#18 (No. 18) Reinforcing Bar - ASTM (US)
#18 (No. 18) is a ASTM (US) reinforcing bar to ASTM A615 / A706 (Table 1). Its nominal diameter is 2.257 in (57.3 mm), giving a cross-section area As of 4 in² (2581 mm²) and a mass of 13.6 lb/ft (20.24 kg/m). It is supplied in grade Gr.60 / Gr.75 / Gr.80 with characteristic yield f_yk = 420 MPa. The tables below give the total steel area for 1-10 bars and the area per foot of width at common bar spacings.
| d | Nominal diameter | 2.257 in / 57.3 mm |
| As | Cross-section area | 4 in² / 2581 mm² |
| m | Mass per metre | 13.6 lb/ft / 20.24 kg/m |
| u | Nominal perimeter | 180 mm |
| f_yk | Characteristic yield | 420 MPa |
| Bars | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |
|---|---|---|---|---|---|---|---|---|---|---|
| As (in²) | 4.00 | 8.00 | 12.00 | 16.00 | 20.00 | 24.00 | 28.00 | 32.00 | 36.00 | 40.00 |
| s (in) | 3 | 4 | 5 | 6 | 8 | 10 | 12 | 18 |
|---|---|---|---|---|---|---|---|---|
| As/ft (in²/ft) | 16.00 | 12.00 | 9.60 | 8.00 | 6.00 | 4.80 | 4.00 | 2.67 |
Source: ASTM A615 / A706 (Table 1). Area = π/4 × d² for metric bars; ASTM areas are the standard's tabulated nominal values. Grades available: Gr.60, Gr.75, Gr.80.
Understanding these values
#18 (No. 18) has a nominal diameter of 2.257 in (57.3 mm) and a tabulated area of 4 in² (2581 mm²). Note that this is the published value from ASTM A615 / A706 (Table 1), not the exact geometric area: pi/4 times 57.3 mm squared gives 2578.7 mm². Use the tabulated figure - it is what the standard specifies and what a checker will expect. Mass is 13.6 lb/ft (20.24 kg/m), and the nominal perimeter used in bond and anchorage checks is 180 mm.
It is supplied in Gr.60, Gr.75, Gr.80, where the grade number is the yield strength in ksi - Grade 60 is 60 ksi, about 420 MPa. A706 is the low-alloy weldable variant specified where bars must be welded or where seismic detailing requires controlled strength. The grade affects the strength you design to, not the geometry: the area of 2581 mm² and the mass of 20.24 kg/m are the same whichever grade of ASTM (US) bar is specified.
For slabs and walls the number you need is area per metre width, which is the bar area times 1,000 divided by the spacing. At 150 mm centres #18 (No. 18) gives 17207 mm²/m, at 200 mm centres 12905 mm²/m, and at 250 mm centres 10324 mm²/m. Compare those against the As,req from your slab design, take the first arrangement that satisfies it, and then check the spacing against the maximum bar spacing ACI 318 allows for crack control - a larger bar at wider centres can meet the area and still fail that limit.
If you are working across standards, the nearest equivalents by area are EN / Eurocode 40 mm at 1256.6 mm² (51% smaller), TCVN (Vietnam) 40 mm at 1256.6 mm² (51% smaller), BS (UK) H40 (40 mm) at 1256.6 mm² (51% smaller). Matching on area is the right first step, but it is not a code-sanctioned substitution: changing standard means re-checking anchorage length, lap length, bond and crack width, because those depend on bar surface geometry and the concrete, not on area alone.