PCB Trace Resistance and Voltage Drop Calculator
Long power traces drop voltage and waste power as heat. This calculator gives DC resistance, voltage drop and loss for a copper trace so you can decide on width, weight or a plane before the layout is frozen.
Your numbers
Your results
Uses nominal copper thickness and DC resistance. Etch tolerance, plating variation and skin effect at high frequency change real resistance.
Trace resistance vs width
Sweeps Trace width from 0.1 to 3, holding your other inputs fixed.
Connect engineering data to your ERP
Optional: discuss carrying released design data into item and BOM records.
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How to use it
Enter the length and width of the trace and choose the copper weight. Set the expected copper temperature, since resistance rises with heat, and the load current. The tool returns resistance at 20 C and at temperature, voltage drop and power loss. Use the chart to see how much resistance falls as width increases.
The formula behind it
Resistance equals resistivity times length divided by cross-sectional area. Copper resistivity is about 1.724e-8 ohm meters at 20 C, which is 1.724e-5 ohm mm. Thickness is copper ounces times 0.035 mm. Temperature correction multiplies by one plus 0.00393 times the difference from 20 C.
- R equals rho x L / (w x t)
- Voltage drop is current times resistance
- Loss is current squared times resistance
Reading the result
A 100 mm long, 0.5 mm wide 1 oz trace measures about 0.1 ohm at 20 C. At 1 A that drops roughly 0.1 V, which matters on a 3.3 V rail and almost nothing on a 24 V rail. Heat raises resistance, so at 85 C the same trace is about 25% higher.
Common mistakes
Ignoring the return path doubles the real drop, since ground current also flows through copper. Another mistake is counting only trace length and forgetting vias, connector contacts and solder joints, each of which adds milliohms. Add those from datasheets for a full budget.
Frequently Asked Questions
What is the resistance per square of 1 oz copper?
About 0.5 milliohm per square at 20 C. Resistance equals this value times length divided by width, which is a quick mental check for any trace.
Does this include skin effect?
No. It is a DC calculation. At high frequency skin depth and roughness raise the effective resistance, which needs a field solver.
How does temperature affect copper?
Resistance rises roughly 0.39% per degree C. At 100 C, copper has about 31% more resistance than at 20 C.
Should I use the final plated copper thickness?
Outer layers gain plating, so finished copper is thicker than the base foil. Use the fabricator's finished thickness for accuracy and the nominal value for a safe estimate.
Netray can help tie design calculations and engineering change approvals into your ERP so released PCB revisions are traceable.
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