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Reviewed · Prices checked October 2026 · How we calculate

Rebar Calculator: How Much Rebar Do I Need for a Slab?

Enter the slab size, the spacing and the bar size. You get the bars each way, the sticks to buy with laps, the weight and how many crossings to tie.

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ft
ft
in
in
Bar size
Stick length
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22sticks

20 ft sticks of #4 rebar for a 20 × 12 ft slab, 18 in on center

Rebar details
Bars the long way8 × 19.5 ft
Bars the short way14 × 11.5 ft
Rebar in all, with laps317 ft (212 lb)
Lap where bars join20 in (40 bar diameters)
Crossings to tie112
Cost at your price—

Also buy: tie wire, and chairs or concrete blocks (dobies) to hold the grid at mid-depth. A structural slab's bar size and spacing come from its plans.

Show the math
Long way: (12 ft × 12 − 2 × 3 in) ÷ 18 in + 1 = 8 bars of 19.50 ftShort way: (20 ft × 12 − 2 × 3 in) ÷ 18 in + 1 = 14 bars of 11.50 ftCut from 20 ft sticks, pieces that fit share a stick: 22 sticks for 317.0 ft

The price you type is used for this estimate only; it isn't saved.

How it works

The formula

bars one way = (span − 2 × cover) ÷ spacing, rounded down, + 1
each bar = the other span − 2 × cover, plus a lap wherever two sticks join
sticks = total length ÷ stick length, rounded up

Assumptions

  • Cover: 3 in from the edges, the ACI 318 minimum where concrete is cast against and permanently exposed to earth.
  • Laps: 40 bar diameters (20 in for #4), a common rule of thumb. Your plans or local code may call for more.
  • Weight: ASTM A615 nominal weights: #3 0.376, #4 0.668, #5 1.043 lb per ft.
  • Each run is cut from full sticks plus one last piece, and last pieces share a stick where two or more fit. A single grid at one level; a two-layer slab takes twice the bar.

Worked example

A 20 × 12 ft slab, #4 at 18 in, 3 in cover, 20 ft sticks

  1. 8 bars the long way and 14 the short way
  2. 317 ft in all with laps (212 lb)
  3. ÷ 20 ft → 22 sticks, 112 crossings to tie

20 ft sticks of #4 at 18 in, by slab

20 ft sticks of #4 rebar at 18 in on center, 3 in cover, by slab size
SlabFeetSticks
10 × 10 ft1338
12 × 20 ft31722
20 × 20 ft54628
24 × 24 ft80544
30 × 40 ft1,69897

Questions people ask

How much rebar for a 20 × 20 slab?

At #4 bar 18 in on center with 3 in cover: about 546 ft, or 28 sticks of 20 ft.

What rebar spacing for a slab?

Driveways and patios often use #3 or #4 bar 12 to 24 in on center each way; structural slabs follow their plans. Wire mesh or fiber is common for light patios.

How long should rebar overlap?

A common rule is 40 bar diameters, about 20 in for #4. Splice lengths in structural work come from the engineer or code and can be longer.

How far from the edge should rebar be?

At least 3 in where the concrete is poured against the ground, so the steel stays protected from moisture.