IPC-2221 · §6.2

PCB Trace Width Calculator

Size a copper trace by the IPC-2221 constant-current method, for external and internal layers at the same time. Solve for the width a current needs, for the current a given width can carry, or for the temperature a trace will reach. Add a trace length and you also get resistance, voltage drop and power loss.

Solve for

Current ?

Temperature rise ?

°C

Copper weight ?

Copper plane (IPC-2152 row) ?

Trace length for resistance & drop ?

Min width · external layer

Basis	Min width	Cross-section	Resistance	V-drop	Power
IPC-2221 external	-	-	-	-	-
IPC-2221 internal	-	-	-	-	-
IPC-2152	-	-	-	-	-

Method based on IPC-2221C · reviewed June 2026 · method rev 1.0

The cross-section that sets it

A trace is copper width × thickness. That cross-sectional area sets both current capacity and resistance; an internal layer adds laminate above, so it traps heat.

How it's calculated

IPC-2221 links a conductor's cross-sectional area to the current it carries and the temperature rise that current produces. One equation ties the three together:

I = k · ΔT^0.44 · A^0.725

I: current, in amperes
ΔT: temperature rise above ambient, in °C
A: cross-sectional area, in mil²
k: 0.048 for external layers, 0.024 for internal

Because the three quantities sit in a single relationship, fixing any two gives the third. That is what the mode switch does. Width comes from the area and the copper thickness: 1 oz/ft² is 1.378 mil, so width in mil is A divided by 1.378 times the copper weight. When you supply a length, resistance follows from R = ρL/A using copper's resistivity at the operating temperature, and voltage drop and power loss come straight from it. The IPC-2152 row goes further: it reads the modern, test-based still-air curve (a published curve-fit of IPC-2152 Figure 5-2) instead of the 1950s IPC-2221 data, which is why it usually sits between the external and internal IPC-2221 numbers. The optional copper-plane setting then scales that row using thermal-simulation ratios from Brooks and Adam - a plane carries heat away, so the trace can be narrower. Treat the plane-adjusted figure as a guide, not gospel: the authors who built those ratios concluded that real trace heating is complex enough that exact optimisation needs a thermal simulation. The still-air IPC-2152 figure is the conservative anchor; the plane setting shows how much room a heat-sinking plane buys you.

2 A on an external 1 oz trace, 10 C rise

Required cross-section A = (I / (k x dT^0.44))^(1/0.725), with k = 0.048 for an external layer.
A = (2 / (0.048 x 10^0.44))^(1/0.725) = 42.4 mil-squared.
Width = area / copper thickness. A 1 oz layer is 1.378 mil thick, so width = 42.4 / 1.378 = 30.8 mil (0.78 mm).
This is the IPC-2221 minimum. Add margin for tolerances, vias and reliability, and cross-check power traces against IPC-2152.

5 A on an internal 2 oz trace, 10 C rise

Internal layers use k = 0.024, half the external value, because the 1950s IPC-2221 model assumes they shed heat less easily.
A = (5 / (0.024 x 10^0.44))^(1/0.725) = 390 mil-squared.
2 oz copper is 2.756 mil thick, so width = 390 / 2.756 = 142 mil (3.6 mm).
Internal high-current traces get wide quickly. IPC-2152 often allows much narrower when copper planes are nearby, so check there before committing the space.

Same 2 A trace, but allow a 20 C rise

Raising the allowed temperature rise lets the trace run hotter, so it can be narrower.
A = (2 / (0.048 x 20^0.44))^(1/0.725) = 27.8 mil-squared.
Width = 27.8 / 1.378 = 20.2 mil (0.51 mm), down from 30.8 mil at a 10 C rise.
Trading temperature headroom for board space is a real design lever, but a hotter trace also has higher resistance and sits closer to its limit.

Quick reference

Minimum width at a 10 °C rise on 1 oz copper, by current. Treat it as a starting point, then confirm with your own copper weight and temperature rise above.

Widths above 400 mil fall outside the range IPC-2221 was tested on.
Current	External	Internal
0.5 A	4.5 mil	11.8 mil
1 A	11.8 mil	30.8 mil
2 A	30.8 mil	80.0 mil
3 A	53.8 mil	140 mil
5 A	109 mil	283 mil
10 A	283 mil	737 mil

Before you commit the width

A clean number on screen is not a finished decision. IPC-2221's curves were fitted to data covering 0.5 to 3 oz copper, a 10 to 100 °C rise, currents to 35 A and widths to 400 mil; step outside that and the result is an extrapolation, which the tool flags as you go. Add 10 to 20 percent for etching tolerance, and do not route below your fabricator's floor - most shops hold about 6 mil minimum, so a 4 mil signal trace should be widened to the floor for yield rather than taken literally. Current-carrying vias are sized on their own, not by this formula. And remember the rise is above the board's working temperature, not the room: a board sealed in a warm enclosure starts hotter, so derate accordingly. For high-current or dense work, cross-check IPC-2152, which models the board's thermal behaviour and usually allows a narrower trace.

Where engineers use this

Motor drives and inverters

Sizing the DC-bus and phase-output traces that carry tens of amps, where a controlled temperature rise keeps the copper and nearby components within their ratings.

Switching power supplies

Width for input and output power traces and pours, where high RMS current and a warm enclosure push you past standard 1 oz copper.

LED lighting and strips

Long low-voltage runs where both trace heating and the resistance-driven voltage drop along the run decide the width.

Automotive and under-hood electronics

Power traces derated for a high ambient, where the temperature rise is added on top of an already-hot board, not room temperature.

Frequently asked questions

Can it work backwards from a trace width?

Yes. Set the mode to Current to find the largest current a given width can carry at your temperature rise, or to Temp rise to see how hot a width runs at a given current. The three quantities share one IPC-2221 equation, so any two fix the third, for both external and internal layers.

Shouldn't an external trace need more copper, since it can lift off the board when it overheats?

It is a fair intuition, but the result is the other way round. An external trace sits in open air and loses heat by convection, so it runs cooler for a given width. An internal trace is wrapped in laminate, which traps heat, so it reaches the same temperature at a wider width. That is why IPC-2221 gives internal layers the smaller constant and the wider trace.

Should I use IPC-2221 or IPC-2152?

IPC-2221 is the long-standing, deliberately conservative method, and it is fine for most general designs. IPC-2152, from 2009, rests on more thorough thermal testing and usually allows a narrower trace, particularly on inner layers or beside copper pours. A common approach is to size with IPC-2221, then cross-check with IPC-2152 when a high-current trace is worth optimising.

The calculator wants very wide thermal-relief spokes. Is that right?

No, and this is a known limit of the method. The IPC-2221 formula assumes a long trace with no special heat sinking. A short spoke connecting a pad to a plane is sunk straight into a large copper area, so it carries far more current than its width alone would suggest. Size spokes by your fabricator's guidance or thermal-relief rules, not by this calculator.

What temperature rise should I design for, and what about a hot enclosure?

10 °C is a safe default for signal and light-power traces; 20 °C is common for power traces and returns a narrower width. The rise is measured above the board's working temperature, so a board running inside a warm enclosure has less headroom - subtract that ambient from your laminate's rating before you choose a rise.

How accurate is the result?

Inside the tested range it is a sound engineering estimate, usually within about ten percent of measured behaviour. A real board drifts from that with nearby copper, airflow, solder mask and laminate type, so keep a margin and, for anything safety-critical, confirm with a thermal simulation or your fabricator.

How this relates to other standards

Standard / tool	Relationship	What it means
IPC-2221B (2012)	Superseded by	IPC-2221C (2023) is the current edition; this constant-current method is unchanged between the two.
IPC-2152	Refined by	IPC-2221C defers current-carrying capacity to IPC-2152; cross-check power traces there, where it often allows a narrower trace.
PCB Via Current	Same standard as	Sizes the plated via barrel by the same IPC-2221 method - size them together so the via is not the bottleneck.

Guides

IPC-2221 vs IPC-2152, explainedWhere the two standards diverge, and which to trust.

Sources: IPC-2221C, Generic Standard on Printed Board Design (conductor characteristics; current capacity deferred to IPC-2152) · IPC-2152, Standard for Determining Current-Carrying Capacity in Printed Board Design · L. Rozenblat, PCB trace width calculation based on IPC-2152 (universal still-air curve-fit of Fig 5-2) · D. Brooks & J. Adam, Trace Currents and Temperatures Revisited (copper-plane thermal-simulation ratios). Verify against the current edition.