How much rebar do I need for a concrete slab?

Count the bars each way: floor((side − 2 × edge clearance) ÷ spacing) + 1, then make each bar as long as the other side less the clearance. A 24 × 16 ft slab with 3 in clearance and an 18 in grid has 11 bars of 23.5 ft and 16 bars of 15.5 ft, 525 ft of bar with laps, which is 33 stock bars of 20 ft with 5% extra. The size and spacing come from your plan; this counts the steel, it does not design the slab.

How far apart should rebar be in a slab?

Your plan or engineer decides. Residential slabs commonly show a grid between 12 and 24 in on center; 16 or 18 in is a frequent middle choice, and closer spacing takes more bar. This calculator offers 12, 16, 18 and 24 in and shows what each costs in bar, but it does not say which one a slab needs.

How far from the edge should rebar stop?

ACI 318 asks for 3 in of cover where concrete is cast against and permanently in contact with the ground; for a slab on grade held up on chairs the edge clearance is commonly 2 to 3 in, and your plan or local code sets it. The calculator starts at 3 in. At 2 in the 24 × 16 ft example still takes 33 bars, but lays 529 ft of bar instead of 525, because every bar is 2 in longer.

How long should a rebar lap be?

The real lap depends on the concrete, the bar grade and size, and the cover, so the plan or an engineer sets it (ACI 318 gives a Class B tension splice of 1.3 times the development length). 40 bar diameters is a common rule of thumb: 20 in for #4, 15 in for #3, 25 in for #5. The calculator starts there and lets you change it. It is an estimate, not a code figure.

How are laps and stock bars counted?

A bar run longer than the stock bar is built from pieces lapped at their ends: pieces = ceil((run − lap) ÷ (stock − lap)). A 23.5 ft run on 20 ft bars with a 20 in lap is 2 pieces, a whole bar and a 5.17 ft piece. The short pieces share bars, three to a bar here, so 11 runs take 15 stock bars. Offcuts are only reused whole and never between the two directions, which is conservative.

How much does the rebar weigh?

Bar laid × weight per foot: #3 is 0.376 lb/ft, #4 is 0.668 lb/ft and #5 is 1.043 lb/ft (ASTM A615 nominal). The 24 × 16 ft example lays 351 lb of #4, and the 33 bars you buy weigh about 441 lb. Switch the metric toggle for kilograms.

How many rebar chairs do I need?

One chair every 4 ft each way across the grid is a typical layout, so 35 chairs for the 24 × 16 ft example. That is a planning figure, not a code number: use closer chairs for heavy bar or a lot of foot traffic while you pour, and pick the chair height your plan gives for the bar’s position in the slab.

Do I need rebar in a slab at all?

That is a design question, not a counting one. Many small residential slabs use wire mesh or fibers, or none; rebar is used for heavier loads, weak soil or where the plan or code asks for it. This calculator only counts the rebar once you know you want it.

Should I buy 10 ft or 20 ft bars?

Pick the length that wastes the least for your sides. Where a run is a little over the stock length, the lap and a short last piece make 20 ft bars better than 10 ft. A 20 × 14 ft slab with no clearance fits 20 ft bars exactly across the length: 10 + 14 bars, 24 stock bars. 20 ft bars are harder to carry and need a longer vehicle; many yards will cut or deliver.

Rebar Calculator for a Concrete Slab

A 24 × 16 ft slab with #4 rebar on an 18 in grid, 3 in clear of every edge, needs 33 bars of 20 ft with 5% extra: 11 bars along the 24 ft side and 16 across it, 525 ft of bar laid, about 351 lb. The 23.5 ft runs are longer than a 20 ft bar, so each is lapped (20 in at 40 bar diameters). Bars each way = floor((span − 2 × edge clearance) ÷ spacing) + 1. This is a material estimate, not a structural design: the plan or an engineer sets the bar size, spacing and lap.

Figures based on the sources listed belowUpdated 2026-10-05How we calculate

Slab length

Slab width

Extra bar

Standard: cutting and bending (5%)Generous: odd cuts and mistakes (10%)Exact (0%)Custom

Bar sizeThe slab’s design sets the size. #3 is the usual light residential bar, #4 and #5 are heavier. Diameter and weight from the ASTM A615 / CRSI standard bar table.

Bar spacing on centerCenter to center, the same each way. Closer spacing takes more bar; the plan or engineer sets it.

Edge clearance (in)

Gap from the slab edge to the end of each bar. 3 in is the ACI 318 cover for concrete cast against earth; 2 in is also common at slab edges. Your plan or local code decides.

Stock bar lengthThe length you will buy. Runs longer than this are lapped.

Lap splice length (bar diameters)

40 bar diameters is a common rule of thumb, not a code value: the engineer or local code sets the real lap. #4 at 40 diameters is 20 in.

Chair spacing each wayChairs hold the bar off the ground. 4 ft each way is a typical layout; follow your plan or the chair maker’s spacing.

Price per bar (optional)

Buy 33 bars

524.83 linear ft + 5% waste = 551.08 linear ft

Bars along the length: 11 bars (each 23.5 ft), 2 pieces each, lapped
Bars along the width: 16 bars (each 15.5 ft), no splices
Lap splices: 11 splices (each 20 in)
Bar laid, laps included: 525 ft (buying 660 ft)
Weight of the bar laid: 350.6 lb (buying 440.9 lb)
Chairs (1 every 4 ft each way, typical layout): 35 chairs

How this number is calculated

524.83 linear ft + 5% waste = 551.08 linear ft ÷ 16.7 linear ft per bar = 33 bars

Shopping list

Rebar chairs or bar supports
Rebar tie wire (16 gauge)
Rebar tying tool
Rebar cutter
Work gloves
Safety caps for rebar ends

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Bars and bar laid for common slabs at 12, 18 and 24 in spacing

#4 bar, 3 in edge clearance, 20 ft stock bars, laps at 40 diameters, no extra. Each cell pair is the bars along the length + the bars along the width, then the linear ft of bar laid, laps included. Worked by this calculator.
Slab	12 in: bars	12 in: ft	18 in: bars	18 in: ft	24 in: bars	24 in: ft
10 × 10 ft	10 + 10	190	7 + 7	133	5 + 5	95
12 × 12 ft	12 + 12	276	8 + 8	184	6 + 6	138
16 × 12 ft	12 + 16	370	8 + 11	251	6 + 8	185
20 × 12 ft	12 + 20	464	8 + 14	317	6 + 10	232
20 × 20 ft	20 + 20	780	14 + 14	546	10 + 10	390
24 × 24 ft	24 + 24	1,208	16 + 16	805	12 + 12	604
30 × 20 ft	20 + 30	1,208	14 + 20	826	10 + 15	604

Stock bars to buy by bar size at 18 in spacing

20 ft stock bars, 3 in edge clearance, laps at 40 diameters, no extra, counted with the conservative offcut method above. A bigger bar needs a longer lap, so it can need more stock bars for the same grid.
Slab	#3 bars	#4 bars	#5 bars
10 × 10 ft	8	8	8
12 × 12 ft	16	16	16
16 × 12 ft	19	19	19
20 × 12 ft	22	22	22
20 × 20 ft	28	28	28
24 × 24 ft	40	44	44
30 × 20 ft	48	48	48

Rebar sizes, weights and laps

Nominal diameter and weight per foot from the ASTM A615 / CRSI standard bar table. The lap column is 40 diameters, the rule of thumb the calculator starts from; your engineer or code sets the real lap.
Bar	Diameter	Weight	Lap at 40 diameters	A 20 ft bar weighs
#3	0.375 in	0.376 lb/ft	15 in	7.5 lb
#4	0.5 in	0.668 lb/ft	20 in	13.4 lb
#5	0.625 in	1.043 lb/ft	25 in	20.9 lb

Methodology & assumptions

This is a material estimate for a single mat of bars in a square grid, not a structural design. The bar size, spacing, edge clearance and lap splice come from your plans or an engineer; the calculator only counts what they ask for. The bars stop short of every edge by the edge clearance, so the grid covers (length − 2 × clearance) by (width − 2 × clearance). Bars each way = floor(span ÷ spacing) + 1, where the span is the grid side the bars are spaced across: one bar on each edge of the grid, and the last gap can be narrower than the spacing but never wider. Each bar is as long as the other side of the grid. A bar run longer than the stock bar is lapped: with stock length S and lap L, n pieces cover n × S − (n − 1) × L, so pieces = ceil((run − L) ÷ (S − L)). The lap is the lap diameters you enter × the bar diameter (40 × 0.5 in = 20 in for #4). Stock bars to buy is a simple, conservative count: every whole piece is a stock bar, and the leftover last pieces of one direction (all one length) share stock bars, as many as fit whole in one bar. Offcuts are never joined together and never moved between the two directions, so the count does not rely on a perfect cut list; a careful cutter may need a bar or two fewer. Then the extra percentage is added and the total rounded up once. The bar laid includes the laps, since they are steel too, and the weight is the bar laid × the weight per foot of the bar (ASTM A615 nominal weights). Chairs are one every 4 ft each way across the grid by default, (ceil(length ÷ spacing) + 1) × (ceil(width ÷ spacing) + 1), a typical layout you can change to 3 or 5 ft; they are not a code figure. Bends, hooks, dowels, bars at slab edges thickened for footings, and tie wire are not counted. Metric toggle: lengths in m, clearance in cm, weights in kg.

ASTM A615/A615M, Standard Specification for Deformed and Plain Carbon-Steel Bars for Concrete Reinforcement, and the CRSI standard reinforcing bar table (Concrete Reinforcing Steel Institute, "Rebar Properties" and "FAQs About Bars" pages, crsi.org): #3 is 0.375 in and 0.376 lb/ft, #4 is 0.500 in and 0.668 lb/ft, #5 is 0.625 in and 1.043 lb/ft. The weights were confirmed by a web search; the crsi.org pages themselves could not be opened from the build environment
ACI 318-19, Building Code Requirements for Structural Concrete, Table 20.5.1.3.1 (specified concrete cover): 3 in for concrete cast against and permanently in contact with ground. A 2 in edge clearance is common practice for slab edges, not a code number; a slab on grade with bars on chairs is usually not cast against earth, so the plan decides
Lap splice: ACI 318-19 Class B tension splice is 1.3 times the development length, which depends on concrete strength, bar grade, size, cover and spacing. "40 bar diameters" is a common simplified rule of thumb (see the Building Club Rebar Calculator, buildingclub.info/rebar-calculator, which states it that way), not a code value. Unverified against the code text: use your plan’s lap
Chair spacing: public-works standard specifications for slabs on grade commonly give bar supports at 4 ft maximum each way (found in City of Wichita public works documents via web search; the exact document was not opened). A typical layout, not a requirement. The CRSI Manual of Standard Practice covers bar supports
The 5% and 10% extra for cuts and mistakes are typical allowances, not published figures: unverified, and the buyer’s choice

How to calculate rebar for a slab

Get the bar size, spacing and edge clearance. Take them from your plan or engineer. They are not something a material calculator can choose.
Find the grid. Subtract twice the edge clearance from each side of the slab. That is the area the bars cover.
Count the bars each way. For each direction, bars = floor(grid side in inches ÷ spacing) + 1, using the side the bars are spaced across. Each bar is as long as the other grid side.
Add laps and find the stock bars. If a run is longer than the stock bar, pieces = ceil((run − lap) ÷ (stock − lap)). Count the whole bars, then share the short leftover pieces between bars, and add the extra for cuts.
Weigh it and count the chairs. Weight = total bar laid × weight per foot of the bar. Chairs are about one every 4 ft each way across the grid.

Worked examples

Project	Result
24 × 16 ft slab, #4 bar at 18 in, 3 in edge clearance, 20 ft bars, 40 diameter laps	33 bars of 20 ft with 5% extra: 11 + 16 in the grid, 525 ft laid, 351 lb
10 × 10 ft slab, #4 bar at 12 in, 20 ft bars (every run fits in one bar, two to a bar)	11 bars of 20 ft with 5% extra: 10 + 10 in the grid, 190 ft laid, 127 lb
12 × 12 ft slab, #3 bar at 24 in, 10 ft bars (each run lapped)	17 bars of 10 ft with 5% extra: 6 + 6 in the grid, 12 laps, 58 lb
20 × 20 ft slab, #5 bar at 16 in, 20 ft bars	32 bars of 20 ft with 5% extra: 15 + 15 in the grid, 585 ft laid, 610 lb
The first slab with 2 in edge clearance instead of 3 in (same bars to buy, a little more bar laid)	33 bars of 20 ft with 5% extra: 11 + 16 in the grid, 529 ft laid, 354 lb

Frequently asked questions

How much rebar do I need for a concrete slab?: Count the bars each way: floor((side − 2 × edge clearance) ÷ spacing) + 1, then make each bar as long as the other side less the clearance. A 24 × 16 ft slab with 3 in clearance and an 18 in grid has 11 bars of 23.5 ft and 16 bars of 15.5 ft, 525 ft of bar with laps, which is 33 stock bars of 20 ft with 5% extra. The size and spacing come from your plan; this counts the steel, it does not design the slab.
How far apart should rebar be in a slab?: Your plan or engineer decides. Residential slabs commonly show a grid between 12 and 24 in on center; 16 or 18 in is a frequent middle choice, and closer spacing takes more bar. This calculator offers 12, 16, 18 and 24 in and shows what each costs in bar, but it does not say which one a slab needs.
How far from the edge should rebar stop?: ACI 318 asks for 3 in of cover where concrete is cast against and permanently in contact with the ground; for a slab on grade held up on chairs the edge clearance is commonly 2 to 3 in, and your plan or local code sets it. The calculator starts at 3 in. At 2 in the 24 × 16 ft example still takes 33 bars, but lays 529 ft of bar instead of 525, because every bar is 2 in longer.
How long should a rebar lap be?: The real lap depends on the concrete, the bar grade and size, and the cover, so the plan or an engineer sets it (ACI 318 gives a Class B tension splice of 1.3 times the development length). 40 bar diameters is a common rule of thumb: 20 in for #4, 15 in for #3, 25 in for #5. The calculator starts there and lets you change it. It is an estimate, not a code figure.
How are laps and stock bars counted?: A bar run longer than the stock bar is built from pieces lapped at their ends: pieces = ceil((run − lap) ÷ (stock − lap)). A 23.5 ft run on 20 ft bars with a 20 in lap is 2 pieces, a whole bar and a 5.17 ft piece. The short pieces share bars, three to a bar here, so 11 runs take 15 stock bars. Offcuts are only reused whole and never between the two directions, which is conservative.
How much does the rebar weigh?: Bar laid × weight per foot: #3 is 0.376 lb/ft, #4 is 0.668 lb/ft and #5 is 1.043 lb/ft (ASTM A615 nominal). The 24 × 16 ft example lays 351 lb of #4, and the 33 bars you buy weigh about 441 lb. Switch the metric toggle for kilograms.
How many rebar chairs do I need?: One chair every 4 ft each way across the grid is a typical layout, so 35 chairs for the 24 × 16 ft example. That is a planning figure, not a code number: use closer chairs for heavy bar or a lot of foot traffic while you pour, and pick the chair height your plan gives for the bar’s position in the slab.
Do I need rebar in a slab at all?: That is a design question, not a counting one. Many small residential slabs use wire mesh or fibers, or none; rebar is used for heavier loads, weak soil or where the plan or code asks for it. This calculator only counts the rebar once you know you want it.
Should I buy 10 ft or 20 ft bars?: Pick the length that wastes the least for your sides. Where a run is a little over the stock length, the lap and a short last piece make 20 ft bars better than 10 ft. A 20 × 14 ft slab with no clearance fits 20 ft bars exactly across the length: 10 + 14 bars, 24 stock bars. 20 ft bars are harder to carry and need a longer vehicle; many yards will cut or deliver.

Handy tools

Feet and Inches Calculator: Add up slab sides measured in feet, inches and fractions, and get decimal feet for the fields above.