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

Updated By

A 10 x 10 ft slab with #4 rebar 12 in on center each way, 3 in from the edges, takes 22 bars 9.5 ft long (209 linear ft): buy eleven 20 ft bars, about 147 lb.

How much rebar do you need?

Enter the slab size and, from your plans, the bar spacing, edge clearance, and lap. We lay a grid both ways with a bar at each edge clearance, splice runs longer than one bar, and count the bars to buy.

ft
ft
in
in
in

For estimating only: your plans, local code, or an engineer set the bar size, spacing, cover, and lap. Weights are ASTM nominal (#4: 0.668 lb per ft). The 30 in lap is the IRC table figure for #4 Grade 60 bars (20 in for Grade 40).

To figure rebar by hand, divide the space between the edge clearances by the bar spacing, round up, and add one bar, in both directions. Each bar is the slab size minus the clearance at both ends, cut from the 10 or 20 ft bars you buy.

The bar size, spacing, edge clearance, and lap come from your plans, your local code, or an engineer. Our rebar calculator for concrete slabs turns them into a cut list and a shopping list; it doesn't decide whether a slab needs steel.

How to measure for rebar

  1. Measure the slab inside the forms and convert to decimal feet (10 ft 6 in is 10.5 ft; see our square footage calculator). Split an L-shaped slab into rectangles and run each one.
  2. Take the bar spacing from your plans. It's center to center ("on center"), the same both ways.
  3. Set the edge clearance, the concrete between the slab's edge and the steel.
  4. Pick the bar size and the bar length your supplier sells, 10 or 20 ft.
  5. Enter the lap from your plans. It only counts when a run is longer than one bar.

How the calculator lays out the grid

  • A bar at each edge. The outer bars sit at the edge clearance, and the space between them is divided by your spacing and rounded up, so no gap is wider than the spacing. The bars are spaced evenly.
  • Splices. A run longer than one bar is full bars overlapped by the lap, plus one shorter piece.
  • Cutting. Pieces are cut longest first, and a shorter piece goes into a leftover when it fits. Leftovers aren't spliced together, and saw cuts aren't deducted.

The result shows the offcuts and what the other bar length would take. It doesn't count chairs or tie wire.

The rebar formula

  • Bars each way = (slab size in inches − 2 × edge clearance) ÷ spacing, rounded up, + 1
  • Bar length (ft) = (other slab size in inches − 2 × edge clearance) ÷ 12
  • Linear ft = bars × bar length, for both directions
  • Pieces per run, if a run is longer than a stock bar = (run − lap) ÷ (stock length − lap), rounded up
  • Weight = bars bought × stock length × lb per ft

Worked example (the calculator's default values)

A 10 x 10 ft slab, #4 bars 12 in on center, 3 in edge clearance, 20 ft bars, and a 30 in lap:

  1. 120 − 2 × 3 = 114 in. 114 ÷ 12 = 9.5, rounded up to 10 spaces, so 11 bars each way, 11.4 in apart.
  2. Each bar is 10 ft − 6 in = 9.5 ft. 22 × 9.5 = 209 linear ft, with no splices.
  3. Two pieces per 20 ft bar: 11 bars (220 ft), 11 ft of offcuts.
  4. 220 ft × 0.668 lb per ft = 146.96 lb, about 147 lb.

The calculator shows: "Buy 11 bars of #4 rebar, 20 ft long (about 147 lb)." and "11 bars 9.5 ft long run along the slab's length, 11.4 in apart, and 11 bars 9.5 ft long along its width, 11.4 in apart, 3 in from the edges: 209 linear ft of #4 bar. Cutting them from 20 ft bars leaves 11 ft of offcuts. With 10 ft bars: 22 bars (220 ft, no splices)."

Rounding down instead would give 10 bars each way and leave the last one 9 in from the far edge instead of 3.

Second example: a 12 x 12 ft slab

It's 13 bars each way, 11.5 ft long: 299 linear ft. Only one piece fits in a 20 ft bar: "Buy 26 bars of #4 rebar, 20 ft long (about 347 lb)." with 221 ft of offcuts. With 10 ft bars, each run is a full bar plus a 4 ft piece, lapped 30 in: "Buy 39 bars of #4 rebar, 10 ft long (about 261 lb)." That's 390 ft instead of 520, but 26 splices.

Third example: a 24 x 24 ft slab

It's 25 bars each way, 23.5 ft long: 1,175 linear ft. Each run is a 20 ft bar plus a 6 ft piece, so 50 splices add 125 ft: "Buy 67 bars of #4 rebar, 20 ft long (about 895 lb)." With 10 ft bars it's 150 bars and 100 splices.

Rebar for a 10 by 10 foot slab: 22 bars 9.5 feet long, cut from eleven 20 foot barsPlan view of a 10 by 10 foot concrete slab with a grid of number 4 rebar. The bars stop 3 inches from every edge, so each bar is 10 feet minus 6 inches, or 9.5 feet long, and the space for bars is 120 minus 6, or 114 inches. At 12 inches on center, 114 divided by 12 is 9.5 spaces, rounded up to 10, so there are 11 bars each way, evenly spaced 11.4 inches apart, with a bar along each edge: 22 bars and 209 linear feet. An enlarged corner shows the 3 inch edge clearance and the 11.4 inch spacing. Each 20 foot bar is cut into two 9.5 foot pieces with 1 foot left over, so 11 bars of 20 feet are bought, 220 feet in all, leaving 11 feet of offcuts. At 0.668 pounds per foot, 11 bars of 20 feet weigh 146.96 pounds, about 147 pounds. The edge clearance in the plan is drawn wider than scale.REBAR GRID FOR A 10 × 10 FT SLAB, #4 AT 12 IN10 ft10 ft9.5 ft barsEdge clearance drawn wider than scale11 bars each way9.5 ft long, 11.4 in apart(no gap over 12 in)Corner, enlarged11.4 in3 in3 inOne 20 ft bar = 2 pieces9.5 ft9.5 ft1 ftSpaces: (120 − 2 × 3 in) ÷ 12 = 9.5 → 10, so 11 bars each wayBar length: 10 ft − 2 × 3 in = 9.5 ft; 22 × 9.5 = 209 linear ft20 ft bars: 2 pieces each → 11 bars (220 ft), 11 ft of offcutsWeight: 11 × 20 ft × 0.668 lb/ft = 146.96 lb → about 147 lb
With 3 in of edge clearance, each bar is 10 ft − 6 in = 9.5 ft long and the bars fill 120 − 6 = 114 in. At 12 in on center that's 9.5 spaces, rounded up to 10, so 11 bars each way, 11.4 in apart: 22 bars, 209 linear ft. Each 20 ft bar gives two 9.5 ft pieces, so you buy 11 bars (220 ft, 11 ft of offcuts), and 220 × 0.668 lb per ft = 146.96 lb, about 147 lb.

Rebar sizes and weights

In the ASTM bar data Caltrans lists (Bridge Design Details, Table 13.1.1), each bar number is the diameter in eighths of an inch: a #4 bar is 0.500 in, or 4/8.

BarDiameterWeight per ft10 ft bar20 ft barIRC lap, Grade 40 / 60
#33/8 in0.376 lb3.76 lb7.52 lbnot listed
#41/2 in0.668 lb6.68 lb13.36 lb20 / 30 in
#55/8 in1.043 lb10.43 lb20.86 lb25 / 38 in
#63/4 in1.502 lb15.02 lb30.04 lb30 / 45 in

Gerdau lists the same weights and typical lengths of 20, 30, 40 and 60 ft. Home Depot lists #3 bars 10 ft long and #4 and #5 bars 20 ft long, and Lowe's a #4 bar 10 ft long. CRSI says a bar's rolled-on marks show the mill, then the bar size, then the steel type, usually "S" for ASTM A615 or "W" for A706.

Rebar for common slab sizes

#4 bars 12 in on center each way, 3 in edge clearance, 30 in lap:

Slab (ft)Bars each wayLinear ft20 ft bars10 ft bars
8 x 89 at 7.5 ft135918
10 x 1011 at 9.5 ft2091122
12 x 1011 at 11.5 ft, 13 at 9.5 ft2501830 (11 splices)
12 x 1213 at 11.5 ft2992639 (26 splices)
16 x 1213 at 15.5 ft, 17 at 11.5 ft3973052 (30 splices)
16 x 1617 at 15.5 ft5273468 (34 splices)
20 x 2021 at 19.5 ft81942105 (84 splices)
24 x 2425 at 23.5 ft1,17567 (50 splices)150 (100 splices)

Runs just under 10 or 20 ft cut cleanly; runs a little over half a bar, like the 11.5 ft runs of a 12 x 12 slab, waste the most, so try both lengths. Spacing drives the count: on the 10 x 10 slab, 16 in spacing takes 9 bars each way (171 linear ft) and 18 in takes 8 (152 linear ft).

Lap splices: how much should bars overlap?

Where a run is longer than your bars, two bars overlap and are tied. The 2018 International Residential Code's lap table for concrete walls, Table R608.5.4(1), as reproduced in Element ICF's bulletin on lap splices, gives 20 in for #4 bars of Grade 40 steel and 30 in for Grade 60, 25 and 38 in for #5, and 30 and 45 in for #6. Section R403.1.3.5.4 points to that table, says wall reinforcement "shall be the longest lengths practical," and limits the gap between lapped bars to the smaller of one-fifth of the lap and 6 in.

The table doesn't list #3, and a slab's laps are part of its design, so use your plans. The 30 in default is the table's #4 Grade 60 figure. Home Depot lists its 20 ft #4 bar (REB/4/615G4/20) as Grade 40 and Weyerhaeuser's 20 ft #4 bar as Grade 60.

Edge clearance and where the bars sit

Marshalltown's slab guide says the bar ends "should be about two- to three-inches away from the edge of your form," and the calculator starts at 3 in. Where concrete is cast against and permanently in contact with the ground, ACI 318-19 requires 3 in of cover, the American Concrete Institute says, citing footings, retaining walls, and grade beams.

Height matters too. The California Residential Code, based on the IRC, says slab reinforcement must be supported "to remain in place from the center to upper one-third of the slab" during the pour: from 2 in to about 1 1/3 in below the top of a 4 in slab. Quikrete's patio plan sets rebar on bolsters, and Marshalltown raises it on rebar chairs.

Does your slab need rebar?

Not always. The residential code's slab section sets rules for reinforcement only "Where provided," and Quikrete's patio plan lists it as optional. Concrete Network says many slabs on a well-compacted base with properly spaced control joints don't need it, suggests rebar or welded wire for slabs more than 5 in deep, and notes that reinforcement won't prevent cracks but helps hold them together.

Footings, garage slabs, and anything that carries a building are structural: follow your plans and local code, and have an engineer or a qualified pro set the bar size, spacing, and laps. For the rest of the order, the concrete calculator puts the default slab at 1.36 cubic yards for 4 in thick with 10 percent extra, and the gravel calculator sizes the base under it.

Tips and common mistakes

  • Rounding the bar count down. You need a bar at each edge clearance.
  • Forgetting the clearance at both ends. A 10 ft slab takes 9.5 ft bars, not 10 ft.
  • Buying by linear feet alone. 299 ft for a 12 x 12 slab isn't 15 bars of 20 ft: each 11.5 ft run needs its own piece.
  • Counting on a full 10 ft. Lowe's gives its "10 ft" #4 bar's actual length as 9.9895 ft.
  • Leaving the grid on the ground. Set it on chairs at or just above mid-depth.
  • Forgetting the walls on top. The concrete block calculator counts the blocks for a wall; its reinforcement comes from your plans.

Frequently asked questions

How much rebar do I need for a 10x10 slab?

With #4 bars 12 in on center and 3 in from the edges, 22 bars 9.5 ft long (209 linear ft): eleven 20 ft bars or twenty-two 10 ft bars, about 147 lb.

How much rebar do I need for a 12x12 slab?

At the same spacing, 26 bars 11.5 ft long (299 linear ft): twenty-six 20 ft bars, or thirty-nine 10 ft bars with 26 splices at a 30 in lap.

How much does rebar weigh per foot?

#3 is 0.376 lb, #4 0.668 lb, #5 1.043 lb, and #6 1.502 lb per foot. A 20 ft #4 bar weighs 13.36 lb.

How far should rebar be from the edge of a slab?

Marshalltown puts the bar ends about 2 to 3 in from the form, and ACI 318-19 requires 3 in of cover where concrete is cast against the ground. Use the figure on your plans.

How much should rebar overlap?

Use the lap on your plans. The IRC's lap table for concrete walls gives 20 in for #4 Grade 40 bars and 30 in for Grade 60.

Does a concrete patio need rebar?

Not always. Quikrete lists reinforcement as optional in its patio plan, and Concrete Network says many slabs on a well-compacted base with proper control joints don't need it.

Spot an error? Tell us.