PCB trace resistance & current calculator
Estimate the resistance and voltage drop of a PCB trace — or the trace width needed for a target current (IPC-2221 / IPC-2221A).
Mode
Results
Trace resistance (R)
ρ × L ÷ (W × t), ρ(Cu) = 1.724e-8 Ω·m
Voltage drop (V)
I × R
Copper loss (P)
I² × R — heats the board
About the model
Resistance comes from R = ρ·L/A: copper resistivity ρ ≈ 1.724e-8 Ω·m at 20 °C, length in metres, cross-section in m² (width in mil × copper thickness). A 1 oz layer is ≈ 35 µm thick, and 1 mil = 0.0254 mm. The IPC-2221 width formula I = k·ΔT^0.44·A^0.725 uses area in mil² and k = 0.048 for external traces — internal traces dissipate heat about half as well, so k drops to 0.024 and the trace must be wider for the same current. Resistivity rises with temperature (~0.4%/°C), and vias, connectors and solder add resistance the model ignores — treat results as estimates.
Designers also reach for PCB connectors and terminal blocks where the trace meets the outside world.
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Frequently asked questions
How is PCB trace resistance calculated?
Trace resistance comes from R = ρ·L/A, with copper resistivity ρ ≈ 1.724e-8 Ω·m at 20 °C, length in metres and cross-section in m² (width in mil × copper thickness in µm). A 25 mm trace at 12 mil wide on 1 oz copper (35 µm) computes to a few milliohms, from which the tool derives the voltage drop (I·R) and copper loss (I²·R).
How do I find the trace width for a given current?
Switch to Width mode and the tool inverts the IPC-2221 current model I = k·ΔT^0.44·A^0.725 to solve the required copper area, then divides by the foil thickness. External traces use k = 0.048 and internal ones k = 0.024, because an internal trace dissipates heat about half as well and needs roughly twice the area for the same current.
Why does copper weight change the resistance?
A heavier copper layer is thicker, so the cross-sectional area rises and resistance falls. The calculator offers 0.5 oz (17.5 µm), 1 oz (35 µm) and 2 oz (70 µm) finished copper. Resistivity also rises about 0.4% per °C, and vias, connectors and solder add resistance the model ignores — treat the results as estimates.
Copper resistivity rises with temperature and varies with the board stack-up; real traces also add via, connector and solder resistance. ICBOMS provides this tool for reference only.