 2026 US national ranges — get local quotes before budgeting a real pour.
- Structural dimensions and frost depths are governed by local building code; verify with your building department.
- This page was last reviewed on 2026-10-01. Ranges are updated as new data lands, so re-check before using them in a contract or a plan.
- Use these numbers as a starting range, not a guarantee — your own historical data always beats a benchmark.
Frequently asked questions
How do you calculate rebar for a concrete slab?
For a grid at 18-inch spacing, divide each slab dimension by 1.5 feet, round up, and multiply by the other dimension. A 20×20 ft slab needs 14 bars each way: 14 × 20 × 2 = 560 linear feet, plus 10% for laps and cuts — about 620 feet, or 31 twenty-foot #4 bars.
Which option pays the most in the worked example: 20×20 ft slab, #4 rebar at 18 in on center table?
20-ft bars to buy, at 616 ÷ 20 (31 bars). That row represents the strongest case in this dataset, so use it as an upper bound rather than an expectation.
What is a realistic low-end figure?
Linear feet (both ways) at 14 × 20 × 2 (560 ft). Plan your costs so the low end still works, then treat anything above it as upside.
Where do these numbers come from?
Grid count = ⌈dimension ÷ spacing⌉ + edge bar per direction. Spacing (12–18 in on center for #4 in residential slabs) and lap lengths follow ACI 318 development-length basics simplified for flatwork.
How can I estimate my own number instead of using a benchmark?
Use the Concrete Slab Material Calculator on RevenueLab — it takes your own inputs and returns a figure specific to your setup, which is always more accurate than a published range.
Model your own numbers
- Concrete Slab Material Calculator
- Concrete Cost Calculator
- Concrete Slab Cost Calculator
- Concrete Slab Material Calculator
Related reading
More answers in this category
- How do you calculate cubic yards of concrete?
- How many bags of concrete make a cubic yard?
- How do you calculate concrete for a slab?
- How do you calculate concrete for a driveway?
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Last updated 2026-10-01.