18 Gauge Wire in mm: Diameter, mm2 & Cable Size
18 gauge wire in mm is 1.02 mm across, with a cross-section of 0.823 mm2. It sits closer to a metric size than any other gauge, and that changes the choice.
18 gauge wire measures 1.02 mm across. Its cross-section is 0.823 mm², or 1,620 circular mils.
This gauge sits closer to a metric size than any other in the range, and that changes which way you should round.
The closest call on the site
Every AWG size lands between two metric sizes. Usually the one below is a long way down and the round-up is obvious. Not here:
| Option | Cross-section | Against 18 AWG |
|---|---|---|
| 0.75 mm² | 0.75 mm² | 9% less |
| 18 AWG | 0.823 mm² | — |
| 1 mm² | 1 mm² | 22% more |
A 9% step down is the smallest gap between an AWG conductor and the metric size beneath it anywhere in the range. Compare 14 AWG, where the step down is 28%, or 8 AWG at 28%. That matters because the resistance test comes out the way you would not expect.
18 AWG passes as 0.75 mm²
A 18 AWG conductor runs about 20.95 Ω/km.
| Metric size | Limit | Result |
|---|---|---|
| 1 mm² | 18.1 Ω/km | fails by 16% |
| 0.75 mm² | 24.5 Ω/km | passes |
So the usual advice — round up and you are safe — buys you nothing extra here. 18 AWG already satisfies 0.75 mm², and rounding up to 1 mm² is optional margin rather than a requirement.
If you are replacing an 18 AWG design with metric cable, 0.75 mm² is a legitimate match. It is 9% less copper, which at this size and these currents is rarely consequential, and the conductor resistance requirement is met.
If the specification is written and strict, 1 mm² removes any argument. But this is one of the few gauges where the smaller metric size is defensible on the number that gets tested.
Ampacity is barely tabulated at this size
18 AWG copper appears in the ampacity table with a rating in the 90 °C column only — 14 A. The 60 °C and 75 °C columns are blank, because the gauge is not intended for general power wiring.
On the metric side, the current-carrying capacity tables start at 1.5 mm². Neither 0.75 nor 1 mm² is listed, for the same reason.
That absence is the answer to "how many amps does 18 gauge carry." It is a control and signal conductor, and the tables that govern power circuits do not cover it. Where it does carry current, the rating comes from the equipment standard or the flexible cord rules, not from the general ampacity tables.
Conductor diameter and terminations
1.02 mm is solid wire. At 1 mm², the maximums by construction:
| Construction | Max diameter | Min strands | Max strand dia. |
|---|---|---|---|
| Class 1, solid | 1.2 mm | 1 | — |
| Class 2, stranded | 1.4 mm | 7 | — |
| Class 5, flexible | 1.5 mm | — | 0.21 mm |
| Class 6, very flexible | 1.5 mm | — | 0.16 mm |
At 0.75 mm² the figures are 1.0, 1.2 and 1.3 mm.
Class 5 and class 6 share the diameter and the resistance limit, and differ only in strand fineness — 0.21 mm against 0.16 mm. At this size that is the whole difference between a conductor that survives a cabinet door loom and one that eventually breaks a strand. See conductor classes 1, 2, 5 and 6.
Where 18 AWG is used
Control panel wiring, relay and contactor coils, thermostat cable, doorbell and low-voltage lighting, appliance internal wiring, and speaker cable.
It is the size where mechanical robustness starts to matter more than current. 18 AWG survives being pulled through a panel and terminated repeatedly in a way that 22 AWG does not.
FAQ
How many mm is 18 gauge wire?
1.02 mm in diameter for solid wire, which is 40.3 mils. Stranded 18 AWG measures wider, up to about 1.5 mm at the equivalent metric size.
What is 18 AWG in mm²?
0.823 mm². The metric sizes either side are 0.75 mm² and 1 mm².
Should I use 0.75 mm² or 1 mm² for 18 AWG?
0.75 mm² is 9% less copper and still meets the conductor resistance requirement, which makes it a defensible match. 1 mm² gives 22% more and removes any argument against a strict specification. This is one of the few gauges where the smaller size is genuinely usable.
Is 18 AWG the same as 1 mm²?
No. 1 mm² is 22% larger, and an 18 AWG conductor misses the 1 mm² resistance limit by 16%.
How many amps can 18 gauge wire carry?
It is rated 14 A in the 90 °C column and is not listed at 60 or 75 °C, because it is not a general power conductor. Where 18 AWG carries current, the rating usually comes from the equipment standard or the flexible cord rules rather than the general ampacity tables.
Is 18 gauge thicker than 20 gauge?
Yes. Lower AWG numbers are thicker. 18 AWG is 1.02 mm against 0.813 mm, and it has 59% more copper. See how the AWG system works.
Reference
| Property | 18 AWG |
|---|---|
| Diameter, solid | 1.02 mm |
| Diameter, solid | 40.3 mils / 0.0403 in |
| Cross-sectional area | 0.823 mm² |
| Area | 1,620 cmil |
| Resistance, solid copper at 20 °C | 20.95 Ω/km |
| Metric size above | 1 mm² (+22%) |
| Metric size below | 0.75 mm² (−9%, and 18 AWG meets its limit) |
Convert any other gauge with the AWG to mm² converter, or read why AWG and metric sizes never line up.
Zakaria El Intissar
Electrical Engineer
Electrical engineer with 12+ years of experience in power systems. I create practical electrical calculators and standards-based technical guides that engineers can apply immediately in their daily work.
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