This calculator determines a PCB’s copper trace resistance from trace length, width, thickness, and operating temperature, which helps to predict voltage drop and heat dissipation.
Calculate PCB Trace Resistance From Copper Trace Dimensions
This tool calculates the electrical resistance of a copper PCB trace from its length, width, thickness, and operating temperature. PCB layout and power-electronics engineers use it to estimate voltage drop and heat dissipation along a trace before a board goes to fabrication.
Entering Your Trace Dimensions
Enter trace length in millimeters, trace width in millimeters, trace thickness in millimeters, and operating temperature in °C. If you only know copper weight (oz), convert it to thickness first — see the reference table below. The calculator returns trace resistance in milliohms, plus cross-sectional area, current density, and voltage drop at 1 A.
How Trace Resistance Is Calculated
The calculator applies Pouillet’s Law for resistance, extended with copper’s linear temperature coefficient, following the formulation set out in Douglas Brooks’ PCB engineering reference Signal Integrity Issues and Printed Circuit Board Design (Chapter 5, Resistance).
Cross-sectional area of the trace:
$$A = W \times T$$
Resistance at the reference temperature (this calculator uses 25°C):
$$R_{ref} = \rho \times \frac{L}{A}$$
where $\rho$ is the resistivity of copper, about $1.7 \times 10^{-6}\ \Omega \cdot cm$.
Resistance at the operating temperature:
$$R = R_{ref} \times \left[1 + \alpha (T_{op} – 25°C)\right]$$
where $\alpha$ is copper’s temperature coefficient of resistivity, about $0.00393$ per °C.
Common input mistake: entering length in millimeters but thickness in mils or oz without converting both to the same unit system first. Because $A = W \times T$ sits in the denominator, a mismatched unit doesn’t cause a small rounding error — it throws the result off by orders of magnitude.
Nuance worth knowing: this formula assumes a rectangular cross-section, but chemical etching undercuts the resist and leaves a trapezoidal trace — narrower at the top than at the base. Actual measured resistance typically runs a few percent higher than the rectangular-model result, and the gap widens with heavier copper weights, where the undercut is larger.
Trace Length, Width, and Thickness on the Board
Copper Weight to Trace Thickness
| Copper Weight | Thickness (µm) | Thickness (mil) |
|---|---|---|
| 0.5 oz | 17.5 µm | 0.7 mil |
| 1 oz | 35 µm | 1.37 mil |
| 2 oz | 70 µm | 2.74 mil |
| 3 oz | 105 µm | 4.11 mil |
Derived from copper’s density (8.96 g/cm³): one ounce of copper spread evenly over one square foot yields roughly 35 µm of thickness. This is the conversion standard used across PCB fabrication to translate a copper-weight spec into a physical trace thickness.
Common Questions About Trace Resistance
What does a high trace resistance value mean for my design?
High resistance means more voltage drop and more heat along that trace under load. For power traces carrying significant current, this can mean widening or thickening the copper, or splitting the current across multiple traces.
Does trace resistance change with temperature?
Yes. Copper’s resistivity rises with temperature, by about 0.393% per °C. A trace running hot from self-heating or a warm enclosure will measure noticeably higher resistance than the same trace at room temperature.
Is trace width or thickness more effective at lowering resistance?
Mathematically they’re equivalent, since resistance depends on the product $W \times T$. In practice, increasing copper weight (thickness) is often easier since it applies to the whole layer, while widening a trace competes with routing density.
How is trace resistance different from trace impedance?
Resistance is the DC opposition to current flow. Impedance also factors in inductance and capacitance and governs how high-speed signals behave. A trace can have low resistance and still have poorly controlled impedance.
Why does my measured resistance differ from the calculated value?
The calculation assumes an ideal rectangular trace. Real traces are trapezoidal from etching undercut, and surface roughness and fabrication tolerance add further variance — so measured resistance usually runs a bit higher than calculated.