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PCB trace width calculator

A PCB trace is a resistor, and the current through it makes heat. Too narrow, and it runs hot and drops volts. Type your current, the temperature rise you'll allow and the copper weight to get the width by the IPC-2221 formula, then add the trace's length to see its ohms and voltage drop.

The steady current the trace carries.

10 °C is the usual choice.

For its resistance and voltage drop.

Minimum trace width (IPC-2221)

0.78 mm
In mils
30.8 mil
Copper thickness
35 µm
On an inner layer
2.03 mm
Squares, length ÷ width
128
Resistance
64 mΩ
Voltage drop
128 mV
Heat in the trace, I² × R
256 mW

This width is for heat. A trace can stay cool and still drop too many volts for its load, so check the drop too, and size the return path the same way: the current comes back through copper as well.

The formula

IPC-2221 fits a curve to how hot traces got in tests:

I = k × ΔT0.44 × A0.725

I is the current in amps, ΔT the rise in °C, and A the copper's cross-section in square mils (1 mil = 0.0254 mm). k is 0.048 for outer layers, which shed heat into the air, and 0.024 for inner layers, buried in the board. Solve for A, then divide by the copper's thickness:

width = A over 1.378 mil × oz

1 oz copper is one ounce spread over a square foot: about 1.378 mil, or 35 µm, thick.

A worked example

2 A on an outer layer of 1 oz copper, for a 10 °C rise:

A = (2 over 0.048 × 10^0.44)1 ÷ 0.725 ≈ 42.4 mil²

42.4 ÷ 1.378 ≈ 31 mil, or 0.78 mm. On an inner layer it would need about 2.0 mm.

Now count squares, as World 19 does. 100 mm of 0.78 mm trace is 100 ÷ 0.78 ≈ 128 squares. At about 0.5 mΩ a square, that's 64 mΩ: 2 A drops 0.13 V along it and turns 0.26 W into heat.

Heat or volts: check both

IPC-2221 sizes a trace for temperature. A trace can stay cool and still drop too much for a sensitive load, so compare the voltage drop with what your load can spare, and widen it if it's too much. Watch for necks too: where a wide trace squeezes between two pads, every amp goes through the narrow bit, and that's where the heat goes.

Free lesson

Now build it yourself

Each lesson comes in three short parts: see the idea, try it on a live board, then build it yourself, with quick checks as you go.

Try it yourself: free lesson

“A trace is a resistor” is lesson 1 of 6 in Copper Cay, PCB Design World 1. Free with an account. It’s open as soon as you sign up.

Or start from the very first lesson, no account needed

The ideas behind it

Questions people ask

How wide should a PCB trace be for 1 A?

On an outer layer of standard 1 oz copper, about 0.3 mm (12 mil) for a 10 °C rise, by IPC-2221. 2 A needs about 0.78 mm and 3 A about 1.37 mm. An inner layer needs about 2.6 times the width, because it sheds heat only into the board.

What is the IPC-2221 trace width formula?

I = k × ΔT^0.44 × A^0.725, where I is in amps, ΔT is the temperature rise in °C, A is the cross-section in square mils, and k is 0.048 for outer layers or 0.024 for inner ones. Solve it for A, then divide by the copper's thickness (1.378 mil per ounce) to get the width.

What is the difference between IPC-2221 and IPC-2152?

IPC-2221's formula comes from old charts and is usually on the safe side for inner layers. IPC-2152 is newer, based on more careful tests, and accounts for the board, planes nearby and copper weight. For anything close to the limit, check IPC-2152's charts.

How much voltage does a PCB trace drop?

Count squares: length ÷ width, times about 0.5 mΩ for 1 oz copper. 100 mm of 0.78 mm trace is 128 squares, about 64 mΩ, so 2 A drops about 0.13 V. 2 oz copper halves it.

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