Skip to main content

PCB trace width calculator

An inner layer runs twice as hot

IPC-2221 gives the cross-section of a trace from the current and the permissible temperature rise. The most important practical detail in it is the difference between layers: a trace inside the board is sandwiched between layers of glass-epoxy with nowhere to shed its heat, and it needs roughly twice the area.

Wtouter layer1 A
An inner layer runs twice as hot: the heat has nowhere to go
Current (A)
Permissible temperature rise
Copper thickness
Trace length (mm)
Outer layer
Required width
Cross-sectional area
Inner layer
Required width
Cross-sectional area
Resistance and voltage drop
Trace resistance
Voltage drop
Dissipated power

📐 The formulas​

IPC-2221:

A[mil²] = ( I / ( k · ΔT^0.44 ) )^(1/0.725)

k = 0.048 — outer layer
k = 0.024 — inner layer

W[mil] = A / thickness[mil]
1 oz of copper = 1.378 mil = 35 µm

Resistance and voltage drop:

R = ρ · L / (W · t)          ρ(Cu) = 1.724·10⁻⁸ Ω·m
U = I·R          P = I²·R

Check points (1 oz copper):​

CurrentΔTLayerWidth
1 A10 °Couter0.30 mm
1 A10 °Cinner0.78 mm
5 A20 °Couter1.82 mm
Where this calculator stops:
  • IPC-2221 is a conservative standard built on mid-twentieth-century data: real boards take more
  • Above 20 A the formula leaves the range of its data — there you need busbars, not traces
  • Neighbouring traces heating each other, and the board's overall thermal situation, are not accounted for
  • For RF traces the width is set by characteristic impedance rather than current — a different problem

🎯 In practice​

  • Laying out power circuits on an amplifier or power supply board.
  • Checking someone else's board: will this trace take the current claimed for it.
  • Working out the voltage drop along long supply traces.
Practical advice:
  1. ΔT = 20 °C is the usual working compromise; 10 °C for critical circuits, 40 °C the limit for FR-4.
  2. Two-ounce copper costs little and halves the width you need.
  3. For heavy currents it is worth leaving the trace free of solder mask and tinning it — the cross-section grows.
  4. Voltage drop often bites harder than heating: check it separately.

© 2026 UR3PKI · CyberDev.Space · Content licensed under CC BY-NC-SA 4.0.

How to cite this calculator
UR3PKI. «PCB trace width calculator». CyberDev.Space. https://cyberdev.space/en/radio/calculators/others/trace_width (licence CC BY-NC-SA 4.0).

The licence lets you use this material freely, including in teaching materials, but only with attribution to the author and a link to the source.

Last updated on