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 September 18, 2026 Editorial check September 2026

Trace width, thickness and copper area

The trace width and copper thickness set its cross-sectional area. Actual temperature also depends on the surrounding board, connected copper and operating conditions.

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.

The legacy relationship uses different fitted constants for external and internal traces. That difference is not a physical rule that buried copper always runs hotter. Heat flows through the board and connected copper as well as into the surrounding air; the actual stackup determines the temperature.

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.

The equation is a historical empirical relationship. Its simplicity does not guarantee a conservative answer for every board.

Its limits, and IPC-2152

The legacy equation omits board-specific thermal context and can overestimate or underestimate the needed conductor size, depending on the construction.

IPC-2152 (2009) provides a broader test-based current-capacity data set and is still a useful reference, but IPC’s document revision table currently marks it “No Longer Maintained”. IPC-2221C remains the actively revised generic board-design standard. Use whichever current method and acceptance criteria your design context requires, and validate critical high-current paths thermally.

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	A 2009 dedicated current-capacity reference with richer thermal data. IPC currently marks IPC-2152 “No Longer Maintained”; it is not an actively revised companion standard.
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?

Where does the IPC-2221 formula come from?

The commonly implemented equation is an empirical fit to historical conductor charts, not a full thermal simulation. Its data origin does not establish accuracy for a modern stackup.

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

IPC-2221C is the actively revised generic printed-board design standard. The familiar trace-current equation is a legacy model associated with the IPC-2221 lineage. IPC-2152 is a separate 2009 test-based current-capacity reference with board-construction corrections, but IPC currently lists it as “No Longer Maintained”.

Is IPC-2221 still used?

IPC-2221 covers general PCB design topics. Its familiar legacy current equation remains useful for comparison, while current-capacity decisions require the applicable IPC-2152 data and actual board conditions.

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.