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AESTECHNO
IPC-2221

PCB trace current capacity calculator

A trace's current capacity follows IPC-2221 section 6.2: I = k x dT^0.44 x A^0.725, where k = 0.048 for an external trace or 0.024 for an internal one, dT is the temperature rise in degC and A is the cross-section in square mils, set by width and copper weight.

Inputs
Trace location

An external trace sheds heat better (k = 0.048) than an internal one (k = 0.024), so it carries more current for the same width.

Copper width. mil/mm/um toggle. 100 mil = 2.54 mm. Above 100 mil it is beyond the IPC-2221 validated range.

Finished copper thickness. 1 oz = 1.378 mil = 35.0 um. Most common is 1 oz; 4 oz exceeds the IPC-2221 range.

Heating above ambient. IPC-2221 often uses 10 degC. Above 45 degC, watch reliability.

Used for resistance, voltage drop and power loss. Does not affect the thermal current limit.

If set, we also report the minimum trace width needed for this current at the same thickness, temp rise and layer.

Air (open surface) Dielectric Copper trace W T External: outer layer, k = 0.048 (sheds heat to air)
Result

4.7 A

Breakdown

A = 137.8 mil² (0.0889 mm²) · R = 9.696 mΩ · Vdrop = 45.58 mV · Ploss = 214.3 mW · k = 0.048 (external)

IPC-2221 estimate (its charts were superseded by IPC-2152). External and internal traces differ; add margin for vias, connectors and sustained loads.

Frequently asked questions

FAQ

Which formula does this calculator use?
We apply the IPC-2221 section 6.2 formula: I = k x dT^0.44 x A^0.725, where I is in amperes, dT is the temperature rise in degC and A is the copper cross-section in square mils. The constant k is 0.048 for an external trace and 0.024 for an internal one. Cross-section A is width x thickness, with thickness derived from copper weight (1 oz = 1.378 mil = 35.0 um).
Why does an internal trace carry less current than an external one?
An external trace (top or bottom layer) sheds heat by convection into the air, while an internal trace is buried in the laminate and dissipates poorly. IPC-2221 captures this by halving k (0.024 instead of 0.048). Note that IPC-2152 testing shows this factor of two is pessimistic, so our internal results are deliberately conservative.
How do I convert copper weight to thickness?
Copper weight expresses an areal thickness. 1 oz equals 1.378 mil, or 35.0 um of finished copper. So 0.5 oz = 17.5 um, 2 oz = 70.0 um and 3 oz = 105.0 um. The calculator converts automatically before computing the cross-section, so you just enter the oz value from your fabricator's stack-up.
What temperature rise should I pick?
IPC-2221 commonly uses a 10 degC rise above ambient as a reference, and that is our default. You can go to 20 or 30 degC for a less critical power trace, but above 45 degC you should watch laminate reliability and the derating of nearby components. Temperature rise is a trade-off between routing density and thermal margin.
What are the validity limits of the IPC-2221 formula?
IPC-2221 data was established for currents up to roughly 35 A, traces 0 to 100 mil wide and copper up to about 3 oz. Beyond that, the calculator flags an extrapolation warning. For high currents, validate with measurement or thermal simulation, or contact us for a routing review.
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