PCB Trace Width & Current Calculator

Find the trace width a given current needs, or the current a given trace can carry, with resistance and voltage drop.

How it works

The calculator uses the IPC-2221 relationship between current, temperature rise and the cross-section area of the copper:

Current: I = k × ΔT0.44 × A0.725
Area (width × thickness) in mil²: A = [ I ÷ ( k × ΔT0.44 ) ]1/0.725
Resistance: R = ρ × L ÷ ( w × t )

I is the current in amps, ΔT the temperature rise in °C, and A the cross-section in square mils. The constant k is 0.048 for outer layers (exposed to air) and 0.024 for inner layers. One ounce of copper is 1.378 mil (35 µm) thick, so the width follows from the area divided by the thickness. Resistivity of copper is 1.72 × 10−8 Ω·m at 20 °C and rises 0.393% per °C; the calculator uses the operating temperature (ambient plus allowed rise).

Worked example

A 3 A supply line on the outside of a 1 oz board, with a 10 °C rise allowed: the area is [3 ÷ (0.048 × 100.44)]1.379 = 74.2 mil². Divided by 1.378 mil of copper, the width is 53.8 mil, or 1.37 mm. At 25 °C ambient plus 10 °C rise the copper is at 35 °C, and 100 mm of that trace has a resistance of about 38.1 mΩ, which drops about 114 mV at 3 A and wastes about 343 mW.

Things to know

It is a thermal limit, not a voltage-drop limit. The width found here keeps the temperature rise within what you allowed. For long traces or low-voltage rails you may need a wider trace just to keep the voltage drop small; the resistance and drop shown help with that.

IPC-2221 is conservative for outer layers. It comes from charts drawn in the 1950s. The newer IPC-2152 standard is based on more testing and accounts for the board’s thickness, nearby copper planes and airflow. A copper plane under or next to a trace carries heat away and allows a higher current than shown here.

Inner layers run hotter. The heat has nowhere to go but through the board, so the same current needs a wider trace than on the outside.

Copper weight matters, but not one-to-one. Doubling the copper thickness at the same width lets the trace carry about 65% more current, not double.

Short bursts and fuses. This is for steady current. Very short pulses can be much higher, and a trace can also act as a fuse, which needs a different calculation.

Vias and connections. A trace is only as good as its weakest spot. Use several vias in parallel for high current and check the pad or connector width too.

Safety margin. For anything where heat could cause harm, leave extra margin and test the board under real load.

Common questions

What temperature rise should I choose?

A 10 °C rise is a common, conservative choice for general electronics. Many designs accept 20 to 30 °C for power traces where the board and nearby parts can tolerate it. Remember the rise adds to the ambient temperature inside the enclosure, not room temperature.

Why does my result differ from another trace width calculator?

Calculators differ in the standard (IPC-2221 or IPC-2152), in how thick they assume one ounce of copper is, and in whether they round. This tool uses IPC-2221 with 1.378 mil per ounce.

Does the trace length change the width?

No. The width comes from the current and temperature rise only. The length is used to find resistance, voltage drop and power loss.

What about high-frequency signals?

This calculator is for DC and low-frequency current. At high frequency, skin effect and impedance control decide the geometry instead.

Sources

  • IPC-2221B, Generic Standard on Printed Board Design, section 6.2 (conductor current capacity).
  • IPC-2152, Standard for Determining Current-Carrying Capacity in Printed Board Design.

Last reviewed: November 2, 2026