PCB Trace Width Calculator
IPC-2221 trace width for a current and temperature rise, with resistance, voltage drop and the clearance table that actually matters for safety.
Two things a bare width figure will not tell you
Internal traces need about 2.6× the width, not 2×. IPC halves the constant for buried copper — 0.024 against 0.048 — because it cannot convect and has only the board to conduct into. But the 0.725 exponent means halving k does not halve the width, it multiplies it by roughly 2.6. Reusing two-layer widths on an inner layer of a four-layer board is a standard way to end up with a hot board.
Clearance is the part that can actually hurt someone, and the table is not smooth. Uncoated external copper needs 0.1 mm up to 30 V and 0.6 mm at 50 V — a sixfold jump across twenty volts, easy to walk into when a design creeps from 24 V to 48 V. Above 3050 m it gets far worse: a 300 V rail needs 12.5 mm rather than 1.25 mm, because thin air breaks down much more readily. If the product might fly or go up a mountain, design for the altitude.
- IPC-2221 sizes for heat, not for voltage drop. It has nothing to say about whether your 3.3 V rail is still 3.3 V at the far end. On a long, low-voltage run the drop often decides the width well before the temperature does.
- The rise is above ambient. A 20 °C rise inside an enclosure already at 60 °C puts the trace at 80 °C. Check that against the laminate rating and against whatever is sitting next to it.
- Copper gets more resistive as it warms — about 0.39% per degree — and more resistance makes it hotter still. The figures here use the trace's own working temperature rather than room temperature.
- IPC-2221 is an empirical fit from the 1950s and is deliberately conservative. IPC-2152 supersedes it with far more variables and generally permits narrower traces. Use 2221 as a safe default and reach for 2152 when the extra copper genuinely costs you something.
How to use
- Enter the current and the temperature rise you are willing to accept.
- Pick external or internal — internal needs about 2.6 times the width.
- Set the copper weight and trace length for resistance and voltage drop.
- Enter the working voltage to get the IPC minimum clearance.
Frequently asked questions
Why do internal traces need so much more width?
Because buried copper cannot convect. It has only the board material to conduct heat into, so IPC halves the constant in the formula — 0.024 for internal against 0.048 for external. The catch is that halving the constant does not halve the width: the 0.725 exponent means you need roughly 2.6 times as much. Reusing two-layer widths on an inner layer of a four-layer board is a standard way to end up with a hot board.
What temperature rise should I allow?
Ten degrees is the usual default for signal and general power routing, and twenty is common where board space is tight. The important thing is that it is a rise above ambient, not an absolute temperature. A twenty degree rise inside an enclosure already sitting at sixty degrees puts the trace at eighty, which needs checking against the laminate rating and against whatever component is sitting next to it.
Does this account for voltage drop?
It reports it, but IPC-2221 does not size for it. The standard is about temperature rise and has nothing to say about whether your 3.3 volt rail is still 3.3 volts at the far end. On a long run at low voltage the drop very often decides the width well before the heating does, so check both figures rather than the width alone.
Why does the clearance table jump so sharply?
Because air breaks down at a threshold rather than gradually. IPC-2221 gives 0.1 mm for uncoated external copper up to 30 volts and 0.6 mm at 50 volts — a sixfold jump across twenty volts. It is easy to walk into when a design creeps up from 24 volts to 48. Coating the board with permanent polymer moves you to a much more permissive class, which is one of the quieter arguments for conformal coating.
Does altitude really matter that much?
Enormously. Thin air breaks down far more readily, so IPC has a separate class for anything above 3050 metres. A 300 volt rail needs 1.25 mm at sea level and 12.5 mm above that altitude — ten times the spacing. If the product might fly, go up a mountain, or ship by air unpressurised, design for the altitude rather than for your bench.
What is the difference between IPC-2221 and IPC-2152?
IPC-2221 is an empirical fit to measurements made in the 1950s and it is deliberately conservative. IPC-2152 supersedes it with far more variables — board thickness, nearby copper planes, whether the trace sits in still air or moving air — and generally permits narrower traces for the same rise. Use 2221 as a safe default, and reach for 2152 when the extra copper genuinely costs you board space or money.
How narrow can I actually go?
Cheap fabricators typically quote 0.15 mm, which is six thousandths of an inch, as their standard minimum for both trace and gap. Going below it is possible and costs noticeably more, so it is usually cheaper to widen a trace or move to two ounce copper than to pay for fine-line etching. Check your fabricator's capability page before committing a layout to anything finer.
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