PCB trace width calculator (IPC-2221)

By Shounak Das · Updated November 2026 · 6 min read

How wide should a PCB trace be?

On 1 oz outer copper with a 10 °C rise, IPC-2221 gives 11.8 mil (0.300 mm) for 1 A and 30.8 mil (0.781 mm) for 2 A. A 10 mil trace carries about 0.89 A. Inner layers need about 2.6 times the width for the same current.

The calculator uses the IPC-2221 curve fit; PCB trace width for current explains where it comes from and how the newer IPC-2152 data differs, and the companion post works through common signal and power widths.

Key results from the calculator

10 mil, 1 oz, outer: 0.89 A at 10 °C rise, 1.20 A at 20 °C; 0.44 A on an inner layer.
1 A: 11.8 mil outer, 30.8 mil inner (1 oz, 10 °C).
2 A: 30.8 mil outer, 80.0 mil inner.
Doubling copper to 2 oz halves the width for the same current, because the formula works on cross-section area.

Six traces

Each example loads the current or width, copper weight, layer and temperature rise.

A 10 mil trace on 1 oz outer copper

Common default width

A 10 mil (0.254 mm) trace has 13.8 mil² of copper. At a 10 °C rise IPC-2221 allows 0.89 A; over 50 mm it measures 102.7 mΩ.

Current 0.89 A At 20 °C rise 1.20 A Inner layer 0.44 A 50 mm drop 90.9 mV

A 6 mil signal trace

Fine pitch

A 6 mil (0.15 mm) trace is typical for fine-pitch signals. It still carries about 0.61 A on the surface at a 10 °C rise.

Current 0.61 A Inner layer 0.31 A Width 0.152 mm 50 mm R 171.1 mΩ

1 A on an outer layer

Small power rail

11.8 mil (0.300 mm) on 1 oz copper keeps the rise to 10 °C. An inner layer would need 30.8 mil.

Width 11.8 mil In mm 0.300 mm Inner layer 30.8 mil 50 mm drop 86.8 mV

2 A on an inner layer

Buried trace

IPC-2221 halves the allowed current for buried traces, so 2 A needs 80.0 mil inside against 30.8 mil on the surface. IPC-2152 data suggests that is conservative.

Inner width 80.0 mil Outer width 30.8 mil In mm 2.03 mm 50 mm drop 25.7 mV

5 A on 2 oz outer copper

Motor or LED supply

Thicker copper keeps it manageable: 54.4 mil (1.38 mm) on 2 oz. Over 100 mm it drops 94.3 mV.

Width 54.4 mil In mm 1.38 mm 100 mm R 18.9 mΩ Drop 94.3 mV

10 A on 2 oz, 20 °C rise

High current

Even with a 20 °C allowance, 10 A needs 2.36 mm of 2 oz copper. Wide pours, planes or a bus bar are the usual answer.

Width 93.0 mil In mm 2.36 mm Inner layer 6.14 mm 100 mm drop 114.5 mV

Current for common widths (1 oz copper, IPC-2221)

Width	mm	Outer, 10 °C	Outer, 20 °C	Inner, 10 °C
6 mil	0.152 mm	0.61 A	0.83 A	0.31 A
8 mil	0.203 mm	0.75 A	1.02 A	0.38 A
10 mil	0.254 mm	0.89 A	1.20 A	0.44 A
12 mil	0.305 mm	1.01 A	1.37 A	0.51 A
16 mil	0.406 mm	1.25 A	1.69 A	0.62 A
20 mil	0.508 mm	1.46 A	1.99 A	0.73 A
30 mil	0.762 mm	1.96 A	2.66 A	0.98 A
50 mil	1.27 mm	2.84 A	3.86 A	1.42 A
100 mil	2.54 mm	4.70 A	6.38 A	2.35 A

IPC-2221 curve fit with k = 0.048 (outer) and 0.024 (inner), 1 oz = 1.378 mil. The charts behind it cover up to 35 A, 400 mil, 10 to 100 °C rise and 0.5 to 3 oz copper.

How the calculator works

IPC-2221 curve fit: I = k × ΔT^0.44 × A^0.725, with A the copper cross-section in square mil and k = 0.048 for outer layers in air, 0.024 for inner layers. These constants come from fitting the IPC-2221 charts, as published with the Advanced Circuits calculator.

Width: cross-section divided by copper thickness, 1.378 mil (35 µm) per ounce per square foot.

Resistance: copper resistivity of 1/58 Ω·mm²/m at 20 °C, rising 0.393% per °C (NBS Handbook 100), at the ambient plus the allowed rise.

Drop and loss: V = I × R and P = I² × R over the length you enter.

What the model leaves out

Nearby copper: planes and pours spread heat and let traces run cooler than IPC-2221 predicts; IPC-2152 accounts for this.
Plating: outer layers get plated copper on top of the foil (about 20 µm on average by the IPC table NCAB describes), so a finished "1 oz" outer trace is often thicker than 35 µm.
Board and enclosure: board thickness, airflow and a hot enclosure change the real temperature.
Short pulses: a trace can carry more current for milliseconds than continuously.

Common misconceptions

"Inner traces are always hotter." IPC-2221 assumes so, but its internal chart was not based on tests; IPC-2152 testing found internal and external traces rise by about the same, and buried ones often run cooler.

"Width alone sets the current." Cross-section does: 2 oz copper doubles the area at the same width.

"The trace only needs to survive." A 10 °C rise is a comfortable design target; voltage drop on long power traces often matters first.

Frequently asked questions

How much current can a 10 mil trace carry?

About 0.89 A on 1 oz outer copper at a 10 °C rise, 1.20 A at 20 °C, and 0.44 A on an inner layer, by IPC-2221.

How wide should a trace be for 1 A?

11.8 mil (0.300 mm) on 1 oz outer copper for a 10 °C rise; 30.8 mil on an inner layer.

How thick is 1 oz copper?

1.378 mil, about 35 µm: one ounce of copper spread over a square foot.

Why do inner layers need wider traces?

IPC-2221 treats buried traces as insulated by the board, so it allows half the current. Newer IPC-2152 tests show this is often pessimistic.

Is IPC-2221 still valid?

It is a conservative rule of thumb that is still widely used. IPC-2152 replaced its charts with test data that accounts for board material and nearby planes.