A 24 × 20 ft garage slab reinforced with #4 bars at 18 inches on center takes 14 bars one way and 17 the other. Two of the directions are longer than a 20 ft bar, so fourteen runs need a lap splice, and the whole grid comes to 684 feet of bar. Cut from 20 ft stock that is 36 bars weighing 481 lb, plus 238 ties and about 1.7 lb of tie wire. The calculator above does this count for a slab, a continuous footing or a tied column, in feet and inches or in metres.
How the calculator works
Pick the structure, enter its dimensions and choose the bars. Everything is converted to one unit system inside, so 23' 6½" is read exactly. The result is the number of stock bars to buy, because that is what you order at the yard; the length placed, the splices, the ties and the weight are listed under it.
Slab
Bars running along the length are spread across the width, so their count is the width minus the edge cover at both sides, divided by the spacing, rounded up, plus one bar for the far edge. The bars running the other way are counted the same way along the length. Each bar is the slab dimension minus the cover at both ends. With two mats, top and bottom, every number doubles. For the default slab: (240 − 6) ÷ 18 = 13, plus one is 14 bars along the length; (288 − 6) ÷ 18 = 15.7, rounded up to 16, plus one is 17 bars across.
Ties are counted at every intersection, or at every other one if you choose it; tying alternate crossings is common practice on slabs on ground. Chairs are placed on a grid, about one every 3 ft each way under each mat; enter 0 if the bars sit on dobies you count separately.
Footing
A continuous footing gets rows of bars along its full length. For a closed footing around a house, enter the perimeter measured along the centerline. Two #4 bars at the bottom are typical for a residential footing; more rows are drawn in a top layer. If the design calls for closed ties, switch them on: each tie goes around the cage inside the cover, at the spacing you enter. A 120 ft footing with two rows needs seven 20 ft bars per row with six splices, which the cutting plan turns into 13 bars to buy.
Column
Each vertical bar is the column height plus a lap into the footing dowels or the next lift. ACI 318-19 25.5.5.1 sets the compression lap for Grade 60 bars at 30 bar diameters and not less than 12 inches, so a #5 bar laps 18¾ in and a #3 bar the 12 in minimum. Ties sit at both ends of the column, which makes one more tie than spaces: a 10 ft column with ties at 10 in gets 13 of them.
Laps and the cutting plan
Stock bars are 20 ft at most yards, with 10 ft and 40 ft also sold. A run longer than one bar is spliced, and every splice adds one lap length. The calculator uses 40 bar diameters by default, a common field minimum for temperature and shrinkage steel in slabs: 15 in for #3, 20 in for #4, 25 in for #5. Structural bars are often spliced longer. ACI 318-19 25.5.2 calls for a Class B lap of 1.3 times the development length, which for a #4 bar in 3,000 psi concrete works out to about 57 bar diameters, or 29 in. If your drawings give a lap, enter it in bar diameters.
The bars to buy are not simply the total length divided by 20. Four column bars of 11' 6¾" are 46¼ ft of steel, which would round to three bars, but only one piece of that length fits in a 20 ft bar, so you need four. The calculator cuts runs that fit one bar as many times as they fit, gives longer runs full bars plus a remainder piece, and nests the remainders the same way. Offcuts are not carried from one group of bars to another, so the count errs slightly on the safe side. The spare percentage is added on top, for a miscut or a bar left at the yard.
Bar sizes and weights
US bar sizes are numbered in eighths of an inch: a #4 bar is ½ in in diameter, a #5 is ⅝ in. Weights are the nominal values from ASTM A615 Table 1:
| Bar | Diameter | Metric size | lb/ft | 20 ft bar |
|---|---|---|---|---|
| #3 | 0.375 in | 10 | 0.376 | 7.5 lb |
| #4 | 0.500 in | 13 | 0.668 | 13.4 lb |
| #5 | 0.625 in | 16 | 1.043 | 20.9 lb |
| #6 | 0.750 in | 19 | 1.502 | 30.0 lb |
| #7 | 0.875 in | 22 | 2.044 | 40.9 lb |
| #8 | 1.000 in | 25 | 2.670 | 53.4 lb |
The weight shown is the weight of the bars you buy, offcuts included, since that is what the yard charges for and the truck carries.
Code checks
The calculator flags the values that most often go wrong on site, with the clause of ACI 318-19 behind each one:
- Slab spacing. Shrinkage and temperature steel is spaced no more than 18 in or five times the slab thickness, whichever is less (24.4.3.3).
- Cover. Concrete cast against and permanently in contact with the ground needs 3 in of cover; concrete exposed to weather or soil needs 1½ in for #5 bars and smaller and 2 in for #6 and larger; interior columns need 1½ in (Table 20.5.1.3.1).
- Column ties. Tie spacing may not exceed 16 longitudinal bar diameters, 48 tie diameters or the least column dimension (25.7.2.1). For #5 bars with #3 ties in a 12 in column that is 10 in.
- Tie hooks. Each closed tie ends in two 135° hooks of 6 bar diameters, at least 3 in each (Table 25.3.2), so a #3 tie around a 9 × 9 in cage is 42 in long.
Common mistakes
- Counting bars by area. A grid has one more bar than spaces in each direction. Dividing the slab area by the spacing squared loses a row each way.
- Forgetting the laps. On a slab longer than 20 ft, every bar running the long way needs a splice; at 40 diameters that is another 20 in of bar per run.
- Dividing total length by the bar length. Offcuts are real steel you pay for. A cutting plan is the honest count.
- Bars on the ground. Rebar lying on the subgrade does nothing. Chairs or dobies keep it in the slab at the cover you designed.
For the concrete itself, use the Concrete Calculator. The structural drawings govern bar size, spacing, cover and laps wherever there are any; this calculator turns them into a shopping list.