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PCB Trace Width Calculator: IPC-2221 Current Capacity

Trace width for current is a daily question for hardware and PCBA engineers. This calculator applies the widely used IPC-2221 formula to give a minimum width for a given current, temperature rise and copper weight.

Your numbers

Trace current

Continuous current the trace must carry.

Allowed temperature rise

10 deg C

Above ambient. 10 deg C is a common conservative target.

Copper weight

Layer type

Your results

Minimum trace width

53.8183

In mils.

Minimum trace width

1.367

In millimetres.

Cross-section area

74.1616

In square mils.

Copper thickness

1.378

In mils.

Width with 20% margin

1.6404

In millimetres.

IPC-2221 curves were derived for currents up to about 35 A, widths up to 400 mil and rises of 10 to 100 deg C. Use the calculator as a starting point and check your design with IPC-2152 or thermal simulation for high current or dense boards.

Save results:

Minimum trace width (mil) vs current

Sweeps Trace current from 0.5 to 10, holding your other inputs fixed.

Connect engineering data to your ERP

An optional review of how design release data reaches purchasing and production in your system.

Optional. The tool and every export work without this. No spam.

How to use it

Enter the continuous current, the temperature rise you can tolerate above ambient, the finished copper weight and whether the trace is on an outer or inner layer. The tool returns the minimum width in mils and millimetres, plus a version with a 20 percent margin. Inner traces need roughly twice the cross-section of outer traces because heat cannot escape as easily.

Use finished copper weight, not base copper, if plating adds thickness
Add margin for manufacturing tolerance and etch factor
Check voltage drop and clearance separately

The formula

IPC-2221 gives I = k x dT^0.44 x A^0.725, where I is current in amps, dT is temperature rise in deg C, A is cross-section in square mils, and k is 0.048 for external and 0.024 for internal layers. Solving for A gives the area, and width equals area divided by copper thickness, with 1 oz copper being about 1.378 mil thick. The calculator solves this directly and converts to mm.

Limits and common mistakes

The IPC-2221 chart is based on older test data and is known to be conservative for some conditions and optimistic for others. IPC-2152 is the newer standard and accounts for planes and board thickness. Do not use these values for fusing, short pulses or very high current busbars. Common mistakes include sizing for the average rather than the peak RMS current, ignoring neighbouring heat sources, and forgetting that vias carry far less current than the trace.

Frequently Asked Questions

How wide should a trace be for 1 amp?

For 1 oz outer copper and a 10 deg C rise, IPC-2221 gives a width of roughly 10 mil, which is about 0.25 mm. Inner layers need about twice the area. Use the calculator for your own conditions and add margin.

Is IPC-2221 still valid?

It remains widely used as a quick guide, but IPC-2152 is the newer current-carrying standard based on more recent data. Many designers use both and rely on thermal testing for critical power paths.

Why are inner layers wider?

Heat from an inner trace must conduct through laminate, while outer traces lose heat to the air. IPC-2221 reflects this by halving the constant k for internal layers, which roughly doubles the required area.

Does copper weight include plating?

On outer layers, plating adds to the base foil, so finished copper can be thicker than the nominal weight. Use the finished thickness from your stackup if you know it, or stay conservative with the base weight.

If design rules and trace data need to feed into your PLM or ERP records for release, Netray can help connect those engineering checks to your product data.

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