PCB Trace Resistance Calculator
Compute DC resistance of a copper PCB trace from its width, length and copper weight (0.5–4 oz), with temperature correction, voltage drop at your current, sheet resistance, and a reverse width/length solver. 100% client-side — nothing leaves your browser.
What this tool is (and is not)
- This tool: rectangular PCB trace geometry — width × length × copper weight (oz) → R = ρL/A.
- Round wires by AWG gauge? Use our wire resistance calculator (AWG 8–40 tables, loop resistance).
- Voltage drop across a discrete resistor? Use the resistor voltage drop calculator.
- Feeder/circuit voltage drop (NEC method)? Use the voltage drop calculator.
Trace resistance
Enter trace geometry and operating conditions.
Reverse solver
Know your resistance budget? Solve for the missing dimension.
Copper weight reference
Standard foil thicknesses (IPC-2221 nominal) and their DC sheet resistance at 20 °C.
| Copper weight | Thickness (µm) | Thickness (mil) | Sheet R (mΩ/□) | 1 m × 1 mm trace |
|---|---|---|---|---|
| 0.5 oz | 17.5 | 0.7 | 0.96 | 960 mΩ |
| 1 oz | 35 | 1.4 | 0.48 | 480 mΩ |
| 2 oz | 70 | 2.8 | 0.24 | 240 mΩ |
| 3 oz | 105 | 4.2 | 0.16 | 160 mΩ |
| 4 oz | 140 | 5.6 | 0.12 | 120 mΩ |
“1 m × 1 mm trace” = 1000 squares, so resistance = 1000 × sheet resistance. Finished thickness varies a few percent with plating and etch; use the custom µm input with your fab's data for critical designs.
Worked examples
1 · The anchor: 100 mm × 1 mm, 1 oz, 20 °C, 1 A
Area A = 1 mm × 35 µm = 3.5×10⁻⁸ m². R = 1.68×10⁻⁸ × 0.1 / 3.5×10⁻⁸ = 48 mΩ. Sheet check: 0.48 mΩ/□ × (100 mm / 1 mm) = 48 mΩ. ✓
At 1 A: voltage drop V = 1 × 0.048 = 48 mV, power P = 1² × 0.048 = 48 mW.
2 · Power trace: 80 mm × 2 mm, 2 oz, 75 °C, 3 A
Area A = 2 mm × 70 µm = 1.4×10⁻⁷ m². R(20 °C) = 1.68×10⁻⁸ × 0.08 / 1.4×10⁻⁷ = 9.6 mΩ. Temp factor at 75 °C = 1 + 0.00393 × 55 = 1.21615, so R(75 °C) = 11.675 mΩ.
At 3 A: drop = 3 × 0.011675 = 35.03 mV, power = 9 × 0.011675 = 0.105 W — plan copper pour or wider trace if that heat matters.
3 · Signal trace: 10 mil wide, 150 mm, 1 oz, 20 °C, 100 mA
Width = 10 × 25.4 µm = 254 µm. Area A = 254 µm × 35 µm = 8.89×10⁻⁹ m². R = 1.68×10⁻⁸ × 0.15 / 8.89×10⁻⁹ = 0.2835 Ω.
At 100 mA: drop = 0.1 × 0.2835 = 28.35 mV — fine for digital, but watch it on precision analog references.
Methodology
R = ρ · L / A · A = w · t · R(T) = R₂₀·[1 + α(T−20)]
Constants: ρ (copper) = 1.68×10⁻⁸ Ω·m at 20 °C; α = 0.00393 /°C (0.393%/°C); 1 oz/ft² = 35 µm = 1.4 mil (IPC-2221 nominal). Sheet resistance R□ = ρ/t gives 0.48 mΩ/□ for 1 oz copper, and R = R□ × (L/w).
Frequently asked questions
What is the formula for PCB trace resistance?
PCB trace resistance is R = ρ × L / A, where ρ (rho) is copper's resistivity (1.68×10⁻⁸ Ω·m at 20°C), L is the trace length in metres, and A is the cross-sectional area in square metres (trace width × copper thickness). For example, a 100 mm long, 1 mm wide trace on 1 oz copper (35 µm thick) has A = 3.5×10⁻⁸ m² and R = 1.68×10⁻⁸ × 0.1 / 3.5×10⁻⁸ = 0.048 Ω (48 mΩ).
How do I convert copper weight (oz) to thickness?
Copper weight on a PCB is specified in ounces per square foot. The standard conversion is 1 oz/ft² = 35 µm (1.4 mil) of copper thickness. So 0.5 oz = 17.5 µm, 1 oz = 35 µm, 2 oz = 70 µm, 3 oz = 105 µm, and 4 oz = 140 µm. This calculator accepts the common oz values directly, plus a custom thickness in µm for plated or non-standard foils.
What is sheet resistance, and why is it useful?
Sheet resistance (R□) is the resistance of one square of copper foil: R□ = ρ / t. For 1 oz copper it is 0.48 mΩ per square. Any trace's resistance is then simply R = R□ × (length / width) — a 100 mm × 1 mm trace on 1 oz is 100 squares, so R = 0.48 mΩ × 100 = 48 mΩ. It is the fastest way to estimate trace resistance by hand.
Does temperature affect PCB trace resistance?
Yes. Copper's resistance rises about 0.393% per °C: R(T) = R(20°C) × [1 + 0.00393 × (T − 20)]. A trace that is 48 mΩ at 20°C becomes about 58.4 mΩ at 75°C (a factor of 1.216). Enter your expected operating temperature in the calculator to get the hot resistance, voltage drop, and dissipated power.
How do I calculate voltage drop on a PCB trace?
Voltage drop is V = I × R: multiply the trace current by the trace resistance at your operating temperature. Enter your current (A or mA) in the calculator and it reports the drop plus the power dissipated (P = I²R). A 48 mΩ trace carrying 2 A drops 96 mV and dissipates 0.19 W — enough to matter in precision analog and power circuits.
How is this different from a wire resistance calculator?
Wire calculators work from AWG gauge tables for round conductors. This tool works from the rectangular geometry of a PCB trace: width, length, and copper weight (oz). The physics (R = ρL/A) is identical — only the way the cross-sectional area is specified differs. For round wires, see our wire resistance calculator.
Does this calculator account for AC skin effect?
No — it computes DC resistance, which is what matters for power delivery, voltage drop, and heating. At typical switching frequencies the skin depth in copper (about 66 µm at 1 MHz) is comparable to or larger than standard foil thicknesses, so DC resistance is a reasonable approximation. For RF traces, use impedance-controlled design tools instead.
Do inner layers and outer layers have the same resistance?
The DC resistance formula is the same for inner and outer layers — it depends only on width, length, and copper thickness. What differs is heat dissipation: inner layers cool worse, so the same current raises the trace temperature more, and the hotter copper has higher resistance. Enter a higher operating temperature for inner-layer estimates.
How accurate is the 1 oz = 35 µm conversion?
35 µm is the nominal IPC-2221 value and is accurate to a few percent for standard foil. Real finished thickness varies with the manufacturer's plating process, etch factor (traces etch slightly trapezoidal, narrowing at the top), and solder mask. For critical designs, measure a test coupon or use the custom µm input with your fab's actual foil data.
Why is my measured trace resistance higher than calculated?
Common reasons: the trace is hotter than 20°C under load (add ~0.4% per °C), etch undercut makes the real cross-section smaller than width × thickness, vias and connectors add series resistance, plating adds a thin different-alloy layer, and probe contact resistance pollutes 2-wire measurements. Use 4-wire (Kelvin) measurement for milliohm-level verification.
Can I use this for flex PCBs or aluminium PCBs?
Flex PCBs use the same copper with the same resistivity, so the calculator applies directly — just enter the actual foil thickness, since flex often uses thinner rolled copper (e.g. 18 µm or 12 µm). Aluminium-core PCBs still use copper traces on top; only the substrate differs, so trace resistance is unchanged.
How wide should a trace be for a given current?
Width for current capacity is a thermal question (IPC-2152 charts), not just resistance — but this tool's reverse solver answers the resistance side: enter your maximum acceptable resistance and trace length, and it returns the minimum width (or maximum length) on your copper weight. Always prototype and thermally verify high-current traces.
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