PCB Trace Resistance Calculator (copper track resistance, voltage drop and loss)

Understanding the PCB trace resistance calculator — concept and principle

This calculator finds the DC resistance of a copper trace on a printed circuit board from its length, width, copper thickness and temperature, and gives the voltage drop, power loss, sheet resistance and cross-section. Electronics designers use it for power traces, sense lines and current paths.

A narrow 1 oz trace is 400 squares long here, each worth about half a milliohm.

How it is calculated

R = ρ L ÷ (w × t)

ρ(T) = 1.68 × 10⁻⁸ Ω·m × [1 + 0.0039 × (T − 20 °C)]

Voltage drop = R × I; power loss = I² × R; sheet resistance = ρ ÷ t

L and w in mm, t in µm (35 µm ≈ 1 oz/ft² copper), T in °C.

The principle

A trace is a long, flat rectangular conductor. Its resistance grows with length and falls with cross-sectional area, scaled by copper's resistivity. Resistivity rises with temperature, by about 0.39% per °C for copper. The sheet resistance is handy: any square piece of the copper layer, whatever its size, has the same resistance, so a trace's resistance is simply its number of squares (length ÷ width) times the sheet resistance.

Worked example

Default inputs: L = 100 mm, w = 0.25 mm, t = 35 µm, 25 °C, 1 A.

ρ = 1.68 × 10⁻⁸ × (1 + 0.0039 × 5) = 17.13 nΩ·m.
Area = 0.25 mm × 0.035 mm = 0.0088 mm².
R = 1.713 × 10⁻⁸ × 0.1 ÷ (8.75 × 10⁻⁹) = 195.74 mΩ = 0.1957 Ω.
At 1 A: drop 195.74 mV, loss 195.74 mW.
Sheet resistance = 0.489 mΩ per square; the trace is 400 squares long.

Each point is the calculator re-run with only “Trace width (mm)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Trace width (mm)	Trace resistance (mΩ)
0.0625	782.98
0.125	391.49
0.188	260.99
0.25	195.74
0.312	156.6
0.375	130.5
0.5	97.87

How to read the result

A drop of about 0.2 V can matter for low-voltage rails and sensing. Widening the trace or using thicker copper reduces resistance proportionally. Heating from I²R raises the temperature and resistance further, so high-current traces are usually sized with a temperature-rise standard.

Common mistakes

Entering thickness in mm instead of µm.
Forgetting the return path, which doubles the loop resistance.
Ignoring plating, which can add thickness on outer layers.
Using DC resistance at high frequency, where skin effect increases it.

Keep in mind: Current capacity depends on allowed temperature rise and board construction. Use IPC trace-sizing guidance and verify with testing.

Frequently asked questions

How thick is 1 oz copper?

About 35 µm.

What is sheet resistance?

The resistance of any square of the copper layer, ρ ÷ t.

How much does temperature matter?

Copper's resistance rises about 0.39% per °C.

Does width or thickness matter more?

Both equally; resistance depends on their product.

Sources: OpenStax, University Physics Volume 2 (2016), section 9.3 "Resistivity and Resistance"

Checked 9 Nov 2026. OpenStax, University Physics Volume 2 (2016), §9.3 “Resistivity and Resistance”: Table 9.1 (copper ρ = 1.68 × 10⁻⁸ Ω·m, α = 0.0039 °C⁻¹ at 20 °C), Eq. 9.7 ρ = ρ₀[1 + α(T − T₀)] and R = ρL/A. 1 oz/ft² copper ≈ 35 µm (standard value).

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