PCB Impedance Calculator — Microstrip & Stripline
Design controlled-impedance PCB traces for RF, USB, LVDS, DDR or any high-speed signal. Enter your trace width, substrate height and dielectric constant to get the characteristic impedance — and see how impedance changes as trace width varies.
Trace type
mm
mm
mm
Target 50 Ω for RF/high-speed signals, 90–100 Ω for USB/LVDS differential pairs
- 1
Effective trace width (mm)
0.8 × 0.15 + 0.035 = 0.155IPC-2141A accounts for trace thickness by adding a fraction to the width. - 2
Logarithm argument
5.98 × 0.2 ÷ 0.155 = 7.7161 - 3
Impedance factor 87 ÷ √(εr + 1.41)
87 ÷ √(4.5 + 1.41) = 35.787 - 4
Characteristic impedance (Ω)
35.787 × ln(7.7161) = 73.1
How does this calculator work?
PCB characteristic impedance is set by trace width, substrate height and dielectric constant. Microstrip: Z₀ = (87/√(εr+1.41)) × ln(5.98H/(0.8W+T)); Stripline: Z₀ = (60/√εr) × ln(4B/(0.67π(0.8W+T))). Target 50 Ω for RF and single-ended signals; 90–100 Ω for USB/LVDS differential pairs. Verify with your PCB manufacturer's actual stackup data.
Formula
How this is calculated
Characteristic impedance (Z₀) is a property of the trace geometry and substrate — it tells you the ratio of voltage to current for a wave travelling along the trace at high frequency. Mismatch between the trace impedance and the driver/load impedance causes reflections that degrade signal integrity; this is why RF, USB, HDMI, DDR and many other standards specify a target impedance (commonly 50 Ω single-ended or 90–100 Ω differential).
For a microstrip trace (sitting on the surface above a ground plane), this calculator uses the IPC-2141A empirical formula: Z₀ = (87 / √(εr + 1.41)) × ln(5.98H / (0.8W + T)). The formula is accurate to within about 2–5% for W/H < 1. For a stripline trace (buried between two ground planes), the IPC-2141 formula Z₀ = (60 / √εr) × ln(4B / (0.67π(0.8W + T))) is used, where B is the distance between the two planes.
The dielectric constant (εr) of FR4 is typically 4.2–4.8 at low frequency but falls to around 3.8–4.2 at GHz frequencies — use your PCB manufacturer's stackup data for critical designs. The curve plot shows how impedance changes across a range of trace widths for your substrate, so you can quickly find the width that hits your target. For production boards, order a controlled-impedance stackup from your fabricator and confirm with TDR measurement.
Frequently asked questions
On a typical 4-layer board with a 0.2 mm substrate height between top copper and the adjacent ground plane and 1 oz copper (0.035 mm thick), a microstrip width of roughly 0.37–0.40 mm gives 50 Ω with εr ≈ 4.5. Your exact stackup will differ — enter your manufacturer's values for a precise answer.
A wider trace is closer in behaviour to a parallel-plate capacitor — more surface area relative to the ground plane means more capacitance per unit length and therefore lower impedance (Z₀ ≈ √(L/C)). Narrower traces have less capacitance, more inductance per unit length, and higher impedance.
The IPC-2141A microstrip formula is accurate to within 2–5% for W/H < 1 and a homogeneous dielectric. A 3D field solver (Sonnet, HFSS, Ansys SI) will give better results for non-ideal conditions such as solder mask, nearby vias, or broadside-coupled traces. For high-volume or high-frequency (> 5 GHz) designs, always validate with a field solver or TDR measurement on a test coupon.
Also known as
TG we-Calculate Editorial Team. (2026). PCB Impedance Calculator — Microstrip & Stripline [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/pcb-impedance-calculator
TG we-Calculate Editorial Team. "PCB Impedance Calculator — Microstrip & Stripline." TG we-Calculate. 2026. https://we-calculate.com/calculator/pcb-impedance-calculator.
TG we-Calculate Editorial Team, "PCB Impedance Calculator — Microstrip & Stripline," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/pcb-impedance-calculator
@misc{wecalculate_pcb_impedance_calculator, title = {PCB Impedance Calculator — Microstrip & Stripline}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/pcb-impedance-calculator}}, year = {2026}, note = {TG we-Calculate} }
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