IPC-2221

IPC-2221 - Generic Standard on Printed Board Design

Generic Standard on Printed Board Design

Maintained by Giorgi Gaprindashvili Published July 3, 2026 Updated July 19, 2026

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.

Key facts

Full title: Generic Standard on Printed Board Design
Published by: IPC
Current edition: IPC-2221C (2023); previous editions IPC-2221B (2012) and IPC-2221A (2003)
Covers: Conductor sizing, electrical clearance and creepage spacing, and general printed-board layout
Trace-current formula: I = k times dT^0.44 times A^0.725 (I in amps, dT in degrees C, A in square mils)
Layer constant k: 0.048 for external traces, 0.024 for internal traces
Origin: The formula is an empirical curve-fit of MIL-STD-275 test data from the 1950s, not a physics derivation

What IPC-2221 covers

IPC-2221 is IPC's generic standard for printed board design. It covers conductor sizing, electrical clearance and creepage spacing, and general layout rules - the broad design base most PCB engineers work from.

Within that scope, the part everyone knows is the trace-current method: a simple formula that estimates how wide a trace must be to carry a given current without exceeding an allowed temperature rise. It is the method behind most quick online trace-width calculators.

The trace-current formula

The formula is I = k times dT^0.44 times A^0.725, where I is the current in amps, dT is the allowed temperature rise above ambient in degrees C, and A is the conductor cross-sectional area in square mils (trace width times copper thickness). The constant k is 0.048 for external traces and 0.024 for internal traces.

Because internal traces are buried in laminate with no air cooling, their k is half that of external traces. Owing to the 0.725 area exponent, that halved constant works out to roughly 2.6 times the cross-sectional area for the same current and temperature rise, which is why internal traces come out noticeably wider.

Where the formula comes from

The equation is not derived from physics; it is an empirical curve-fit of test data that traces back to MIL-STD-275 (and the older IPC-D-275) from the 1950s. Those tests used single traces with no adjacent copper, and the results were summarised in a small set of charts.

That origin explains the formula's character: it is simple, familiar and deliberately conservative, but it does not know anything about the modern board around the trace.

Its limits, and IPC-2152

Because it comes from isolated single-trace tests, the IPC-2221 formula ignores the factors that matter most on real multilayer boards: nearby copper planes, board thickness and laminate material. It therefore tends to over-size traces.

For current-carrying capacity specifically, IPC-2221C defers to the newer, test-based IPC-2152, which accounts for those factors and often allows a narrower trace. IPC-2221 remains the standard for the broader design topics that IPC-2152 does not cover. The related guide compares the two head to head.

IPC-2221 trace-current formula terms

The formula is an empirical curve-fit of 1950s data. Internal traces use half the k of external traces because they shed heat less effectively.
Symbol	Meaning	Units or value
I	Maximum continuous current	amperes
dT	Allowed temperature rise above ambient	degrees C
A	Conductor cross-sectional area (width times copper thickness)	square mils
k	Layer constant	0.048 external, 0.024 internal
0.44	Temperature-rise exponent	fixed
0.725	Area exponent	fixed

Tools that use this standard

PCB trace width calculator PCB via current calculator

Related standards

Standard	Relationship	What it means
IPC-2152	Refined by	For current-carrying capacity IPC-2221C defers to the newer test-based IPC-2152, which accounts for planes, board thickness and material and often allows a narrower trace.
MIL-STD-275 (IPC-D-275)	Derived from	The 1950s military and IPC-D-275 test data that the trace-current formula was curve-fit from.
IPC-2221B (2012)	Supersedes	IPC-2221C (2023) is the current edition; the trace-current method is unchanged between the two.

Frequently asked questions

What is IPC-2221?

IPC-2221 is IPC's generic standard for printed board design, covering conductor sizing, electrical spacing and general layout. It is the standard behind the trace-current formula most quick calculators use. The current edition is IPC-2221C (2023).

What is the IPC-2221 trace-width formula?

I = k times dT^0.44 times A^0.725, where I is current in amps, dT is the allowed temperature rise in degrees C, A is the conductor cross-sectional area in square mils (width times copper thickness), and k is 0.048 for external traces or 0.024 for internal traces. Rearranged, it gives the cross-sectional area, and hence the width, needed for a target current and temperature rise.

Why do internal traces need to be wider than external ones?

Internal traces are buried in laminate with no air cooling, so they shed heat far less effectively. In the formula their constant k is half that of external traces (0.024 versus 0.048), which, because of the 0.725 area exponent, works out to roughly 2.6 times the cross-sectional area for the same current and temperature rise.

Where does the IPC-2221 formula come from?

It is an empirical curve-fit of test data that traces back to MIL-STD-275 (and the older IPC-D-275) from the 1950s, based on single traces with no adjacent copper. It is not derived from physics, which is part of why it is conservative for modern multilayer boards.

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

IPC-2221 is the generic design standard with a single conservative formula; IPC-2152 (2009) is the dedicated current-carrying-capacity standard built on modern testing, with correction factors for planes, board thickness and material. IPC-2221C defers current capacity to IPC-2152, which often allows a narrower trace. The related guide compares them in full.

Is IPC-2221 still used?

Yes. Its trace-current formula remains the fast, conservative sanity check that most online calculators implement, and IPC-2221 still governs the broader design topics such as electrical spacing and layout that IPC-2152 does not cover. For final power-trace sizing, cross-check against IPC-2152.

Which edition of IPC-2221 is current?

IPC-2221C, published in 2023, is the current edition; IPC-2221B (2012) and IPC-2221A (2003) preceded it. The trace-current method is unchanged across these revisions.

Source: IPC (shop.ipc.org). An overview for design reference - verify against the current edition before relying on it for compliance.