What does the rebar number mean?

In inch-pound sizes the bar number is the nominal diameter in eighths of an inch: a #4 bar is 4/8 = 0.500 in. diameter, a #8 bar is 8/8 = 1.000 in. That rule holds exactly through #8. Above #8 the bars are sized so the area comes out to a round number — #9 is 1.00 sq in., #10 is 1.27, #11 is 1.56 — so the diameter is 1.128, 1.270 and 1.410 in. rather than 9/8, 10/8 and 11/8.

What is the area of a #5 rebar?

A #5 bar has a nominal diameter of 0.625 in., a cross-sectional area of 0.31 sq in., and a weight of 1.043 lb per linear foot. Its soft-metric designation is #16.

How do you find steel area per foot of width for a slab?

Multiply the single-bar area by 12 divided by the spacing in inches: As = Ab × 12/s. For example #5 bars at 12 in. on center give 0.31 × 12/12 = 0.31 sq in. per foot; the same bars at 6 in. on center give 0.62 sq in. per foot. Halving the spacing doubles the steel.

What is the minimum bend diameter for rebar?

ACI 318 sets the minimum inside bend diameter for standard hooks at 6 bar diameters for #3 through #8, 8 diameters for #9 through #11, and 10 diameters for #14 and #18. Stirrups and ties bent from #3 through #5 use 4 bar diameters, and #6 through #8 use 6 diameters.

How much concrete cover does rebar need?

ACI 318 requires 3 in. of cover for concrete cast against and permanently in contact with ground. For concrete exposed to weather or in contact with ground after forming, cover is 2 in. for #6 through #18 and 1.5 in. for #5 and smaller. For interior members not exposed to weather, cover is 0.75 in. for slabs, joists and walls with #11 and smaller, and 1.5 in. for beams and columns.

What is the difference between ASTM A615 and A706 rebar?

A615 is plain carbon-steel rebar with no chemistry limits aimed at weldability. A706 is low-alloy rebar with a controlled carbon equivalent, a tensile-to-yield ratio of at least 1.25, and a capped actual yield strength, which makes it weldable and suitable where capacity design or ductility governs. Seismic force-resisting systems generally require A706.

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Rebar Sizes & Areas — Reference

By Michael Flynn, PE · water resources & dam safety engineer

ASTM A615/A706 deformed bar properties, the two tables you actually reach for on a detail sheet (total area for n bars, and area per foot of width by spacing), plus ACI 318 hook geometry and cover. Bar number = nominal diameter in eighths of an inch, exactly through #8.

Bar Properties (ASTM A615 / A706, inch-pound)

Bar	Diameter d_b (in)	Area A_b (in²)	Weight (lb/ft)	Perimeter (in)	Soft metric	Metric area (mm²)
#3	0.375	0.11	0.376	1.178	#10	71
#4	0.500	0.20	0.668	1.571	#13	129
#5	0.625	0.31	1.043	1.963	#16	199
#6	0.750	0.44	1.502	2.356	#19	284
#7	0.875	0.60	2.044	2.749	#22	387
#8	1.000	0.79	2.670	3.142	#25	510
#9	1.128	1.00	3.400	3.544	#29	645
#10	1.270	1.27	4.303	3.990	#32	819
#11	1.410	1.56	5.313	4.430	#36	1006
#14	1.693	2.25	7.650	5.319	#43	1452
#18	2.257	4.00	13.600	7.091	#57	2581

There is no #12, #13, #15, #16 or #17. Sizes #14 and #18 are mill-order items, are not generally available in short lengths, and cannot be bent in the field.

Total Area for n Bars (in²)

Bar	1	2	3	4	5	6	7	8
#3	0.11	0.22	0.33	0.44	0.55	0.66	0.77	0.88
#4	0.20	0.40	0.60	0.80	1.00	1.20	1.40	1.60
#5	0.31	0.62	0.93	1.24	1.55	1.86	2.17	2.48
#6	0.44	0.88	1.32	1.76	2.20	2.64	3.08	3.52
#7	0.60	1.20	1.80	2.40	3.00	3.60	4.20	4.80
#8	0.79	1.58	2.37	3.16	3.95	4.74	5.53	6.32
#9	1.00	2.00	3.00	4.00	5.00	6.00	7.00	8.00
#10	1.27	2.54	3.81	5.08	6.35	7.62	8.89	10.16
#11	1.56	3.12	4.68	6.24	7.80	9.36	10.92	12.48

Area per Foot of Width by Spacing (in²/ft)

Bar	3″	4″	6″	8″	9″	10″	12″	18″
#3	0.44	0.33	0.22	0.165	0.147	0.132	0.110	0.073
#4	0.80	0.60	0.40	0.300	0.267	0.240	0.200	0.133
#5	1.24	0.93	0.62	0.465	0.413	0.372	0.310	0.207
#6	1.76	1.32	0.88	0.660	0.587	0.528	0.440	0.293
#7	2.40	1.80	1.20	0.900	0.800	0.720	0.600	0.400
#8	3.16	2.37	1.58	1.185	1.053	0.948	0.790	0.527
#9	4.00	3.00	2.00	1.500	1.333	1.200	1.000	0.667

A_s (in²/ft) = A_b × 12 / s s (in) = A_b × 12 / A_s,req

Grades & Material

Spec	Grades (f_y, ksi)	Notes
ASTM A615 — carbon steel	40, 60, 80, 100	The default. No weldability limits — welding requires a carbon-equivalent check per AWS D1.4.
ASTM A706 — low-alloy	60, 80, 100	Weldable, controlled chemistry, f_t/f_y ≥ 1.25 and a capped actual yield. Required in most seismic force-resisting systems.
ASTM A996 — rail/axle	40, 50, 60	Legacy; limited availability.
ASTM A1035 — low-carbon chromium	100, 120	High strength, high corrosion resistance; design provisions are limited in ACI 318.
ASTM A775 / A934 epoxy-coated	—	Coating applies a development-length factor ψ_e = 1.2 or 1.5. Handle with padded slings.

E_s = 29,000 ksi for all grades. Yield strain ε_y = f_y/E_s: 0.00138 (Gr 40), 0.00207 (Gr 60), 0.00276 (Gr 80), 0.00345 (Gr 100). Deformed bars are furnished in 20, 40 and 60 ft stock lengths.

ACI 318 Standard Hooks & Minimum Bend Diameters

Hook type	Bar sizes	Min. inside bend dia.	Straight extension
90° standard hook (development)	#3–#8	6 d_b	12 d_b
90° standard hook (development)	#9–#11	8 d_b	12 d_b
90° standard hook (development)	#14, #18	10 d_b	12 d_b
180° standard hook	#3–#18	same as above	4 d_b, ≥ 2.5 in.
90° stirrup / tie hook	#3–#5	4 d_b	6 d_b
90° stirrup / tie hook	#6–#8	6 d_b	12 d_b
135° seismic hook	#3–#8	4–6 d_b	6 d_b, ≥ 3 in.

Minimum Concrete Cover (ACI 318, cast-in-place, non-prestressed)

Exposure	Bar sizes	Cover
Cast against and permanently in contact with ground	All	3 in.
Exposed to weather or in contact with ground (formed)	#6–#18	2 in.
Exposed to weather or in contact with ground (formed)	#5 and smaller	1½ in.
Not exposed — slabs, joists, walls	#11 and smaller	¾ in.
Not exposed — slabs, joists, walls	#14, #18	1½ in.
Not exposed — beams, columns, ties, stirrups	All	1½ in.

Spacing & Minimum Steel Rules of Thumb

Rule	Requirement
Minimum clear spacing, parallel bars in a layer	Greatest of 1 in., d_b, and (4/3)·d_agg
Maximum spacing, flexural steel in slabs & walls	Lesser of 3h and 18 in.
Shrinkage & temperature steel, Gr 60	ρ = 0.0018 · A_g (0.0020 for Gr 40/50)
Minimum flexural steel, beams	A_s,min = max(3√f′_c/f_y, 200/f_y) · b_w d
Weight of a bar mat	lb = (lb/ft from the table) × total linear feet of bar; add 5–10% for laps and waste

Nominal diameter is not the ordering diameter. The values above are the nominal plain-round equivalents used for design. The deformations make the actual outside dimension larger — roughly d_b + 0.05 to 0.10 in. — which matters for clear spacing at congested joints, for coupler and sleeve fits, and for cover at the outermost bar. Detail with the nominal value, but check tight conditions with the mill's actual deformation height.

Sources: ASTM A615/A615M, A706/A706M, A996, A1035; ACI 318-19 Ch. 20 (cover), Ch. 25 (hooks, bend diameters, spacing); CRSI Manual of Standard Practice. Areas and weights are the standard nominal values; unit weight of steel taken as 490 pcf.

Sizing a section? Open the concrete flexure calculator → · Need l_d? Development length card.

Related cheat sheets and tools

Pair the bar areas here with the ACI 318 development-length card and the concrete flexure tool. Section geometry comes from section properties, dead load from the unit weights card, and footing sizing from presumptive bearing values and bearing capacity. For hydraulic structure design end to end, see HydroComplete.

Rebar Sizes & Areas — Reference

Bar Properties (ASTM A615 / A706, inch-pound)

Total Area for n Bars (in²)

Area per Foot of Width by Spacing (in²/ft)

Grades & Material

ACI 318 Standard Hooks & Minimum Bend Diameters

Minimum Concrete Cover (ACI 318, cast-in-place, non-prestressed)

Spacing & Minimum Steel Rules of Thumb

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