Enter values and press Calculate.
Uses the same width, copper weight, current and temperature as the calculator above — run a calculation there first, then view how voltage drop grows with trace length.
Copper is an excellent, but not infinite, conductor. Every trace has a small resistance determined by its geometry: R = ρ×L/(W×t), where ρ is copper's resistivity, L is the trace's length, and W×t is its cross-sectional area (width times thickness). This resistance does two things: it drops voltage along the trace (Vdrop=I×R), so the far end of a long power trace sees less voltage than the source, and it dissipates power as heat (Ploss=I²×R), which is separate from (and in addition to) the current-carrying-capacity heating covered by the Trace Width calculator.
It is worth being precise about the difference between these three PCB trace concerns, because they are often confused:
A trace can easily pass the IPC-2221 thermal check (it won't overheat) while still causing an unacceptable voltage drop on a long, thin run — especially for low-voltage, high-current supplies (like a 1.0 V core-voltage rail) where even a 20 mV drop is a meaningful percentage of the supply. That is exactly why voltage drop needs its own separate calculation.
| Quantity | Formula |
|---|---|
| Trace resistance | R = ρ × L / (W × t) |
| Voltage drop | Vdrop = I × R |
| Power loss | Ploss = I² × R |
| Copper resistivity at 20 °C | ρ = 1.72×10⁻⁸ Ω·m |
| Temperature correction | ρ(T) = ρ20×(1 + 0.00393×(T−20)) |
Use R = ρ×L/(W×t), where ρ is copper resistivity (about 1.72×10⁻⁸ Ω·m at 20 °C), L is the trace length, W is its width, and t is its copper thickness (from the oz weight).
Multiply the trace resistance by the current: Vdrop = I×R. This is simple Ohm's law applied to the trace as if it were a small resistor.
A common design target is keeping voltage drop under about 2–5% of the supply voltage for power traces. Low-voltage rails (below ~2 V) need much tighter absolute drop budgets since the same millivolt drop is a larger percentage.
No, they are related but different checks. Overheating (ampacity, IPC-2221) asks whether the trace itself gets too hot; voltage drop asks whether the far end of the trace still receives enough voltage. A trace can pass one check and fail the other.
Make the trace shorter, wider, or thicker (heavier copper weight) — resistance is proportional to length and inversely proportional to width and thickness. For very high-current paths, a copper pour or busbar reduces resistance dramatically compared with a routed trace.
Copper resistivity is about 1.72×10⁻⁸ Ω·m at 20 °C, and it rises about 0.393% per °C above that. A trace that has heated up from its own current carries slightly more resistance than the cold calculation suggests.
Use Ploss = I²×R, the standard resistive (I²R) heating formula, using the trace's resistance found from its geometry.
They are connected (both derive from the same trace geometry) but are usually calculated with different formulas: ampacity uses the empirical IPC-2221 thermal formula, while resistance and voltage drop use simple Ohm's law once you know R.
Because the same absolute voltage drop is a much larger percentage of a low-voltage rail. A 100 mV drop is negligible on a 12 V rail (0.8%) but significant on a 1 V core-voltage rail (10%), which can push a chip outside its operating tolerance.
Both scale linearly with length — doubling the length doubles the resistance, the voltage drop, and the power loss (for the same current), since resistance is directly proportional to length.
Resistance is inversely proportional to cross-sectional area, so doubling the copper weight (thickness) halves the resistance for the same trace width — a common way to cut voltage drop without widening a trace.
Yes, generally. A copper plane or pour has a vastly larger effective width than any practical trace, giving much lower resistance, lower voltage drop, and lower power loss for the same current — the standard approach for power and ground distribution on multi-amp designs.
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