Rebar sizes

The rebar page already prints diameter, kilograms per meter, and pounds per foot for metric 6 mm through 32 mm and for US #3 through #11, #14, and #18. Copying that table here would be a second chart. This page is the part the chart does not answer: which name is which bar, the cross-sectional area, and how a spacing you were given becomes the count and the length you type.

US #3 through #11

South Dakota DOT’s construction-management chapter prints Table 7.1 as properties of ASTM standard reinforcing bars. The inch-pound size, the nominal diameter, and the area below are that table. The metric size in the same row is a soft conversion of the US bar. It is not the diameter on this site’s millimeter menu. #9, #10, and #11 are not whole-inch bars. There is no standard #12, #13, or #15 through #17, which the calculator page already says.

Table 7.1 also lists #14 and #18. Those two, and the pounds per foot for every row, stay on the calculator. #3 and #4 match that table exactly: 0.376 lb/ft and 0.668 lb/ft. The tool’s #5 is 1.044 lb/ft where the table prints 1.043. The difference is the 7,850 kg/m³ circle, not a second bar. When you order from this site, read the calculator.

Metric 8 mm through 32 mm

The millimeter menu is the nominal diameter you select. Area in the table is the circle π ÷ 4 × diameter². It is geometry of that nominal size, not a row from Table 7.1. Weight is the calculator at 7,850 kg/m³. A 12 mm bar is 0.888 kg/m. The shop shortcut d² ÷ 162 prints 0.889 kg/m for the same bar. The tool keeps 0.888. Ten of those bars at 12 m are 106.54 kg.

Do not read the soft-metric column backward. Table 7.1’s metric #10 is the US #3 bar, 9.5 mm, not this menu’s 10 mm bar at 0.617 kg/m. Table 7.1’s metric #32 is the US #10 bar, 32.3 mm and 1.27 square inches, not this menu’s 32 mm bar at 6.313 kg/m. The 6, 14, 18, 22, and 28 mm rows are on the calculator chart with the same density. They are omitted here so this table stays the sizes people mix up.

Spacing is a maximum, a minimum, or a plan

No public CRSI page opened for this article printed a residential slab grid, so this page does not cite CRSI and does not invent “#4 at 18 inches.” The code text that did open is ACI 318-95, from the law.resource.org copy. A later edition can read differently. Section 7.6.1 sets the minimum clear space between parallel bars in a layer at the bar diameter, but not less than 1 inch. Section 7.6.2 puts at least 1 inch clear between layers, with the upper bars over the lower bars.

Section 7.6.5 says primary flexural reinforcement in walls and slabs, other than concrete joist construction, shall be spaced not farther apart than three times the wall or slab thickness, nor 18 inches. Section 7.12.2.2 says shrinkage and temperature reinforcement shall be spaced not farther apart than five times the slab thickness, nor 18 inches. The commentary on 7.12 says those provisions are for structural slabs and are not intended for soil-supported slabs on grade. A driveway or garage slab on grade is the case the commentary sets aside. The slab calculator already tells you to follow the plan for spacing and cover. It counts concrete, not bars.

Section 10.5.4, for structural slabs and footings of uniform thickness, says the maximum spacing of the minimum tensile steel shall not exceed the lesser of three times the thickness and 18 inches. That is still a maximum, and it does not name #4 or #5. Concrete in the footing is the footing calculator. Cover in 7.7.1 is 3 inches for concrete cast against and permanently exposed to earth. Bars exposed to earth or weather are 2 inches for No. 6 through No. 18, and 1-1/2 inches for No. 5 and smaller. In slabs not exposed to weather, No. 11 and smaller are 3/4 inch. Measure cover to the outside of the steel the section names.

From a spacing to a weight

The calculator asks for a size, a cut length, and a piece count. It does not look up a spacing schedule. If the plan already says 12 inches on center across a 10 foot width, the count with a bar at each edge is 120 ÷ 12 = 10 spaces, plus one, which is 11 bars. Eleven bars times 10 feet is 110 feet. At 0.668 lb/ft that is 73.48 lb. Switch the menu to #4, the length unit to feet, the length to 10, and the count to 11. A second direction is a second run. A 40-diameter lap on a 12 mm bar is 480 mm, about 0.43 kg at 0.888 kg/m. Add that lap to the piece length before you multiply.

South Dakota’s same chapter tells a bridge inspector to check spacing against the plans, within 1/2 inch, and not to substitute a bar size without the bridge office. That tolerance is an inspection note for their work, not a license to move a house slab’s bars by half an inch. Mesh, when the plan calls for sheets instead of bars, is the wire mesh calculator. Plate, pipe, and beams are the steel weight calculator, at the same 7,850 kg/m³.

Sources

• South Dakota DOT, Construction Management Training, Chapter 7, Reinforcing Steel. Table 7.1, ASTM standard bar diameter, area, and the soft-metric size name. Pounds per foot in that table match this site’s #3 and #4 rows.
• ACI 318-95, sections 7.6.1, 7.6.2, 7.6.5, 7.7.1, 7.12.2.2, and 10.5.4, and the commentary that 7.12 is not for soil-supported slabs on grade. This is the 1995 edition. A later ACI 318 was not opened past a table of contents.

Frequently asked questions