Transmission Line Impedance Calculator
Calculates PCB microstrip and stripline characteristic impedance using IPC-2141 formulas, and compares the result against common 50/75/90/100Ω targets.
FreeOnline Tool
Loading…
How to Use
- Select the transmission line type: Microstrip (outer-layer trace) or Stripline (inner-layer trace).
- Enter the dielectric constant εr (typical FR-4 value is 4.2–4.8, default 4.5).
- Enter the dielectric thickness H (distance from signal layer to reference plane, in mil).
- Enter the trace width W (in mil).
- Enter the copper thickness T (in mil; 1oz copper = 1.37 mil).
- The characteristic impedance Z0 is calculated and displayed in real time, along with the percentage deviation from four common target impedances (50Ω/75Ω/90Ω/100Ω). Deviations ≤5% show green (within recommended range), 5–15% show yellow (significant deviation), and >15% show red (redesign recommended).
- Click "Load Sample Data" to fill in the default example: εr=4.5, H=10mil, W=18mil, T=1.37mil.
Features
- Toggle between Microstrip and Stripline transmission line types, each computed with its corresponding IPC-2141 empirical formula.
- Microstrip formula: Z0 = (87 / √(εr+1.41)) × ln(5.98H / (0.8W+T)).
- Stripline formula: Z0 = (60 / √εr) × ln(4H / (0.67π(0.8W+T))).
- Four adjustable parameters — dielectric constant εr, dielectric thickness H, trace width W, copper thickness T — all support decimal input with live recalculation.
- Automatically compares the computed Z0 against 50Ω/75Ω/90Ω/100Ω common targets, showing percentage deviation with a three-tier color indicator (in range / significant deviation / large deviation).
- Displays the exact formula text used for the calculation for verification.
Use Cases
Controlled-impedance high-speed trace design
When routing USB, HDMI, or Ethernet traces requiring 50Ω/90Ω/100Ω single-ended or differential impedance control, work backward to find the needed trace width and dielectric thickness combination.
Stack-up estimation before aligning with the fab house
Estimate approximate trace width/spacing with a standard formula before submitting impedance control requirements to a PCB fabricator, reducing back-and-forth on engineering documents.
Comparing inner-layer (stripline) vs. outer-layer (microstrip) impedance
For the same layer count and dielectric thickness, compare impedance differences between microstrip and stripline at the same trace width to help decide which layer to route signals on.
RF/antenna feedline impedance verification
Verify whether the trace width design for a 50Ω feedline on an RF board meets the target impedance, avoiding reflection losses from impedance mismatch.
FAQ
Will the result differ from a PCB fab's own impedance calculator?
Yes, somewhat. This tool uses IPC-2141 empirical formulas, which are engineering estimates typically accurate to about ±10%. PCB fabricators' professional tools (e.g., Polar Si9000, Saturn PCB Toolkit) use more precise field-solver algorithms. Before final tape-out, defer to the fab's tool or a 2D field solver.
What value should I use for εr (dielectric constant)?
It depends on the substrate material. Standard FR-4 typically ranges from 4.2 to 4.8, and the tool defaults to 4.5 as a typical value. For high-frequency laminates (e.g., Rogers series), use the actual Dk value from the manufacturer's datasheet, which is usually much lower than FR-4.
How do I choose between microstrip and stripline?
Microstrip applies to traces on the outermost PCB layers (top/bottom), with air above and dielectric below. Stripline applies to inner-layer traces, sandwiched between dielectric and copper planes on both sides. The two use different formulas, so selecting the wrong type will produce inaccurate results.
What happens if trace width or copper thickness is 0 or negative?
The tool validates the denominator (e.g., 0.8W+T for microstrip); if invalid input makes the denominator ≤0, Z0 displays as "--" with a prompt to enter valid parameters, and no impedance value is given.