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Rebar Size Converter

Convert reinforcing bar designations — imperial bar numbers, soft metric sizes, diameters, cross-sectional area and mass per length. Enter what you know, choose what you want and press Convert.

Written and reviewed by SM Arijul Islam · Last reviewed 29 October 2026 · Editorial standards · Report an error

About the rebar size converter

Reinforcing bar is sold by bar number in North America and by millimetres almost everywhere else. The imperial number is simply the nominal diameter in eighths of an inch, so a #4 bar is 4/8 in = 0.5 in = 12.7 mm across. The bar number is a designation, not a measurement: on a deformed bar you measure the shank between the ribs.

The area figure quoted by suppliers is the nominal area from that nominal diameter, and the mass per length follows from it at steel's density of 7,850 kg/m³. This converter derives every value from the diameter, which is why the numbers here are internally consistent — published nominal areas are rounded.

US rebar sizes

Imperial rebar sizes with soft metric equivalents, nominal diameter, area and mass
Bar	Soft metric	Diameter	Area	Mass
#2	#6	6.35 mm (0.250 in)	32 mm²	0.249 kg/m (0.167 lb/ft)
#3	#10	9.53 mm (0.375 in)	71 mm²	0.561 kg/m (0.376 lb/ft)
#4	#13	12.70 mm (0.500 in)	129 mm²	0.996 kg/m (0.668 lb/ft)
#5	#16	15.88 mm (0.625 in)	200 mm²	1.556 kg/m (1.043 lb/ft)
#6	#19	19.05 mm (0.750 in)	284 mm²	2.240 kg/m (1.502 lb/ft)
#7	#22	22.23 mm (0.875 in)	387 mm²	3.049 kg/m (2.044 lb/ft)
#8	#25	25.40 mm (1.000 in)	509 mm²	3.982 kg/m (2.670 lb/ft)
#9	#29	28.65 mm (1.128 in)	645 mm²	5.071 kg/m (3.400 lb/ft)
#10	#32	32.26 mm (1.270 in)	819 mm²	6.418 kg/m (4.303 lb/ft)
#11	#36	35.81 mm (1.410 in)	1006 mm²	7.924 kg/m (5.313 lb/ft)
#14	#43	43.00 mm (1.693 in)	1452 mm²	11.41 kg/m (7.650 lb/ft)
#18	#57	57.33 mm (2.257 in)	2581 mm²	20.28 kg/m (13.60 lb/ft)

Metric bar sizes

Common metric reinforcing bar diameters with area and mass
Diameter	Area	Mass
6 mm	28.3 mm²	0.222 kg/m
8 mm	50.3 mm²	0.395 kg/m
10 mm	78.5 mm²	0.617 kg/m
12 mm	113 mm²	0.888 kg/m
16 mm	201 mm²	1.579 kg/m
20 mm	314 mm²	2.467 kg/m
25 mm	491 mm²	3.855 kg/m
32 mm	804 mm²	6.316 kg/m
40 mm	1257 mm²	9.868 kg/m

Handy formulas

Metric mass per metre: kg/m = d² ÷ 162, where d is the diameter in millimetres. A 12 mm bar is 144 ÷ 162 = 0.889 kg/m.
Imperial mass per foot: lb/ft ≈ 2.67 × (bar number ÷ 8)². A #5 bar is 2.67 × 0.3906 = 1.043 lb/ft.
Cross-sectional area: A = π × d² ÷ 4.

Frequently asked questions

Why is there no #12 or #13 bar?

The US series jumps from #11 to #14 and then to #18. Larger sizes were added later for heavy civil work and follow the same eighths-of-an-inch rule.

Is "soft metric" the same as metric rebar?

No. Soft metric simply relabels the imperial bar with its nominal millimetre size, so #13 means the #4 bar. True metric rebar is made to 6, 8, 10, 12 mm and so on.

Should I use nominal or actual diameter?

Design codes use the nominal diameter for area and weight, while a micrometer on the ribs reads a little larger. Use nominal values for quantities and bar spacing checks.

Does the same table apply to GFRP bar?

The geometry is similar but the density is not — glass fibre reinforced polymer is around 2,000 kg/m³, so it weighs roughly a quarter of the steel figure shown here.

Common mistakes to avoid

Reading the bar number as millimetres. A #4 bar is 12.7 mm, not 4 mm.
Using the deformed outer dimension for area instead of the nominal diameter.
Mixing soft metric and true metric designations in one bar schedule.
Estimating steel weight from length alone — always convert through area, or use 0.888 kg/m for a 12 mm bar as a sanity check.

Sources and standards

Bar designations, nominal diameters and nominal mass per metre follow ASTM A615/A615M for the US series; the metric sizes in the table are the same bars under their soft-metric designations. Mass is calculated from the nominal diameter with steel at 7,850 kg/m³, so a delivered bar can differ slightly from the nominal figure.

Rebar size chart

Imperial bar sizes are eighths of an inch: #3 is 3/8 in, #4 is 1/2 in, and #8 is 1 in. Metric sizes are the nominal diameter in millimetres, so #4 is close to 12 mm but not identical — they are separate standards, not conversions.

Rebar size chart with weight per foot and metric equivalents
Imperial bar	Diameter	Metric bar	Weight
#3	3/8 in	10 mm	0.376 lb/ft
#4	1/2 in	12 mm	0.668 lb/ft
#5	5/8 in	16 mm	1.043 lb/ft
#6	3/4 in	20 mm	1.502 lb/ft
#7	7/8 in	22 mm	2.044 lb/ft
#8	1 in	25 mm	2.670 lb/ft
#10	1¼ in	32 mm	4.303 lb/ft

Rebar calculator

For a slab, divide the span by the spacing and add one bar for the edge, then multiply by the number of rows in the other direction. A 6 m × 4 m slab at 200 mm spacing each way needs 21 bars at 6 m and 31 bars at 4 m — 126 m plus laps of about 40 diameters. Multiply metres by the weight per metre from the chart to get the order weight.