PCB Trace Resistance Calculator
Enter your trace dimensions, material, and current to instantly calculate DC resistance, cross-section area, voltage drop, and power dissipation. Temperature correction follows the IPC-2221B formula. Supports copper, silver, gold, and aluminum, with standard copper weight presets (0.5 oz through 3 oz). Reverse-solve: leave one geometric dimension blank and the calculator fills it in.
What is PCB trace resistance and why does it matter?
Every copper trace on a printed circuit board has a small but real resistance. For signal traces at low frequencies this resistance is usually negligible, but for power distribution lines, high-current motor drivers, battery management circuits, and precision analog designs, even a few milliohms can cause a meaningful voltage drop, unexpected heating, or inaccurate current sensing. Trace resistance is governed by the same physics as any conductor: longer traces have more resistance, wider and thicker ones have less, and resistance rises with temperature. Understanding and calculating trace resistance before layout lets you size traces correctly and avoid board-level failures.
The PCB trace resistance formula
The DC resistance of a rectangular PCB trace is given by the formula from IPC-2221B Appendix A: R = (rho x L) / (T x W) x (1 + alpha x (T_amb - 25)), where rho is the resistivity of the conductor material in Ohm-meters, L is trace length in meters, T is trace thickness in meters, W is trace width in meters, alpha is the temperature coefficient of resistance in inverse degrees Celsius, and T_amb is the ambient temperature in degrees Celsius. For copper at 25 degC, rho is 1.724 x 10^-8 Ohm-m and alpha is 0.00393 /degC. The temperature correction term matters for operating temperatures far from 25 degC: at 85 degC the resistance of a copper trace is about 23.6 percent higher than at room temperature.
Voltage drop and power dissipation
Once you have the trace resistance, Ohm's Law gives the voltage lost across it: V_drop = I x R. A 10 mOhm trace carrying 2 A drops 20 mV, which may or may not matter depending on the supply rail. Power dissipation follows P = I^2 x R. That same 10 mOhm trace at 2 A dissipates 40 mW as heat in the copper. At higher currents the heating can become significant: at 5 A the loss jumps to 250 mW, which will raise the temperature of the trace and the nearby board material. The current-squared dependence means doubling current quadruples the heat, so power traces deserve careful resistance budgeting during layout.
How to reduce trace resistance in your PCB design
Four levers control trace resistance. First, increase trace width: doubling the width halves resistance. Second, use heavier copper: moving from 1 oz/ft2 (35 um) to 2 oz/ft2 (70 um) also halves resistance. Third, shorten the route: trace length is linearly proportional to resistance, so a more direct routing path directly reduces it. Fourth, add copper pours or vias in parallel: a wide copper pour or a cluster of vias sharing a current path acts like resistors in parallel, reducing the effective resistance significantly. For very low-resistance requirements, multiple parallel traces or thick bus-bar connections are used in industrial power electronics.
Material choices beyond copper
Copper is by far the most common conductor for PCB traces because of its excellent balance of resistivity, solderability, and cost. Silver has slightly lower resistivity (1.59 x 10^-8 Ohm-m vs. 1.72 x 10^-8 for copper) but is used mainly for screen-printed flex circuits and specialized RF applications. Gold is used for plating contact surfaces and bondpads, not for long current-carrying traces, because of its higher resistivity (2.44 x 10^-8 Ohm-m) and cost. Aluminum is used in some power semiconductor packages and was historically used in ICs, but almost never appears as a PCB conductor.
Standard PCB copper weights
| Copper weight | Thickness (um) | Thickness (mil) | Typical application |
|---|---|---|---|
| 0.5 oz/ft2 | 17.5 | 0.69 | Fine-pitch signal traces, HDI boards |
| 1 oz/ft2 | 35 | 1.38 | General-purpose signal and power traces |
| 2 oz/ft2 | 70 | 2.76 | Higher-current power distribution |
| 3 oz/ft2 | 105 | 4.13 | Heavy-current bus bars, thermal management |
IPC-2221 standard copper plating thicknesses and their typical use cases.
Frequently asked questions
What is the resistivity of copper used in PCB traces?
The accepted value for annealed copper at 25 degC is 1.724 x 10^-8 Ohm-m (often cited as 1.68 to 1.72 x 10^-8 in different references, with small variations depending on purity and processing). The temperature coefficient of resistance for copper is approximately 0.00393 per degC, meaning resistance increases about 0.393 percent for every 1 degC rise above the 25 degC reference point.
What does 1 oz/ft2 mean for PCB copper weight?
Copper weight is a historical way of specifying foil thickness: one ounce per square foot of copper spread evenly over a one-square-foot area gives a thickness of approximately 35 micrometers (1.38 mils). Standard options are 0.5 oz/ft2 (17.5 um), 1 oz/ft2 (35 um), 2 oz/ft2 (70 um), and 3 oz/ft2 (105 um). Most double-sided and multilayer boards use 1 oz/ft2 for signal layers. Power layers often use 2 oz or even 3 oz copper for lower resistance and better current capacity.
How does temperature affect PCB trace resistance?
Resistance increases linearly with temperature for metals. For copper the increase is about 0.393 percent per degC above the 25 degC reference. At 85 degC a copper trace has about 23.6 percent more resistance than at 25 degC. This matters for designs that operate over a wide temperature range, such as automotive electronics (-40 to +125 degC), where resistance variation can be more than 60 percent from cold start to full operating temperature.
What is sheet resistance and how is it related to trace resistance?
Sheet resistance (also called surface resistivity or rho per square) is the resistivity of the material divided by the thickness of the layer: Rs = rho / T. It is expressed in mOhm per square (sometimes written as Ohm/sq). Once you know Rs, the resistance of any trace is simply Rs x (L / W), where L / W is the number of squares in the trace. This makes sheet resistance a useful rule-of-thumb for quickly comparing trace resistance across different copper weights without converting every time.
Why does my simulated resistance differ slightly from the measured value?
Several factors cause small discrepancies. Actual copper purity and grain structure affect resistivity slightly. Etching tolerances mean the as-built width and thickness differ from the nominal design values, typically a few micrometers narrower due to undercut. Via resistance, connector contact resistance, and solder joint resistance add series resistance not captured by the trace formula. For precision current sensing, a calibration measurement on the actual board is the most reliable approach.
How do I calculate voltage drop across a PCB trace?
Voltage drop is V = I x R, where I is the current in amperes and R is the trace resistance in ohms. Using milliohms: V (mV) = I (A) x R (mOhm). For example, a 20 mOhm trace carrying 3 A drops 60 mV. Whether this is acceptable depends on the supply rail: a 60 mV drop on a 12 V rail is 0.5 percent, likely fine, but on a 1.0 V core rail the same drop is 6 percent, which is almost certainly a problem.