YepCalc

Rebar Calculator

Find how many bars of rebar to buy for a concrete slab or footing, laid in a grid both ways. Enter the slab, the bar size, and the spacing to see the bars to buy, the total length with laps, and the weight.

Section 1

Add another rectangle for an L-shaped slab, or use Quantity for identical footings.

Rebar

0.668 lb/ft (0.9941 kg/m)

Default: 12 in

Center to center, between the bars that run along the length.

Default: 12 in

Center to center, between the bars that run along the width.

Default: 3 in

From the edges of the concrete to the bar ends and the outermost bars.

Stock bars and laps

Default: 20 ft

The length the bars are sold in, such as 20 ft or 40 ft.

Leave blank to use 60 bar diameters (30 in). Use the lap on your plans if they give one.

Cost

per

Result

Enter the length and width of the slab to see the rebar you need.

How to calculate rebar for a slab

The calculator lays straight bars in a grid, with the ends and the outermost bars at the edge distance from the edges of the concrete. Bars that run along the length are spaced across the width, and bars that run along the width are spaced along the length:

Bars along the length = (width − 2 × edge distance) ÷ spacing, rounded up, + 1

Length of each bar = length − 2 × edge distance

The bars across the width are found the same way, with the length and width swapped. The spacing is the most the bars can be apart, center to center, like #4 @ 12 in o.c. on a plan. Some guides round the number of spaces down instead: Hercules's guide How to Easily Calculate the Rebar Needed for Your Concrete Slab puts 20 bars across 19.5 ft, which leaves them about 12.3 in apart. This calculator rounds up and gives 21.

Laps and splices

When a run of rebar is longer than one stock bar, the bars overlap by the lap length and are tied together:

Bars in one run = (run length − lap) ÷ (stock length − lap), rounded up

Total length = run length + (bars in the run − 1) × lap, for every run

All but the last bar in a run are full stock bars, and the last one is cut to fit. A run as long as one stock bar, or shorter, needs no splice. Where to put the splices along a run is up to your plans.

Bars to buy

Bars to buy = the full stock bars in spliced runs + the stock bars the cut pieces come from. Pieces of the same length are cut from one stock bar as many times as they fit, across every section and both directions. Pieces of different lengths aren't combined, so you may need a few fewer bars if you plan the cuts: from a 20 ft bar, for example, a 144 in piece leaves 96 in, enough for a 54 in piece. The count doesn't add extra bars for cutting mistakes, so add a few if you want a margin.

Spacing and edge distance

The 12 in default is the closer end of the spacing in Hercules's table for home projects: 12 in both ways for a garage floor or a foundation slab, 12 to 18 in for a driveway, and 18 in for a patio or walkway. The 300 mm Metric default is 12 in rounded down. Closer spacing means more bars, so enter the spacing on your plans.

The 3 in (75 mm) edge distance is the cover the US residential code (IRC R403.1.3.5.3) requires for concrete cast against the earth. Concrete cast in forms that will be exposed to the earth or weather needs 1 1/2 in for No. 5 bars and smaller and 2 in for larger bars. The calculator measures the edge distance to the bar ends and to the center of the outermost bars, so entering the cover from your plans gives a slightly high count.

Stock bar length

The defaults are 20 ft and 6 m, the shortest lengths in stock at the suppliers checked, such as Steel Supply, L.P. in the US and Jewson in the UK. Rebar also comes in 30, 40, and 60 ft lengths, and mills sell 12 m and longer. A longer bar means fewer splices, so enter the length you buy.

Lap length

If you leave the lap blank, the calculator uses 60 bar diameters, such as 30 in for a #4 bar or 720 mm for a 12 mm bar. That matches the lap splice lengths for Grade 60 bars in the US residential code's table for concrete walls (IRC Table R608.5.4(1)): 30 in for No. 4, 38 in for No. 5, and 45 in for No. 6. The same table gives 40 diameters for Grade 40 bars, and Hercules gives 40 diameters as the rule of thumb. CRSI notes that the lap depends on the concrete strength and the bar's grade, size, and spacing. The default is an estimate, not a code value: if your plans give a lap, enter it. A lap of 0 counts bars joined end to end, such as with mechanical couplers.

Weight and cost

Total weight = total length × weight per foot (or per meter)

The weight is for the rebar in the grid, laps included, using the nominal weight of each size in the table below. The cut-off ends of the stock bars aren't included. Real bars can differ from the nominal weight by a few percent: the Indian standard IS 1786:2008 allows 3 to 7% for a batch, depending on the size.

Price the rebar per bar if you buy stock bars, or by weight if your supplier sells cut bars by the pound, ton, kilogram, or tonne. A price by weight leaves out the offcuts, so it comes out a little low for stock bars bought by weight.

Rebar sizes and weights

The calculator uses the nominal diameter and weight of each size. For #3 to #8, the US bar number gives the diameter in eighths of an inch, so a #4 bar is 4/8 = 1/2 in. Larger bars don't follow this rule: a #9 bar is 1.128 in. The US sizes and weights are from the AASHTO M 31 table in the Oklahoma Department of Transportation's Materials Reference Document 5005-R1, which matches the ASTM A615 table from Russel Metals.

US sizes (ASTM A615)

Size Diameter lb/ft kg/m
#3 3/8 in 0.376 0.5595
#4 1/2 in 0.668 0.9941
#5 5/8 in 1.043 1.552
#6 3/4 in 1.502 2.235
#7 7/8 in 2.044 3.042
#8 1 in 2.670 3.973
#9 1.128 in 3.400 5.06
#10 1.270 in 4.303 6.404
#11 1.410 in 5.313 7.907
#14 1.693 in 7.65 11.38
#18 2.257 in 13.60 20.24

Metric sizes

Size Diameter lb/ft kg/m
8 mm 8 mm 0.2654 0.395
10 mm 10 mm 0.4146 0.617
12 mm 12 mm 0.5967 0.888
16 mm 16 mm 1.062 1.58
20 mm 20 mm 1.66 2.47
25 mm 25 mm 2.587 3.85
28 mm 28 mm 3.246 4.83
32 mm 32 mm 4.24 6.31
36 mm 36 mm 5.369 7.99
40 mm 40 mm 6.626 9.86

Canadian sizes (CSA)

Size Diameter lb/ft kg/m
10M 11.3 mm 0.5275 0.785
15M 16.0 mm 1.055 1.570
20M 19.5 mm 1.582 2.355
25M 25.2 mm 2.637 3.925
30M 29.9 mm 3.692 5.495
35M 35.7 mm 5.275 7.850
45M 43.7 mm 7.912 11.775
55M 56.4 mm 13.19 19.625

The Metric sizes and weights are from Table 1 of IS 1786:2008, which takes the weight of a bar of the nominal diameter at 7,850 kg/m³. British BS 4449 bars have the same weights. Canadian bars are named 10M, 15M, and so on, and are heavier than the Metric size with the same number: a 10M bar is 11.3 mm across and weighs about 27% more than a 10 mm bar. Their weights are from Russel Metals' CSA G30.18 table. The values in the other system's unit are converted and rounded to four significant digits.

For a size that isn't listed, such as an Australian N24 bar or a 4, 6, 45, or 50 mm bar, choose Custom size and enter the nominal diameter. The calculator works out the weight from the same 7,850 kg/m³: for a #4 bar, that gives 0.668 lb/ft, as in the table.

Examples

A 20 × 20 ft slab with #4 bars

With bars 12 in apart both ways and 3 in from the edges, the inside of the grid is 240 − 6 = 234 in. That is 234 ÷ 12 = 19.5, rounded up to 20 spaces, so 21 bars each way, each 19 ft 6 in long. Every bar fits in one 20 ft stock bar, so you need 42 bars: 819 ft of rebar, about 547 lb.

A 40 × 30 ft slab with splices

The same grid needs 31 bars 39 ft 6 in long and 41 bars 29 ft 6 in long. With 30 in laps, a 39 ft 6 in run takes (474 − 30) ÷ (240 − 30) = 2.11, rounded up to 3 bars, two full and one cut to 54 in. A 29 ft 6 in run takes 2 bars, one full and one cut to 144 in. That is 103 splices, plus 41 bars for the 144 in pieces and 8 bars for the 54 in pieces, four from each bar: 152 bars of 20 ft, 2,691.5 ft, about 1,798 lb.

Footings

Four 3 × 3 ft footings take 4 bars each way in each footing, 2 ft 6 in long, or 32 pieces. Eight of them come from one 20 ft bar, so they need 4 bars.

A Metric slab

A 6 × 4 m slab with 12 mm bars 300 mm apart and 75 mm from the edges takes 14 bars 5,850 mm long and 21 bars 3,850 mm long. Each fits in one 6 m bar, so you need 35 bars: 162.75 m, about 145 kg.

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