IPC-2221C · conductor geometry

PCB Trace Resistance Calculator

Find the DC resistance, voltage drop and power loss of a copper PCB trace from its width, copper weight, length and temperature - with the substituted formula shown, not a black box.

Trace width ?

Copper weight ?

Trace length ?

Temperature ?

°C

Current ?

Method based on IPC-2221C · reviewed June 2026

The cross-section that sets it

A trace is copper width × thickness. That cross-sectional area sets both current capacity and resistance; an internal layer adds laminate above, so it traps heat.

DC resistance of a 1 oz trace, 2 in long, at 25 C

Resistance scales inversely with width and copper weight, and linearly with length. Halve the width or the copper, and the resistance doubles.
Trace width	Cross-section	Resistance
5 mil	6.9 mil²	201 mΩ
10 mil	13.8 mil²	100 mΩ
15 mil	20.7 mil²	67 mΩ
20 mil	27.6 mil²	50 mΩ
30 mil	41.3 mil²	34 mΩ
50 mil	68.9 mil²	20 mΩ

How trace resistance is calculated

The current flows through the copper cross-section of the trace, which is its width times its thickness. Copper weight sets the thickness: 1 oz/ft2 of copper is about 1.378 mil (roughly 35 micrometres), so a 10 mil wide, 1 oz trace has a cross-section of about 13.8 square mil.

Resistance is then the classic conductor relation R = rho x L / A: resistivity times length divided by area. Annealed copper has a resistivity of about 1.724e-8 ohm-metre at 20 C (the International Annealed Copper Standard, 100% IACS). The calculator converts your width, length and copper weight into consistent units and substitutes them, showing the worked figure rather than a black-box result.

Why temperature matters

Copper's resistivity rises with temperature by about 0.39% per degree C (a temperature coefficient of 0.00393 per C). A trace running at 50 C therefore has roughly 12% more resistance than the same trace at 20 C, which is why the same geometry reads 100 m-ohm at 25 C but 110 m-ohm at 50 C.

For a power trace, that extra resistance feeds back into self-heating: more resistance means more power dissipated for the same current, which raises the temperature further. Size power traces with that loop in mind, and check current capacity separately with the trace-width calculator.

Resistance, voltage drop and power

Once you have the resistance, two practical numbers follow directly. Voltage drop along the trace is V = I x R, which matters for sense lines, references and low-voltage rails where a few tens of millivolts shift a regulation point. Power dissipated in the trace is P = I-squared x R, the heat the copper has to shed.

These are DC, steady-state figures. They do not include skin effect at high frequency, the resistance of vias and connectors in the path, or the cooling effect of adjacent copper pours - all of which a real design has to account for.

Worked examples

Real sizing calls, and the number that decides each one.

Scenario	Result	Why
10 mil, 1 oz, 2 in trace at 25 C carrying 1 A	100 mΩ, 100 mV, 100 mW	The baseline case: about 0.5 m-ohm per square, 100 squares of length, so 50 m-ohm per inch, doubled for 2 in.
The same trace running warm at 50 C	110 mΩ, 110 mV, 110 mW	Copper resistivity rises about 0.39% per C, so 30 C hotter adds roughly 12%.
6 mil, 1 oz, 1 in signal trace at 25 C, 0.5 A	84 mΩ, 42 mV, 21 mW	Narrow trace, but short and low current - the drop stays small.
25 mil, 2 oz, 3 in power rail at 25 C, 5 A	30 mΩ, 151 mV, 753 mW	Wide, heavy copper keeps resistance low, but at 5 A the power loss is still three-quarters of a watt.

How this relates to other standards

Standard / tool	Relationship	What it means
PCB Trace Width	Counterpart of	Trace width sizes the conductor for current capacity to IPC-2221C; this tool gives the resistance, drop and loss of whatever width you settle on.
PCB Via Current	Shares the model	A plated via is a short trace; its resistance uses the same R = rho x L / A on the barrel cross-section.
IPC-2152	Companion standard	The test-based current-capacity standard IPC-2221C defers to; trace resistance is the physics underneath both.
Annealed copper (IACS)	Material basis	Resistance uses 100% IACS copper, 1.724e-8 ohm-metre at 20 C, rising about 0.39% per degree C.

A DC, steady-state estimate for one straight trace at a single temperature. It does not include skin effect at high frequency, the resistance of vias and connectors in the path, or the cooling effect of adjacent copper pours. Use it for first-pass drop and loss budgets, and check current capacity separately with the trace-width calculator.

Where engineers use this

Voltage-drop budgeting

Low-voltage, high-current rails where a few milliohms of trace resistance eat into regulation and have to be accounted for end to end.

Current sensing and shunts

Knowing the parasitic copper resistance that sits in series with a sense path before it reaches the amplifier.

Kelvin and 4-wire layout

Estimating the trace resistance you are deliberately routing around with separate force and sense connections.

Frequently asked questions

Is trace resistance defined by IPC-2221?

Not directly. IPC-2221C governs conductor sizing for current capacity and gives the copper-weight-to-thickness convention (1 oz is about 1.378 mil). The resistance itself is basic conductor physics, R = rho x L / A, using copper's resistivity. This tool uses the same geometry as the IPC trace-width method so the two agree.

Does this include vias, connectors or skin effect?

No. It is the DC resistance of one straight copper trace at a single temperature. Real paths add via resistance, connector resistance and, at high frequency, skin effect, which all raise the effective resistance. Add those separately.

How much does temperature change the answer?

Copper rises about 0.39% per degree C. From 25 C to 85 C that is roughly a 23% increase in resistance. For anything carrying real current, use the temperature the copper actually reaches, not ambient.

Why does 1 oz copper give about 1.378 mil?

Copper weight is specified in ounces per square foot of board. One ounce of copper spread over a square foot works out to a thickness of about 1.378 mil (about 35 micrometres). Two ounce copper is twice as thick, so it has half the resistance for the same width.

What is a reasonable voltage drop to allow?

It depends on the rail. For a logic supply a few tens of millivolts is usually fine; for a sense or reference line it can be too much; for a high-current rail the power loss often matters more than the drop. The tool gives you all three numbers so you can judge against your own budget.

Guides

IPC-2221 vs IPC-2152Which standard sizes a trace, and how resistance fits in.

Sources: IPC-2221C, Generic Standard on Printed Board Design (conductor geometry, copper-weight to thickness) · Copper resistivity 1.724e-8 ohm-metre at 20 C and temperature coefficient 0.00393 per C (International Annealed Copper Standard, 100% IACS). Verify against the current edition.