Kumar IronUpdated
In this guide
What SWG means
SWG stands for Standard Wire Gauge, a long-used table that matches a gauge number to a wire diameter. It is the one most dealers in North India still quote for GI and mild steel wire. The rule to remember is simple: 8 SWG is thick and stiff, 20 SWG is very fine.
Some sellers quote wire in mm instead, and some use other gauge systems. Always ask for the diameter in mm on the bill, because the gauge number on a roll can be read differently by different people.
SWG to mm chart, with weight per 100 m
| SWG | Diameter (mm) | Weight per 100 m (kg) | Typical use |
|---|---|---|---|
| 8 | 4.064 | about 10.2 | Heavy stays, strainers, thick farm and gate tie wire |
| 9 | 3.658 | about 8.2 | Heavy tie wire, some fencing |
| 10 | 3.251 | about 6.5 | Fencing strands, heavy binding and gate ties |
| 11 | 2.946 | about 5.4 | Fencing, strong ties |
| 12 | 2.642 | about 4.3 | Common fencing wire, heavy GI tie wire |
| 13 | 2.337 | about 3.4 | Light fencing, lacing |
| 14 | 2.032 | about 2.5 | Light fencing, fence ties, general GI tie |
| 15 | 1.829 | about 2.1 | Light ties, lacing wire |
| 16 | 1.626 | about 1.6 | Light GI tie wire, tying lath and light plaster mesh |
| 17 | 1.422 | about 1.2 | Fine ties, light lacing |
| 18 | 1.219 | about 0.92 | Binding wire for TMT bars, usually soft annealed |
| 19 | 1.016 | about 0.64 | Fine ties, light craft and lacing work |
| 20 | 0.914 | about 0.52 | Very fine ties and lacing |
Check the gauge your mesh or plaster job asks for. Plaster and lath wire is often specified as a light GI wire, and the mesh drawing or the mason should state the exact gauge.
How the weight is worked out
A wire is a long cylinder, so its weight per metre is its cross-section area times its length times the density of the metal. For plain steel the density is 7850 kg per cubic metre. The area of a round wire is pi/4 times d squared, with d in mm.
Step by step for 12 SWG, with d = 2.642 mm: area = 0.7854 x 2.642 x 2.642 = 5.48 mm squared. Weight per metre = 5.48 mm squared x 1000 mm x 7850 kg/m cubed, converted to m cubed (divide by 1 000 000 000) = 0.043 kg per metre. For 100 m that is 4.3 kg.
The same working simplifies to a quick formula. Weight per metre is about 0.00617 x d squared kg, with d in mm, because 0.7854 x 7850 x 10 to the power minus 6 is 0.00617. Multiply by 100 for a 100 m roll, which gives weight per 100 m = 0.617 x d squared kg.
Zinc coating on GI wire is very thin compared with the steel, so for everyday purchase it adds only a small amount, typically around 1 percent or less. Ask the supplier if you need the exact coated weight for a large order, because the coating mass depends on the process.
Galvanised or black, and which gauge for what
- GI (galvanised) wire: zinc coated, so it resists rust outdoors and in damp walls. Use it for fencing, plaster support and anywhere the wire stays wet.
- Black annealed binding wire: soft and easy to twist, but it rusts. Use it inside slabs and beams where concrete covers it, which is the normal use for TMT tying.
- Heavier gauges (8 to 12 SWG) are for strength, such as strainers and farm fencing. Finer gauges (16 to 20 SWG) are for ties and lacing, where you need to twist by hand.
- A gauge that is too thin will snap while you tension it. A gauge that is too thick is hard to twist and wastes money.
What to ask when buying
- The gauge in SWG and the diameter in mm, both on the bill.
- Whether the wire is GI (galvanised) or black annealed, and for GI, what zinc coating it has.
- The net weight of the roll or coil, and whether it is the net or gross weight.
- Whether it is soft annealed, if you are tying bars by hand.
- A test or quality certificate if the job is large or for a public or institutional work.
Tell us the job
Tell us what you are tying or fencing, and the length, and we will suggest a gauge. Wire prices change with the mill and the zinc market, so call or WhatsApp the store for the current rate per kg before you order.
Frequently asked questions
- Is a higher SWG number thicker or thinner?
- Thinner. 8 SWG is about 4.06 mm, while 20 SWG is about 0.91 mm.
- How much does 100 m of 12 SWG wire weigh?
- About 4.3 kg, worked out from its diameter of 2.642 mm and the density of steel.
- Which gauge is used for TMT binding?
- Usually 18 SWG, about 1.2 mm, in soft annealed wire. Heavier bars may need thicker wire, so check the binding wire guide.
- Does galvanising change the weight much?
- Very little. The zinc layer is thin, so the weight stays close to the plain steel figure.