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Rebar Calculator

Lay out a rebar grid for a slab: bars each way, linear feet with lap splices, stock bars to order, weight and cost.

Enter a number from 0 to 1000

Enter a number from 0 to 1000

Enter a number from 2 to 48

Use the spacing on your plans or from your engineer and local code.

Enter a number from 0 to 12

Distance from the slab edge to the first bar.

Enter a number from 1 to 60

Enter a number from 0 to 120

Overlap where two bars join. Use the lap length on your plans or from your engineer; 20 in is only a placeholder.

%

Enter a number from 0 to 50%

$

Enter a number of at least 0

20 ft bars to order
64
1,268.4linear feet
847 lbweight
24 × 24bars each way
Bars along the length · 1 lap per bar24 × 25.17 ft
Bars along the width · 1 lap per bar24 × 25.17 ft
With 5% waste · 847 lb of #41,268.4 ft
Rebar cost · 64 × $9.00$576.00
Material cost$576.00

Add chairs or dobies to hold the grid at mid-depth of the slab, plus tie wire.

See breakdown

How the rebar calculation works

  1. Subtract the edge clearance

    With 3 in at each edge, a 24 ft slab has 23.5 ft of usable length and width.

  2. Bars each way = usable width × 12 ÷ spacing + 1

    At 12 in on center, 23.5 ft gives 23 spaces and 24 bars running the length, and the same the other way.

  3. Add a lap wherever a run is longer than a stock bar

    A 23.5 ft run with 20 ft bars needs one 20 in lap, so each bar run is 25.17 ft.

  4. Total feet, waste, sticks and weight

    48 runs × 25.17 ft = 1,208 ft, or 1,268.4 ft with 5% waste: 64 twenty-foot bars, about 847 lb of #4.

Worked example. A 24 × 24 ft slab with #4 bar at 12 in on center and 3 in edge clearance needs 24 bars each way. Each 23.5 ft run gets one 20 in lap, so the grid is 1,208 ft, or 1,268.4 ft with 5% waste. That is 64 twenty-foot bars weighing about 847 lb, costing $576 at $9 a bar.

Rebar FAQ

How much rebar do I need for a slab?

Count the bars each way from the spacing, multiply by their length, add laps and waste. A 24 × 24 ft slab at 12 in centers takes about 1,268 ft of bar.

How much does rebar weigh per foot?

#3 is 0.376 lb/ft, #4 is 0.668 lb/ft, #5 is 1.043 lb/ft and #6 is 1.502 lb/ft. These are the nominal weights Nucor lists for standard US bar sizes (ASTM A615).

How much does a 20 ft rebar weigh?

Multiply the weight per foot by 20: about 7.52 lb for #3, 13.36 lb for #4, 20.86 lb for #5 and 30.04 lb for #6.

What lengths does rebar come in?

Mills such as Nucor sell most sizes cut to 20, 40 or 60 ft. Set the stock bar length to match what your supplier stocks.

What spacing should rebar be in a concrete slab?

There is no single answer. Spacing depends on slab thickness, loads and soil, so use the spacing on your plans or from your engineer and local code.

How long should a rebar lap splice be?

Lap length depends on bar size, concrete strength and the design code. Take it from your structural plans or engineer; the 20 in default is only a placeholder.

How many 20 ft bars do I need?

Divide total linear feet including laps and waste by 20 and round up. 1,268.4 ft needs 64 bars.

Sources: Nucor: rebar sizes, weights and stock lengths

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