PCB Way Impedance Calculator: How It Works

PCB Way’s impedance calculator is a free online tool that estimates the characteristic impedance of a printed circuit board trace based on the stack-up geometry and material properties you enter. You input trace width, thickness, dielectric constant, and the distance to the reference plane, and it returns the impedance in ohms for common transmission line types like microstrip and stripline. This is a first-pass design check, not a substitute for a controlled-impedance fab’s final stack-up validation.

Formula

The calculator uses two standard approximations from IPC-2141A. For a surface microstrip (trace on an outer layer with one reference plane below), the characteristic impedance is:

Z₀ = (87 / √(Er + 1.41)) × ln(5.98 × H / (0.8 × W + T))

where:

• Zâ‚€ = characteristic impedance in ohms (Ω)
• Er = relative permittivity (dielectric constant) of the substrate
• H = distance from trace bottom to the reference plane (dielectric thickness) in mils
• W = trace width in mils
• T = copper thickness in mils

For an embedded stripline (trace on an inner layer with reference planes above and below), the approximation is:

Z₀ = (60 / √Er) × ln(4 × H / (0.67 × (0.8 × W + T)))

Here H is the distance from the trace to the nearest reference plane, and the formula assumes the trace is centered between two planes. All dimensions must be in the same unit (the calculator uses mils; 1 mil = 0.001 inch = 25.4 µm).

Worked Example

Using the default values from the calculator block:

• W = 6 mil, T = 1.4 mil, H = 8 mil, Er = 4.3

For microstrip:

Z₀ = (87 / √(4.3 + 1.41)) × ln(5.98 × 8 / (0.8 × 6 + 1.4))

√(5.71) = 2.389, so 87 / 2.389 = 36.42

0.8 × 6 + 1.4 = 6.2; 5.98 × 8 = 47.84; 47.84 / 6.2 = 7.716

ln(7.716) = 2.044

Z₀ = 36.42 × 2.044 = 74.4 Ω (rounded to 74.4)

For the same geometry as a stripline (H = 8 mil to nearest plane):

Z₀ = (60 / √4.3) × ln(4 × 8 / (0.67 × 6.2))

60 / 2.074 = 28.93; 32 / 4.154 = 7.703; ln(7.703) = 2.042

Z₀ = 28.93 × 2.042 = 59.1 Ω

What the Calculator Is For

This tool is for quick estimates during schematic capture or layout planning. It helps you decide whether a 50 Ω trace is feasible with your chosen stack-up, or what width you need for a differential pair (though differential impedance is not directly computed here). PCB manufacturers often provide their own impedance calculators based on their exact prepreg and core materials; those numbers are the ones you should trust for production.

How to Choose the Right Inputs

To get a useful result, you need the actual stack-up from your PCB fab. Common values:

• FR-4 dielectric constant: typically 4.2–4.8 depending on frequency and resin content. Many fabs use 4.3 for 1 GHz and below.
• Copper thickness: 1 oz/ft² = 1.4 mil (35 µm), 0.5 oz = 0.7 mil, 2 oz = 2.8 mil.
• Dielectric height: ask your fab for the prepreg thickness between the signal layer and the adjacent ground plane. It is often 4–8 mil for standard 1.6 mm boards, but can vary.

For a 50 Ω microstrip on standard 1.6 mm FR-4 (Er ≈ 4.3), a trace width of around 8–10 mil with 1 oz copper and an 8 mil dielectric gives roughly 50 Ω. But always verify with the fab’s own calculator.

Common Mistakes and Misconceptions

The most frequent error is using the total board thickness instead of the distance to the nearest reference plane. If you have a 4-layer board, the signal layer on top uses the distance to the internal ground plane, not the 1.6 mm total thickness. Another mistake is ignoring solder mask: the formulas above assume bare copper. Solder mask can lower impedance by 2–5 Ω at high frequencies. Also, these formulas are approximations valid for W/H between 0.1 and 3.0 and Er between 1 and 15. For extreme geometries, use a field solver.

Many designers also confuse microstrip and stripline. Microstrip has one reference plane; stripline is buried between two planes. Stripline generally offers better signal integrity (less crosstalk, no radiation), but requires more layers and has a higher cost.

Limitations of the Formula

The IPC-2141A approximations assume a uniform dielectric, infinitely wide ground plane, and negligible trace thickness effects (though T is included in the width correction). They do not account for:

• Frequency-dependent dielectric constant (Er decreases slightly with frequency)
• Surface roughness of copper
• Adjacent traces or ground fills
• Non-uniform dielectric (e.g., prepreg vs. core)

For controlled impedance (e.g., 50 Ω ±10%), most fabs require a specific stack-up and will tune the trace width themselves. The calculator only gives a starting point.

Alternatives to PCB Way’s Tool

Other free impedance calculators include those from EEWeb, Sierra Circuits, and various online PCB fabs like JLCPCB or Eurocircuits. Many EDA tools (KiCad, Altium, Eagle) have built-in impedance calculators or integrate with field solvers. For high-speed design (above 1 GHz), use a 2D field solver like Polar Si8000 or open-source tools like ATLC or KiCad’s transmission line calculator.

Frequently Asked Questions

What is the difference between microstrip and stripline impedance?

Microstrip is a trace on an outer layer with one reference plane below; stripline is an inner-layer trace sandwiched between two reference planes. Stripline impedance is lower for the same trace width because the electric field is more confined. The calculator uses different formulas for each, so you must select the correct type.

Why does my fab’s calculator give a different number?

PCB manufacturers use their own stack-up materials and often use more accurate field-solving methods. They also account for solder mask, etch factor, and glass weave. Always use the fab’s recommended width for controlled impedance.

Can I use this calculator for differential pairs?

No, the formulas here are for single-ended impedance only. Differential impedance depends on the spacing between the two traces and coupling effects. You need a differential impedance calculator or a field solver.

What is the correct Er for FR-4?

It varies between 4.2 and 4.8 depending on frequency and resin content. At 1 GHz, 4.3 is a common approximation. Ask your fab for the exact value they use.

How do I convert mils to millimeters?

1 mil = 0.0254 mm. So a 6 mil trace is 0.1524 mm. Most calculators accept mils, but you can convert if needed.

Does solder mask affect impedance?

Yes, solder mask adds a thin dielectric layer on top of the trace, which lowers impedance slightly (typically 2–5 Ω). The formulas ignore this, so for tight tolerances, use a calculator that includes solder mask or rely on your fab.

Related

• NTSC vs PAL DVD: Which Format Should You Burn?
• COUNTIFS with Variable Table Column?
• Forza Horizon 6 Tokyo Tower Photo Challenge Location

in w | Trace width (W) | mil | 6 | 0.1 | 1000
in t | Copper thickness (T) | mil | 1.4 | 0.01 | 100
in h | Dielectric height (H) | mil | 8 | 0.1 | 1000
in er | Dielectric constant (Er) | - | 4.3 | 1 | 100
in d | Distance to next plane (H2) | mil | 20 | 0.1 | 1000
out z0 | Microstrip impedance | ohm | (87 / sqrt(er + 1.41)) * ln(5.98 * h / (0.8 * w + t)) | 1
out z0s | Stripline impedance | ohm | (60 / sqrt(er)) * ln(4 * h / (0.67 * (0.8 * w + t))) | 1