Rebar sizes chart: bar numbers, diameters and weights

The chart

Reading a bar number

For #3 through #8, the bar number is the nominal diameter in eighths of an inch: #3 is ⅜ in, #4 is 4/8 = ½ in, #8 is 8/8 = 1 in. From #9 up the pattern breaks — the bars are sized so their areas come out to round numbers (#9 is 1.00 sq in) — so a #9 is 1.128 in across rather than 1⅛.

The metric names are soft conversions of the same bars, named for their diameter in millimetres: #10 is ⅜ in bar, #13 is ½ in bar, #16 is ⅝ in. A drawing that calls for #13 bars is asking for what a US yard sells as #4.

Nominal diameter is measured without the ribs (deformations) that help the bar grip the concrete. Calipers across the ribs will read a bit more.

Weight is about area, not diameter

Weight goes with the square of the diameter. That is why stepping up one size adds so much steel: #4 to #5 is only ⅛ in larger but 56% heavier (0.668 to 1.043 lb/ft), and #5 to #6 adds another 44%. The weights are worked through on how much does #4 rebar weigh per foot, how much does #5 rebar weigh per foot and how much does #3 rebar weigh per foot.

Grade and stock lengths

Bars are also marked with a grade — the minimum yield strength in thousands of psi, such as Grade 40 or Grade 60 — rolled into the bar along with the mill’s mark and the size. Your plans specify the grade along with size and spacing. Steel suppliers list stock lengths such as 20, 30, 40 and 60 ft; home stores often carry 10 and 20 ft sticks of #3 to #5.

What the chart can’t tell you

Which size, how far apart, how far from the edges, how long the laps are and whether the bars need hooks are all structural design decisions. They come from your plans, an engineer or your local building code, not from a chart. Once you have them, the rebar calculator counts bars for a slab grid — see how much rebar for a 20×20 slab and how much rebar for a 30×40 slab — and the wire mesh calculator handles welded mesh, as in how much wire mesh for a slab.

Handling and cutting

• Carry long sticks near their middle and watch the ends — 20 ft of #4 whips when you turn.
• A manual cutter-bender handles #3 to #5; a grinder with a cut-off wheel or a portable band saw cuts anything. Wear eye protection; grinding throws sparks.
• Cap exposed bar ends on site. Rebar caps are cheap and stop impalement injuries.
• Keep bars off the ground until they are placed. Mud and loose rust weaken the bond; light, tight surface rust is normal.