This tool covers two related but distinct PCB trace design questions: how wide does a trace need to be to carry a given current without overheating (IPC-2221 current-capacity method), and what characteristic impedance does a trace of a given width actually have (IPC-2141 microstrip/stripline formulas) — the second question matters for any high-speed digital or RF signal trace that needs a controlled impedance to avoid reflections.
IPC-2221 (the successor to the older IPC-D-275) gives an empirical relationship between trace cross-sectional area, allowable temperature rise, and current-carrying capacity: I = k × ΔT^0.44 × A^0.725, where k is 0.048 for traces on an external (outer) layer exposed to open air, and 0.024 for traces buried on an internal layer, which can't dissipate heat as effectively. This calculator inverts the formula to solve for the required area (and therefore width, given a copper thickness) for a target current and allowable temperature rise. A lower allowable temperature rise or an internal-layer trace both require a proportionally wider trace for the same current.
A microstrip trace runs on an outer layer with a single reference plane below it and air (or solder mask) above; a stripline trace is buried between two reference planes. Both geometries are described by IPC-2141's widely-used empirical formulas, which relate trace width, dielectric height/spacing, dielectric constant, and trace thickness to characteristic impedance (Z0). Common target impedances are 50Ω single-ended (most RF and high-speed digital signals) and 100Ω differential (common for USB, HDMI, Ethernet, and other differential pairs, achieved by placing two 50Ω-ish traces close enough together that their fields couple) — this calculator computes single-ended impedance; differential impedance additionally depends on trace spacing and requires a separate differential-pair formula.
Both the IPC-2221 current-capacity formula and the IPC-2141 impedance formulas are curve-fit approximations to more complex electromagnetic field behavior, valid within specific geometric ranges (IPC-2141's microstrip formula, for example, is documented as accurate for 0.1 < W/H < 2.0 and 1 < εr < 15). For most everyday PCB design they are accurate enough to specify a trace and move on. For controlled-impedance boards where a fabricator guarantees a specific tolerance (commonly ±10%), or for high-frequency RF work, use your board fabricator's own stack-up-specific impedance calculator (most PCB fabs provide one, calibrated to their actual process) or a full field-solver tool rather than relying solely on the closed-form approximation here.
For current capacity: enter the current the trace must carry, an allowable temperature rise (10°C is a common conservative default; less critical traces sometimes use 20°C), the copper weight (1oz or 2oz are most common), and whether the trace is on an external or internal layer — internal traces need to be noticeably wider for the same current since they can't shed heat into open air. For impedance: pick microstrip or stripline, enter your board's dielectric constant (FR-4 is typically quoted around 4.2–4.8 depending on frequency and resin content), the relevant dielectric height or spacing from your stack-up, and the trace width and copper thickness — the tool reports the resulting characteristic impedance so you can iterate on width until you hit your target (commonly 50Ω).
An external (outer layer) trace can dissipate heat directly into the surrounding air (and, in an enclosure, eventually to ambient), while an internal trace is sandwiched between layers of dielectric material with much lower thermal conductivity than air convection provides at the board surface — heat has to conduct through the PCB stack-up before it can escape. IPC-2221 captures this with a lower k constant for internal traces (0.024 vs 0.048 for external), which is why the same current and temperature-rise target computes to roughly double the cross-sectional area — and therefore, for the same copper weight, roughly double the width — for an internal trace.
FR-4's dielectric constant isn't a single fixed number — it varies by resin/glass ratio, manufacturer, and notably by frequency (it decreases somewhat at higher frequencies, a property called dielectric dispersion). A commonly cited working range is roughly 4.2–4.8 for general-purpose FR-4 at typical digital signal frequencies; if you need precision beyond a rough estimate, get the specific dielectric constant from your PCB fabricator's stack-up documentation for the exact material and frequency you're designing for, rather than assuming a generic textbook value.
This calculator's impedance tab computes microstrip and stripline separately because they have genuinely different formulas and typically need different trace widths for the same target impedance and dielectric constant — a stripline trace (sandwiched between two planes) generally needs to be narrower than a microstrip trace (single plane below, open air above) to hit the same characteristic impedance, because the stripline's fields are more tightly confined by the second nearby plane. Make sure you've selected the correct topology matching your actual stack-up layer, not just reused microstrip dimensions for an inner-layer trace.
Trace thickness affects both, but its impact on impedance is comparatively small — it appears in the denominator of the impedance formulas as an addition to the effective width (0.8W + T), so a thicker trace slightly lowers impedance, all else equal, but the trace width and dielectric height/spacing dominate the result. For current capacity, thickness matters much more directly, since a thicker trace has proportionally more cross-sectional area for the same width, and the current-capacity formula depends on area, not width alone.
Use it for initial design and estimation, but for a board you're specifying as controlled-impedance to a fabricator, use their own impedance calculator tuned to their actual process stack-up (copper weights, prepreg/core thicknesses, and measured dielectric constant all vary by fab and material lot) — most PCB fabricators provide this as a free tool specifically because generic formulas like IPC-2141, while a solid engineering approximation, don't capture fab-specific process variation precisely enough to guarantee a tight impedance tolerance.
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