How Much Rebar Do I Need for a Concrete Slab?
Part of Concrete & Masonry
Quick answer
Reinforce a slab with a two-way grid of rebar at 12 to 18 inches on center. Count the bars each way (slab side ÷ spacing, then add one), add up their lengths, and add about 10% for lap splices. A 20 × 20 ft slab at 12 in on center needs roughly 900 linear feet of #4 rebar — about 46 twenty-foot sticks. Use #4 bar for typical 4-inch residential slabs.
Rebar keeps a slab from cracking apart once concrete (which is strong in compression but weak in tension) flexes under load. For most residential slabs you build a flat grid — a 'mat' — of bars running both directions, tied where they cross, suspended in the middle third of the slab thickness.
The five-step estimate
- Pick a spacing: 12 in on center for driveways and footings, 16–18 in for patios and light slabs.
- Bars each way = slab side ÷ spacing, rounded up, plus one for the closing bar at the far edge.
- Bar length = the slab side minus about 6 in total, so the steel stays 3 in back from each edge (concrete cover).
- Total length = (bars one way × their length) + (bars the other way × their length).
- Add ~10% for lap splices and cut-offs, then divide by 20 ft for the number of sticks to buy.
Worked example — a 20 × 20 ft slab at 12 in
At 12 in (1 ft) on center, a 20 ft side takes 20 ÷ 1 + 1 = 21 bars. Each bar is about 19.5 ft long (20 ft minus 3 in of cover at each end). The grid is 21 × 19.5 + 21 × 19.5 ≈ 819 linear feet. Add 10% for laps and you are ordering about 900 linear feet — 46 twenty-foot sticks of #4, weighing roughly 600 lb.
| Slab size | Spacing | Approx. linear feet (#4, +10%) | 20 ft sticks |
|---|---|---|---|
| 10 × 10 ft | 12" | ~230 lf | 12 |
| 12 × 12 ft | 16" | ~250 lf | 13 |
| 20 × 20 ft | 12" | ~900 lf | 46 |
| 24 × 24 ft | 16" | ~980 lf | 50 |
| 30 × 30 ft | 18" | ~1,360 lf | 69 |
These are planning figures for a single-layer mat with 3 in of cover and a 10% lap allowance. The rebar calculator runs your exact dimensions, spacing and bar size and returns the bar count, total length, sticks to buy and the steel weight.
A few field rules
- Keep the steel in the middle third of the slab thickness on chairs or bricks — rebar lying on the sub-grade does nothing.
- Lap (overlap) splices at least 40 bar diameters: ~20 in for #4, ~25 in for #5, and tie them.
- For thin slabs (≤4 in) some pros use welded wire mesh instead of a rebar grid; rebar is stronger and the norm for driveways and footings.
FAQs
Do I need rebar in a 4-inch slab?
For anything that carries load — driveways, garage floors, footings, structural slabs — yes, use #4 rebar in a grid. Welded wire mesh is sometimes used in thin, lightly loaded slabs like a shed floor, but a tied rebar mat gives more control over crack widths and is the standard for a 4-inch slab that will see vehicles.
Should I order extra rebar?
Add about 10% to the bare grid length to cover lap splices (where bars join end to end) and cut-off waste. If your slab is much longer than your bar stock — so you have lots of end-to-end splices — bump it toward 12–15%. The calculator's lap allowance is set to 10% by default.