PCB Trace Resistance Calculator

Compute the DC resistance of a copper PCB trace from its length, width, copper weight, and temperature, plus the resulting voltage drop and power loss at a given current.

Quick Facts

Formula
R = ρ × L / (W × T)
Copper resistivity ρ ≈ 1.7241×10⁻⁸ Ω·m at 20°C; 1 oz copper ≈ 35 µm thick.

Your Results

Calculated
Trace resistance
-
R = ρ×L / (W×T)
Voltage drop
-
V = I × R at entered current
Power dissipated
-
P = I² × R (heat in the trace)
Cross-sectional area
-
Width × copper thickness

Ready

Enter trace geometry and current, then press Calculate.

About the PCB Trace Resistance Calculator

Every copper trace on a printed circuit board is a resistor, whether the designer thinks of it that way or not. This calculator applies the standard resistance formula, R = ρ × L / (W × T), to a trace's length, width, and copper thickness so you can check whether a trace is thin enough to matter — for signal integrity, IR drop, or self-heating — before the board goes to fabrication.

The formula

Resistance of a uniform conductor is resistivity times length divided by cross-sectional area: R = ρ × L / A. On a PCB, the cross-sectional area is the trace width (W) times the copper thickness (T), so the working formula is:

R = ρ × L / (W × T)

  • ρ (resistivity): for annealed copper at 20°C, ρ₂₀ ≈ 1.7241 × 10⁻⁸ Ω·m (equivalent to 100% IACS conductivity, about 5.8 × 10⁷ S/m).
  • L (length): the trace's routed length, not the straight-line distance between its endpoints.
  • W (width): the trace's copper width, as drawn in the PCB layout.
  • T (thickness): the finished copper thickness, set by copper weight. 1 oz/ft² of copper foil finishes at roughly 35 µm (1.37 mils); thickness scales linearly with weight, so 2 oz ≈ 70 µm and 0.5 oz ≈ 17.5 µm.

Correcting for temperature

Copper's resistivity rises with temperature, so a hot trace has more resistance than the same trace at room temperature. This calculator applies the standard linear temperature-coefficient model:

ρ(T) = ρ₂₀ × [1 + α × (T − 20)]

with α = 0.00393 per °C, the accepted temperature coefficient of resistance for annealed copper referenced to 20°C. Raising a trace from 20°C to 80°C increases its resistance by roughly 24%.

Voltage drop and power loss

Once resistance is known, Ohm's law and the power law give the practical numbers a designer actually cares about: voltage drop V = I × R and heat dissipated in the trace P = I² × R. A trace with even a fraction of an ohm can drop meaningful voltage or generate noticeable self-heating once current climbs into the amps.

Practical context

This calculator gives DC trace resistance for a straight, uniform-width run. It does not model AC skin effect at high frequency, corner and via resistance, or temperature rise from current-carrying capacity (that requires an IPC-2221 trace-width/current chart, a related but separate calculation). For power traces or long runs, always leave margin — validate against your fab's actual copper thickness tolerance and your board's real operating temperature.

Frequently Asked Questions

What formula does this PCB trace resistance calculator use?
R = ρ × L / (W × T), where ρ is copper resistivity, L is trace length, W is trace width, and T is copper thickness (derived from the copper weight in oz/ft²). Resistivity is adjusted for temperature using ρ(T) = ρ₂₀ × [1 + α × (T − 20)] with α = 0.00393 per °C, the standard temperature coefficient for annealed copper.
How thick is 1 oz copper on a PCB?
1 oz/ft² copper weight corresponds to a finished thickness of about 35 micrometers (1.37 mils). Common PCB copper weights scale linearly from that: 0.5 oz is about 17.5 micrometers, 2 oz is about 70 micrometers, and 3 oz is about 105 micrometers.
Why does temperature affect trace resistance?
Copper is a normal metallic conductor, so its resistivity rises roughly linearly with temperature. A trace running hot from self-heating or a hot enclosure has measurably higher resistance than the same trace at 20°C, which is why entering an accurate operating temperature matters for tight power budgets.
How is voltage drop calculated from trace resistance?
Voltage drop follows Ohm's law: V = I × R, where I is the current you enter and R is the computed trace resistance. Power dissipated as heat in the trace follows P = I² × R.

Related calculators