PCB Trace Width Calculator (IPC-2221)

Calculate minimum PCB trace width and copper thickness for internal and external layers to carry target current without overheating.

Continuous Current (Amps)

Allowed Temperature Rise (°C)

Copper Thickness

External Trace Width (Top/Bottom layers)

11.89 mils

Internal Trace Width (Inner layers)

30.94 mils

Metric (mm) Equivalent

External Trace0.302 mm

Internal Trace0.786 mm

Calculation Steps

IPC-2221 Formula for Area (sq. mils)Area = ( I / (k × ΔT^0.44) )^(1/0.725)
Thickness from Cu WeightThickness = 1 oz × 1.37 mils/oz = 1.37 mils
Required Internal WidthW_int = 42.39 sq.mils / 1.37 mils = 30.94 mils (0.786 mm)
Required External WidthW_ext = 16.30 sq.mils / 1.37 mils = 11.89 mils (0.302 mm)

How it works

This tool uses the IPC-2221 formulas to determine the minimum trace width required to carry a given continuous current without exceeding a specific temperature rise.

The standard formula is based on empirical data from the Association Connecting Electronics Industries (IPC).

I = k × ΔT^0.44 × A^0.725

Where I is current (Amps), ΔT is temperature rise (°C), and A is the cross-sectional area (sq mils). k is a constant: 0.048 for external traces and 0.024 for internal layers.

Worked example

Suppose you need to carry 1 Amp on a standard 1 oz copper outer layer, and you allow a 10°C temperature rise above ambient.

First, calculate required area (A) for an external layer (k=0.048):
A = (1 / (0.048 × 10^0.44))^(1/0.725) ≈ 16.2 sq mils

Standard 1 oz copper thickness is ~1.37 mils.
Width = A / Thickness = 16.2 / 1.37 = 11.8 mils (≈ 0.3 mm).

Frequently Asked Questions

Why do internal traces need to be wider?

Internal traces are surrounded by FR4 (or other substrate materials) which acts as a thermal insulator. They cannot shed heat via convection to the air like external traces, so they must be wider to keep their resistance (and thus self-heating) lower.

Is IPC-2221 still accurate for modern PCBs?

IPC-2221 is an older standard but is extremely widely used and generally conservative (safe). The newer standard, IPC-2152, is more complex and depends on board thickness, thermal vias, and plane layers, but IPC-2221 remains a trusted rule of thumb for standard 2-4 layer boards.

Sources

IPC-2221 "Generic Standard on Printed Board Design" section 6.2.

Results are for engineering assistance. Verify important calculations and check official datasheets. See the Engineering Disclaimer.