AWG to mm² Converter: Chart, Formula and Worked Examples

This guide gives the formula, a chart from 30 AWG to 4/0 AWG, three worked examples and the rules for using the result in a real project. To skip the arithmetic and size cable with derating factors, open the cable sizing workflow in GorkyCAD.

What AWG actually measures

AWG (American Wire Gauge) is a logarithmic scale that describes diameter, not area. The scale runs backwards: a larger AWG number means a thinner wire. Each step of three gauge numbers roughly halves or doubles the cross-sectional area, and each step of ten changes it by about a factor of ten — the basis of the “10-gauge rule”.

The reference points are fixed by definition:

• 36 AWG = 0.0050 in = 0.127 mm diameter
• 0 AWG (1/0) = 0.4600 in = 11.684 mm diameter
• The ratio between consecutive sizes is 92^(1/39) ≈ 1.1229

The scale is geometric, so either direction is computable — a table is simply faster on site.

The AWG to mm² formula

Diameter in millimetres:

d (mm) = 0.127 × 92^((36 − n) / 39)

Cross-sectional area in square millimetres:

A (mm²) = 0.012668 × 92^((36 − n) / 19.5)

where n is the AWG number. For sizes larger than 1/0, use negative values: 1/0 → n = 0, 2/0 → n = −1, 3/0 → n = −2, 4/0 → n = −3.

Going the other way, from mm² to AWG:

n = 36 − 19.5 × log₉₂(A / 0.012668)

The constant 0.012668 is just π/4 × 0.127². It converts the squared diameter into area, so the formula is nothing more than A = πd²/4 with the AWG diameter substituted.

Conversion chart: 30 AWG to 4/0 AWG

Values below are nominal, calculated with the formula above and rounded to three decimals for diameter and area; odd intermediate gauges are omitted for readability. "kcmil" is thousand circular mils (1 kcmil = 0.5067 mm², so kcmil ≈ mm² × 1.9735), included because North American ampacity tables are often indexed by it.

Two sanity checks: 4/0 must be 0.4600 in = 11.684 mm, which the table matches, and 10 AWG must be 0.1019 in ≈ 2.588 mm. If your spreadsheet disagrees in the third decimal, check whether it uses 92^(1/39) or the older 1.1229 approximation.

Worked examples

Example 1 — 12 AWG to mm². n = 12, so exponent = (36 − 12)/19.5 = 1.2308. 92^1.2308 = 261.2, times 0.012668 = 3.31 mm². A European 2.5 mm² conductor is 24% smaller in area; 4 mm² is 21% larger. Neither is a drop-in electrical equivalent without checking ampacity.

Example 2 — 4/0 AWG to mm². n = −3, exponent = 39/19.5 = 2. 92² = 8464, times 0.012668 = 107.2 mm². The nearest metric sizes are 95 mm² (−11%) and 120 mm² (+12%).

Example 3 — 18 AWG to mm². n = 18, exponent = 18/19.5 = 0.9231. 92^0.9231 = 65.0, times 0.012668 = 0.823 mm². This sits between 0.75 mm² and 1.0 mm², which is why control-panel wiring often crosses between the two systems without an exact match.

Stranded vs solid: same mm², different cable

The formula describes the total metallic area, so a stranded 12 AWG and a solid 12 AWG have the same nominal 3.31 mm². What differs is everything around it:

• Overall diameter. Stranded conductors need voids between strands, so the finished cable is slightly larger for the same area.
• Flexibility and bend radius. Class B stranding is stiff; Class K and D are fine-stranded for moving parts and panel doors.
• Terminal compatibility. Fine-stranded cable generally needs ferrules or terminals rated for fine stranding.
• Metric "class" equivalence. A 2.5 mm² Class 2 conductor and a 14 AWG solid are close in area (2.08 vs 2.5 mm²) but not interchangeable in a terminal torque table.

When a drawing mixes systems, put both the AWG and the mm² on the cable schedule, and let the specification state which one governs. GorkyCAD's documentation and export options support schedules that carry both columns.

Size is not ampacity — the warning that matters

This is the part people get wrong. Two conductors with the same cross-section can have completely different current ratings depending on insulation, ambient temperature, number of loaded conductors, installation method, and the standard you are working to.

NEC Table 310.16 (copper, 30 °C ambient, not more than three current-carrying conductors in a raceway or cable) gives, for example:

Under IEC 60364-5-52 the same 2.5 mm² copper with PVC insulation is commonly tabulated around 20 A for two loaded conductors in conduit (method B2) and higher when clipped direct. Always size from the ampacity table of the code edition you are actually working to, then apply correction factors for ambient and grouping.

Code editions change. Verify the current edition of NFPA 70 (NEC) or IEC 60364 and any local amendments before you freeze a cable size. This article is technical guidance, not a compliance certificate.

How to use the conversion in practice

1. Convert, then round up, not down. If a US spec calls for 8 AWG (8.37 mm²), do not substitute 6 mm² without rechecking ampacity and voltage drop.
2. Check the terminal range. A 107 mm² equivalent may not fit a lug listed for 95 mm².
3. Keep voltage drop in the loop. A 30 m run at 16 A on 1.5 mm² behaves very differently from the same run on 4 mm².
4. Document the basis. State "8 AWG (8.37 mm²), 75 °C terminals" so the next engineer does not have to guess, and re-verify device coordination after any substitution.

For a full workflow — plan, circuits, cable lengths, derating, then BOM — see house wiring plan software and the electrical takeoff guide. More guides: GorkyCAD blog.

Ready to stop hand-converting? Open the free GorkyCAD demo, draw one circuit, and let the cable schedule carry both units.

Frequently asked questions

Sources

• NFPA 70, National Electrical Code — Table 310.16 (ampacities) and Article 240 (overcurrent protection). Free access at nfpa.org/freeaccess.
• IEC 60364-5-52, Electrical installations of buildings — Selection and erection of wiring systems (current-carrying capacity).
• ASTM B258, Standard Specification for Nominal Diameters and Cross-Sectional Areas of AWG Sizes of Solid Round Wires.
• BIPM, The International System of Units (SI Brochure), 9th edition — for the mm² unit definition.
• NIST Special Publication 811, Guide for the Use of the International System of Units.

Next step: check current pricing or read the full documentation before you build your next cable schedule.