PCB trace width and spacing calculator, the IPC-2221 current and voltage rules in one place
Current sets the width of a copper trace, voltage sets the spacing between traces, and the two are worked out from entirely separate formulas in IPC-2221B. The tool keeps them apart, adds the via and impedance calculations that use the same underlying rules, and always applies the production class minimum on top.
Open the PCB calculator A Pro account opens this tool. The interface is in English and nothing you enter leaves your browser.How it works
Three steps. Current and voltage answer two different questions, keep them separate.
Enter the board's copper weight, layer count and production class
Copper weight in ounces per square foot sets the cross-section available at any given width, one ounce is exactly 35 microns, and the production class, Class 2 or Class 3, sets the manufacturing floor that gets applied on top of every calculated width.
Enter current and voltage for the trace you are sizing
Current drives the width calculation, voltage drives the spacing calculation, and the two never feed into each other. Enter both even if only one matters for the trace in question.
Read the trace width, spacing, via sizing and impedance results together
Each result shows which IPC-2221B table or chart it comes from, and whether the manufacturing class minimum, rather than the physical formula, ended up being the limiting number.
Every number traces back to IPC-2221B
Chart 2a and 2b for current, Table 6-1 for voltage spacing, IPC-2141A for controlled impedance tolerance.
Built to keep current, voltage and manufacturing apart
Most PCB sizing mistakes are not arithmetic errors, they come from applying the wrong one of these rules to the wrong quantity.
Current sets width, voltage sets spacing, never the other way round
A trace carrying 1 amp at a 10 degree Celsius rise on 1 ounce external copper needs about 0.30 millimetres of width, 3 amps needs about 1.37 millimetres, straight from IPC-2221B Chart 2a. An internal trace needs roughly 2.6 times the width of an external one for the same current, because it is surrounded by FR4, a poor thermal conductor, instead of open air. None of that depends on voltage.
Voltage spacing read straight from IPC-2221B Table 6-1
Up to 50 volts the minimum spacing is 0.1 millimetres, from 51 to 300 volts it is 0.4 millimetres, from 301 to 500 volts it is 0.8 millimetres, for an indoor, uncoated board below 3050 metres altitude. FR4's comparative tracking index, typically 175 to 400, is what limits how close two different potentials can sit before the board itself starts to track.
Via current capacity and annular ring, not just trace width
A via with a 0.3 millimetre diameter and typical 0.025 millimetre plating carries about 1.79 amps at a 10 degree rise, the same IPC-2221 area formula applied to a cylinder instead of a flat trace. The minimum annular ring is 0.05 millimetres for Class 2 and 0.1 millimetres for Class 3, and doubling a via's diameter does not double its current capacity, several vias in parallel do that job properly.
Manufacturing class minimum applied automatically, not left for you to remember
No standard fab house reliably etches narrower than 0.1 millimetres with the ordinary subtractive photoimaging process, even when the current formula alone would allow less. The tool always returns the larger of the physical minimum and the manufacturing class floor, so a low-current signal trace never comes out narrower than what can actually be built.
Frequently asked questions
Does trace width depend on voltage?
No. Width comes entirely from current, through the IPC-2221 area formula for a given copper weight and allowed temperature rise. Voltage governs the separate question of spacing between traces, from IPC-2221B Table 6-1. Mixing the two up, for example assuming a 12 volt trace needs a wider gap because it is closer to the 50 volt threshold, produces the wrong number for both.
Why does an internal trace need to be so much wider than an external one for the same current?
An external trace sits on the surface and radiates heat into open air. An internal trace is sandwiched inside FR4, which conducts heat poorly, so the same current raises its temperature more for a given width. IPC-2221B's k constant reflects that directly, 0.048 for external layers against 0.024 for internal ones, which works out to roughly a 2.6 times wider internal trace for the same current and temperature rise.
Can I just double the via diameter if I need more current through it?
No, and this is a common mistake. Via current capacity grows with diameter through the same area formula, but the drilling and plating geometry does not scale the same way, and a larger via is also harder to place accurately and can make the annular ring requirement difficult to meet. The correct fix for extra current is two or three vias in parallel at the original diameter, not one larger one.
Is 0.05 millimetres annular ring actually safe?
It is the Class 2 minimum, and it is legal, but drilling tolerance and layer positioning tolerance are each around plus or minus 0.05 millimetres, and combined they can theoretically reduce a ring right down to nothing. That risks the copper around the via cracking after repeated thermal cycling. For anything that will see real thermal cycling, staying at 0.1 millimetres is the safer choice even where Class 2 would technically allow less.
Why does my 50 ohm trace width from one board not work on another?
Because the Schneider microstrip formula depends heavily on the substrate height to the reference plane, not just the trace width. A thick two-layer board with a large substrate height needs a much wider trace for the same 50 ohm impedance than a dense four-layer stack with thin layers. A width copied between two different stackups without recalculating is not a valid impedance match.
What is the difference between Class 2 and Class 3?
Class 2 is the ordinary industrial production class, the most common one in commercial manufacturing. Class 3 is the tighter class used for military and medical work, and it doubles the minimum annular ring to 0.1 millimetres. The manufacturing class sets a real production floor that exists independently of what the physics would otherwise allow.
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