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Microstrip Impedance Calculator

Enter the trace and substrate dimensions to estimate Z0.

How it is calculated

For traces up to twice as wide as the dielectric is thick (w/h ≤ 2) the tool uses the IPC-2141 / Wheeler form: Z0 = 87 / √(εr + 1.41) × ln(5.98 h / (0.8 w + t)), in ohms. w is the trace width, h the dielectric height between trace and reference plane, t the copper thickness, and εr the dielectric constant. For wider traces (w/h > 2) that formula fails, so a Hammerstad-Jensen wide-strip formula with an effective width for the copper thickness is used instead. The three lengths must share one unit; the default copper thickness 0.035 is 35 µm (1 oz) in millimetres. If t or εr are left empty, 0.035 and 4.3 are used. The ratio shown is the argument of the logarithm. The two formulas do not meet exactly at w/h = 2, so expect a jump of a few ohms there. This is an approximation, not a field solver.

Worked example

Take w = 0.3 mm, h = 0.2 mm, t = 0.035 mm and εr = 4.3. Here w/h = 1.5, so the IPC-2141 form applies. The denominator is 0.8 × 0.3 + 0.035 = 0.275, so the ratio is 5.98 × 0.2 / 0.275 = 4.3491 and ln(4.3491) ≈ 1.470. The square root is √(4.3 + 1.41) = √5.71 ≈ 2.390, so 87 / 2.390 ≈ 36.41. Multiplying gives 36.41 × 1.470, which the tool shows as 53.52 Ω, close to the common 50 Ω target. The next step would be to widen the trace slightly: w = 0.4 mm gives 44.22 Ω, so the 50 Ω width lies between the two.

Try this example →

Common mistakes

  • Mixing units, for example width in mils and height in millimetres. All three lengths must match, and the default copper thickness assumes millimetres.
  • Entering the whole board thickness as h. It must be only the dielectric thickness between the trace layer and its reference plane.
  • Reading the ratio line as a second result. It is only the number inside the logarithm; for wide traces (w/h above 2) a different formula is used and it is no longer part of the calculation.
  • Leaving εr at the default 4.3 for every board. It is only a typical FR-4 value, and the real one varies with the material and frequency.
  • Treating the result as final. It does not account for solder mask, a differential pair partner or the trace edge shape, so confirm with the board manufacturer's stack-up data.

About this calculator

A microstrip is a flat copper trace running above a grounded reference plane, separated by a dielectric substrate — the standard way to route controlled-impedance signals on a printed circuit board. Its characteristic impedance (Z0) depends on the trace width, the substrate's dielectric constant (εr) and thickness, and the copper thickness; this calculator applies the widely used Wheeler / IPC-2141 approximation to estimate Z0 from those physical dimensions.

Getting Z0 right matters whenever a trace needs to match the impedance of a driver, cable, or connector — commonly 50Ω for RF and general digital signaling or 100Ω differential for USB and Ethernet. PCB designers and RF/hardware engineers use impedance calculators like this one during stack-up planning, before finalizing trace widths in tools like Altium or KiCad, to avoid signal reflections that cause data errors or radiated emissions at high frequencies.

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