Wire Gauge (AWG) Calculator
Size a wire from load current, run length and voltage drop, with AWG diameter, mm², resistance and NEC ampacity.
- Recommended size
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- Limited by
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- Voltage drop
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- Drop as a percentage
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- Voltage at the load
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- Circuit resistance
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- Ampacity of that size
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- Size if length didn't matter
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Planning aid only — check ampacity derating and your local code.
AWG reference table
Diameter, area and resistance come from the AWG definition; ampacity follows NEC 310.16 for the material and insulation rating you picked above. The recommended size is highlighted, and the table scrolls inside its own box for the other sizes.
| AWG | Ø mm | Ø in | Area mm² | Area cmil | Ω / 1000 ft | Ω / km | Ampacity | Max breaker |
|---|
A planning aid, not an electrical design. Real installations also depend on ambient temperature and bundling derating, conduit fill, continuous-load factors, terminal ratings and your local code. Have anything you are unsure about checked by a licensed electrician.
How it works
Wire sizes are defined by a formula, not a table: the diameter of gauge n is
d = 0.127 mm × 92^((36 − n) ÷ 39), where 1/0, 2/0, 3/0 and 4/0 count as
n = 0, −1, −2 and −3. Cross-section follows as A = π d² ÷ 4, and the area in
circular mils is simply the diameter in mils squared. DC resistance is
R = ρ L ÷ A with ρ = 1.724 × 10⁻⁸ Ω·m for annealed copper and
2.826 × 10⁻⁸ Ω·m for aluminium at 20 °C, corrected for temperature with
ρ_T = ρ₂₀ (1 + α (T − 20)), α = 0.00393 /°C for copper and 0.00403 /°C for
aluminium. At 75 °C that reproduces the familiar NEC Chapter 9, Table 8 values — 1.93 Ω per
1000 ft for 12 AWG copper, 0.764 Ω for 8 AWG.
Voltage drop uses Vdrop = k × I × ρ_T × L ÷ A with L the one-way run
length, k = 2 for DC and single-phase two-wire circuits (current goes out and comes back) and
k = √3 for three-phase. The calculator walks the standard sizes from smallest to largest and
returns the first one that both carries your current — using the NEC 310.16 ampacity column for
your insulation rating, capped by the 240.4(D) small-conductor limits of 15 A for 14 AWG copper,
20 A for 12 AWG copper and 30 A for 10 AWG copper — and stays inside your voltage-drop budget.
Values are computed from the geometric AWG definition, so they can differ by a percent or two
from stranded-conductor table entries. Everything runs in your browser: no sign-up, no ads and
nothing you type ever leaves the device.
Frequently asked questions
What wire gauge do I need for 20 amps?
For a 20 A circuit, 12 AWG copper is the smallest size allowed: NEC 310.16 rates it at 25 A in the 75 °C column, and 240.4(D) caps 12 AWG copper at a 20 A breaker. Length matters too. Over a 100 ft one-way run at 120 V, 12 AWG drops about 6.4 V (5.4%), so this calculator steps up to 8 AWG to stay inside a 3% voltage-drop budget.
How is AWG wire diameter calculated?
AWG is a formula, not a lookup table. The diameter of gauge n is d = 0.127 mm × 92^((36 − n) ÷ 39), where 1/0, 2/0, 3/0 and 4/0 count as n = 0, −1, −2 and −3. Cross-section is A = π d² ÷ 4, and the area in circular mils is the diameter in mils squared, so 12 AWG works out to 2.053 mm, 3.309 mm² and 6530 cmil.
How do you calculate voltage drop on a wire run?
Use V_drop = k × I × ρ × L ÷ A, with L the one-way run length, k = 2 for DC and single-phase two-wire circuits and k = √3 for three-phase. ρ is 1.724 × 10⁻⁸ Ω·m for copper and 2.826 × 10⁻⁸ Ω·m for aluminium at 20 °C, corrected with ρ_T = ρ₂₀ (1 + α (T − 20)). At 75 °C that reproduces the NEC Chapter 9 Table 8 figures, such as 1.93 Ω per 1000 ft for 12 AWG copper.