Microstrip Impedance Calculator (IPC-2141)
Characteristic impedance for a trace on an outer layer above a ground plane, per IPC-2141.
- Characteristic impedance
- 53.52 Ω
- W/H ratio
- 1.500
- Formula
- IPC-2141A microstrip
IPC-2141A microstrip: Z₀ = (87 / √(εr + 1.41)) × ln(5.98·H / (0.8·W + T)).
W = trace width, H = dielectric height above the reference plane,T = copper thickness (1 oz = 0.035 mm). Valid for W/H roughly 0.1–3.0. Outside that range, use a field solver (e.g. Polar SI9000) for critical designs.
50 Ω single-ended microstrip on FR4
Typical 4-layer stackup: 0.3 mm (12 mil) trace over 0.2 mm (8 mil) dielectric to inner ground plane with 1 oz copper gives ~53 Ω on FR4 (εr ≈ 4.3). Tune width down to ~0.35 mm or increase dielectric height for exactly 50 Ω.
When you need controlled impedance
Any signal with edge rate faster than ~2 ns needs impedance control. This includes USB, Ethernet, PCIe, DDR, SATA, HDMI, MIPI, and CSI/DSI. Low-speed signals (I²C, SPI, UART) usually don't require it, though long traces still benefit from careful routing.
Frequently asked
- What is characteristic impedance?
- The ratio of voltage to current in a traveling wave on a transmission line. For PCB traces at high edge rates, matching source/trace/load impedance prevents reflections that corrupt signals.
- How to get 50 Ω on FR4?
- On a typical 4-layer stackup with 1 oz copper and 0.2 mm dielectric to inner ground, a trace around 0.35 mm wide gives ~50 Ω. Exact number depends on your stackup — use this calculator with your fab's stackup.
- Do I need controlled impedance for USB 2.0?
- Yes — USB uses a differential pair with 90 Ω differential impedance. Use the differential pair calculator for that.
- Microstrip vs stripline — which to use?
- Microstrip is on an outer layer over one ground plane. Stripline is buried between two ground planes. Stripline has less EMI radiation and better impedance control; microstrip is easier to route and probe.
Impedance only holds if the board is built to the stackup you calculated. Set the layer count and widths up front and PCBEditor routes to them.
Design a controlled-impedance board →Related free tools
- Stripline Impedance Calculator (IPC-2141)Characteristic impedance for a trace buried between two ground planes, per IPC-2141.
- Differential Pair Impedance CalculatorDifferential and common-mode impedance for edge-coupled microstrip and stripline pairs (USB, Ethernet, LVDS).
- PCB Layer Stackup Planner (2/4/6/8-Layer)Plan a PCB layer stack with copper weights, dielectric heights, FR4 Dk values, and finished thickness — 2 to 8 layers.