Skip to main content

Free calculator

Use this calculator to

  • Convert any AWG gauge from 0 to 40 into diameter in mm and inches
  • Get the cross-sectional area in mm² to compare against UK metric cable sizes
  • Estimate copper resistance per metre for voltage-drop checks
  • Translate US equipment datasheets into BS 7671 friendly units

Wire Gauge Calculator

Convert AWG wire gauge to diameter (mm/inches), area (mm²), and resistance per metre.

AWG
Result
—

Formulas

  • d(mm) = 0.127 × 92^((36−AWG)/39)
  • A(mm²) = π × (d/2)²
  • R/m (Ω/m) = ρ_Cu / A = 1.72×10⁻⁸ / A

Related Calculators

mm²

Cable Ampacity

Current carrying capacity by CSA, material, install method and grouping

Open calculator
V↓

Voltage Drop

Calculate voltage drop over wire length and load

Open calculator
MCB

Circuit Breaker Sizing

MCB rating from load watts, voltage, PF and safety factor

Open calculator

Common scenarios

Select one to run it in the calculator above.

For business

Why this matters for businesses

Cable sizing decides whether an installation is compliant, safe and economic over its 25-year life. Picking the conductor cross-section from the current draw, the run length and the acceptable voltage drop is what BS 7671 asks the design engineer to do. Undersize and the cable runs hot, the insulation degrades, and the install fails the next periodic inspection. Oversize and the capital cost of copper is higher than it needs to be, eating into the payback on the project that triggered the new circuit in the first place.

The calculation matters most in long cable runs: warehouses with 200-metre runs to remote loading bays, EV charging installations spread across a depot, factory expansions that feed new lines from an existing substation across the site. In all three cases, the voltage drop limit (3% on lighting, 5% on power under BS 7671) is the constraint that drives the conductor size, not the current-carrying capacity. A 32A circuit run 150 metres at 400V three-phase will need 16mm squared, not the 6mm squared the current rating alone would suggest.

Purely Energy works with mid-market clients adding capacity across multi-site portfolios. We connect facilities and operations teams with the design engineers and contractors who sign off the install against BS 7671, and we make sure the supply contract reflects the new load profile once the cabling work is complete. That coordination across procurement, capacity and on-site design is what we mean when we say we manage energy as a strategic cost, not a commodity purchase.

Common questions

What is AWG and how does the numbering work?

American Wire Gauge is the US standard for conductor sizes, and it runs backwards: the larger the number, the thinner the wire. Each step changes the diameter by a constant ratio, captured in the formula d = 0.127 × 92^((36 - AWG) / 39) millimetres. Going down three gauge numbers roughly doubles the cross-sectional area, and going down six doubles the diameter, which is handy for quick mental checks.

How do I convert AWG to mm²?

Calculate the diameter from the gauge number, then the area: A = pi × (d / 2)². Useful anchors: 14 AWG is about 2.08 mm², 12 AWG about 3.31 mm², 10 AWG about 5.26 mm² and 6 AWG about 13.3 mm². These rarely match UK metric sizes exactly, so you normally map to the next metric size up, for instance 12 AWG to 4 mm².

Why does the UK use mm² instead of AWG?

British and European cable standards, including the tables in BS 7671, define conductors directly by cross-sectional area in mm², which feeds straight into current-carrying capacity and voltage-drop calculations. AWG appears in UK work mainly on imported equipment, US datasheets and electronics. Convert to mm² first, then check the metric size against the regulations: final cable selection is a job for a qualified electrician working to BS 7671.

How is the resistance per metre calculated?

From the conductor's resistivity and area: R per metre = rho / A, using 1.72 × 10^-8 ohm metres for copper at 20°C. A 12 AWG conductor at 3.31 mm² works out near 5.2 milliohms per metre. Resistance rises with temperature by roughly 0.4 percent per degree for copper, which matters for voltage-drop checks on long, heavily loaded runs.

Can I choose a cable size from the AWG number alone?

No. The gauge fixes the conductor geometry, but safe current depends on insulation type, installation method, grouping, ambient temperature and the length of the run. Two cables with identical conductors can have very different ratings once one is buried in insulation. Use the cable ampacity calculator for capacity and the voltage drop calculator for long runs, and have a qualified electrician confirm the final selection to BS 7671.

Related calculators

The conversions that procurement and engineering use alongside this one.

Wire Gauge Calculator (BS 7671) | Purely Energy