Formula
Bars that span the length are spaced across the width: floor(width / spacing) + 1, which puts a bar on both edges. Bars that span the width are spaced across the length the same way. A 10 × 20 ft grid at 12 inches is 11 bars of 20 ft and 21 bars of 10 ft, 430 ft, before any lap.
splices = ceil((span − stock) / (stock − lap))
bar length = span + splices × lap
weight = total feet × ASTM A615 lb/ft
If the span fits on one stick, the splice count is zero. A 30 ft bar on 20 ft stock with a 2 ft lap needs one splice and is 32 ft. A 60 ft bar needs three splices and is 66 ft. The lap is an input. This is not an ACI development length. The concrete calculator is the yards and the bags under the steel.
Worked examples
10 × 20 ft, #4 at 12 in both ways, 20 ft stock. 11 + 21 bars, no splice, 430 ft. At 0.668 lb/ft that is 287.24 lb.
One 30 ft bar, 20 ft stock, 2 ft lap. One splice, 32 ft of #4, which is 21.38 lb. The grid multiplies that rule by every bar that needs it.
#5 instead of #4 on the 430 ft grid. The length stays 430. The weight uses 1.043 lb/ft.
Reference
ASTM A615 nominal weights. #4 is 0.668 lb/ft.
| Size | lb/ft | kg/m | Diameter, in |
|---|---|---|---|
| #3 | 0.376 | 0.560 | 0.375 |
| #4 | 0.668 | 0.994 | 0.500 |
| #5 | 1.043 | 1.552 | 0.625 |
| #6 | 1.502 | 2.235 | 0.750 |
| #7 | 2.044 | 3.042 | 0.875 |
| #8 | 2.670 | 3.973 | 1.000 |
| #9 | 3.400 | 5.060 | 1.128 |
| #10 | 4.303 | 6.404 | 1.270 |
| #11 | 5.313 | 7.907 | 1.410 |
| #14 | 7.650 | 11.380 | 1.693 |
| #18 | 13.600 | 20.240 | 2.257 |
Questions
Does this compute a lap from the code?
No. You type the lap. ACI development length depends on the bar, the concrete, the cover, and the bar position. This page will not invent that length.
Why is #4 listed at 0.668 lb/ft?
That is the ASTM A615 nominal weight. A few supplier charts print 0.688. The 0.668 figure matches the standard’s nominal table as reprinted by mill charts and by state DOT figures.
How are the bars counted?
A bar on both edges, then one more every spacing that fits. Ten feet at 12 inches is 11 bars. The bars that run the long way are spaced across the short way.
Sources
- ASTM A615 nominal unit weights, as reprinted on mill charts (Russel Metals, InterMetal) and the NCDOT area-and-weight figure. #4 is 0.668 lb/ft. A chart that prints 0.688 is not the one used here.
- Lap length is an input. This page does not compute ACI 318 development or splice length.