Use the uniform-conductor equation
For a uniform conductor, DC resistance is R = rho × L / A. For a rectangular trace, A = width × thickness. Use resistivity, length and area in a consistent unit system. The equation describes the conductor represented by those dimensions, not every pad, via and joint in the circuit.
As an illustrative calculation, use copper resistivity of 1.68 × 10^-8 ohm-meters near 20°C, length 0.1 m, width 0.001 m and thickness 35 × 10^-6 m. The estimated resistance is 0.048 ohm. The chosen thickness and temperature are assumptions, not a finished-board tolerance guarantee.
Translate resistance into design consequences
For the example above, 1 A would produce about 48 mV of DC drop and 48 mW of dissipation, using V = I × R and P = I² × R. At another current, recalculate both values. Power dissipation alone does not determine the resulting temperature without a thermal model.
Copper resistance rises with temperature. Use the material properties and operating range appropriate to the analysis. A room-temperature calculation can understate the drop in a conductor that runs substantially hotter.
Include the real geometry and connections
Account for changes in width or thickness by dividing the route into suitable segments. Include relevant via barrels, pads, contacts and joints in the end-to-end path. Finished etching and plating introduce variation that may matter when the resistance budget is tight.
At higher frequencies, current distribution and parasitic behavior can make a DC resistance model insufficient. Skin and proximity effects, inductance, capacitance and transmission-line behavior may need analysis. Do not use the DC equation as an impedance calculator.
Choose a measurement method that resolves the value
For low resistance, test-lead and contact resistance can dominate a two-wire measurement. A suitable four-wire method separates current injection from voltage sensing and can improve the result. Define the measurement points, current and thermal state.
Compare the measurement with the same path and conditions represented by the calculation. Record the board revision and copper definition. Use the result to validate the model and tolerances, while keeping current capacity and thermal acceptance as separate questions.