PCB Trace Resistance Calculator

PCB Trace Resistance Formula

Trace resistance depends on the resistivity of the conductor material, its geometry, and operating temperature. The resistivity of copper increases with heat, so ambient conditions directly affect your final resistance value.

• R — Resistance in ohms (Ω)
• ρ — Resistivity of the trace material at 25°C (Ω·m)
• L — Length of the trace in meters
• T — Trace thickness (height) in meters
• W — Trace width in meters
• α — Resistivity temperature coefficient (per °C)—how much resistivity changes per degree
• T_ambient — Ambient temperature in degrees Celsius

Understanding PCB Trace Geometry

Trace resistance scales inversely with cross-sectional area. A thicker or wider trace carries current with less opposition; a longer trace accumulates more resistance. For example, doubling trace width halves the resistance, while doubling length doubles it. This relationship is why high-current designs demand wider traces and shorter runs.

Copper is standard for PCBs, but some designs use gold plating (lower conductivity than copper) or aluminum in specialized applications. Each material has its own resistivity value and temperature coefficient. Always verify your material's properties from the PCB manufacturer or material datasheet before relying on calculated values for thermal or signal-integrity analysis.

Temperature Effects on Resistance

Copper's resistivity increases approximately 0.39% per degree Celsius above 25°C. This temperature coefficient (α) becomes critical in high-power circuits where traces dissipate significant heat. A trace carrying 10 A in a 60°C enclosure may have 15–20% higher resistance than the same trace at room temperature.

The calculator accounts for this automatically. If your PCB operates in a warm environment or near power components, always enter the realistic ambient temperature. Neglecting temperature compensation can lead to underestimated voltage drop and unexpected thermal runaway in marginal designs.

Voltage Drop and Current Calculation

Once you have trace resistance, Ohm's Law directly gives you voltage drop: V = I × R. The calculator reverses this relationship too—enter current and get voltage drop, or provide voltage drop and current will appear. This bidirectional feature speeds up verification workflows.

For instance, a 0.5 m copper trace with 0.5 mm width and 0.035 mm thickness carrying 5 A might drop 0.2 V at room temperature. The same trace at 50°C could drop 0.23 V due to temperature-induced resistivity increase. Always check whether your power supply, signal integrity budget, or thermal limits tolerate these drops before committing to layout.

Common Pitfalls and Design Tips

Avoid these frequent mistakes when calculating or designing PCB traces.

1. Forgetting temperature derating — Many designers use room-temperature resistivity without accounting for operating conditions. Interior electronics heat up; outdoor or automotive designs face worse extremes. Always include at least a 20–30°C margin above ambient in your calculations.
2. Confusing trace thickness with copper weight — PCB manufacturers specify copper weight in ounces per square foot (typically 0.5, 1, 2, or 4 oz), not thickness in millimeters. A 1 oz copper layer is approximately 0.035 mm thick. Check your stackup drawing to confirm the actual thickness before calculating.
3. Ignoring current-carrying capacity limits — Even if your trace geometry yields acceptable voltage drop, high current density (amperage per square millimeter) generates heat and can fail solder joints. IPC-2221 and similar standards define maximum current limits by trace width and allowed temperature rise.
4. Neglecting via resistance and connections — The trace itself is only part of the resistance path. Vias, solder joints, connector contacts, and layer transitions add non-trivial resistance. For high-current paths, sum these in series to get the true system resistance.