Introduction
Impedance is how much a circuit resists the flow of alternating current (AC). It combines resistance, which blocks current equally at all speeds, with reactance, which changes based on the signal frequency. Getting impedance right is a key part of electrical engineering — it affects everything from PCB trace design to filter circuits. Our Impedance Calculator helps you solve for impedance quickly in two common situations: PCB trace impedance and RLC circuit impedance.
In PCB trace mode, you can pick from 13 trace models, including microstrip, stripline, coplanar waveguide, and differential pairs. Enter your trace width, dielectric height, copper thickness, and dielectric constant, and the tool gives you the characteristic impedance, effective dielectric constant, propagation delay, and per-unit-length capacitance and inductance. You can also flip the calculation to find the trace width needed for a target impedance — a common task when designing controlled-impedance boards.
In RLC circuit mode, you can choose series or parallel configurations, or simple RL, RC, and LC combinations. Enter your resistance, inductance, capacitance, and frequency, and the calculator returns the total impedance magnitude, complex impedance, phase angle, inductive and capacitive reactance, resonant frequency, quality factor, and bandwidth. Both modes include interactive charts so you can see how impedance changes as you sweep trace width or frequency.
How to Use Our Impedance Calculator
This calculator has two modes: PCB Trace Impedance and RLC Circuit Impedance. Enter your circuit or trace details, and the tool will compute impedance values, phase angles, and other key electrical properties.
PCB Trace Impedance Mode
Calculation Direction: Choose whether you want to calculate the impedance from a known trace width, or find the trace width needed to hit a target impedance.
Trace Model: Pick the type of PCB trace you are working with. Options include microstrip, stripline, coplanar waveguide, and differential pairs. Use the filter buttons to narrow down your choices by category. A cross-section diagram will appear to show you the layout of your selected model.
Trace Width (W): Enter the width of the copper trace in millimeters. If you chose "Calculate Trace Width" mode, enter your target impedance instead.
Dielectric Height (H): Enter the distance in millimeters between the trace and the ground plane (or between ground planes for stripline models).
Copper Thickness (T): Enter the thickness of the copper trace in millimeters. A common value is 0.035 mm (1 oz copper).
Dielectric Constant (εr): Enter the relative permittivity of your PCB substrate material. For example, FR-4 is typically around 4.3.
Trace Spacing (S): For differential pair models, enter the edge-to-edge gap between the two traces in millimeters.
Gap to Coplanar Ground (G): For coplanar waveguide models, enter the distance in millimeters between the signal trace and the adjacent ground pour on the same layer.
Additional Height Parameters (H1, H2, Hc, Hd): Some models require extra dimensions such as asymmetric layer heights, coating thickness, or the dielectric spacing between broadside-coupled traces. Enter these values in millimeters as shown in the diagram.
Coating Dielectric Constant (εrc): For coated microstrip models, enter the relative permittivity of the solder mask or conformal coating layer.
RLC Circuit Impedance Mode
Circuit Configuration: Select how your components are connected. Choose from Series RLC, Parallel RLC, RL Only, RC Only, or LC Only. A circuit diagram will update to show your selected layout.
Resistance (R): Enter the resistance value and select the unit (Ω, kΩ, MΩ, or mΩ). If you need help understanding resistance relationships, our Ohm's Law Calculator can assist with basic voltage, current, and resistance calculations.
Inductance (L): Enter the inductance value and select the unit (H, mH, μH, or nH).
Capacitance (C): Enter the capacitance value and select the unit (F, mF, μF, nF, or pF). For standalone capacitor calculations such as energy storage and charge, see our Capacitor Calculator.
Frequency (f): Enter the signal frequency at which you want to calculate impedance, and select the unit (Hz, kHz, MHz, or GHz). If you need to convert between frequency and wavelength, our Wavelength Calculator can help.
After entering your values, click Calculate to see results including total impedance magnitude, complex impedance, phase angle, reactance, resonant frequency, quality factor, and bandwidth. An interactive chart will also display how impedance and phase change across a range of frequencies.
What Is Impedance?
Impedance is a measure of how much a circuit resists the flow of alternating current (AC). It is similar to resistance, but it also accounts for the effects of capacitors and inductors, which store and release energy as the current changes direction. Impedance is measured in ohms (Ω) and is written as a complex number with a real part (resistance) and an imaginary part (reactance). The real part represents energy lost as heat, while the imaginary part represents energy that is stored and returned to the circuit.
PCB Trace Impedance
When electrical signals travel along a copper trace on a printed circuit board (PCB), the trace acts like a tiny transmission line. The shape of the trace, the thickness of the copper, the height of the dielectric material (the insulating layer between the trace and the ground plane), and the dielectric constant of that material all affect how signals move through it. This is called the characteristic impedance of the trace, and it is usually written as Z₀.
Getting the right trace impedance matters a lot in high-speed digital and radio-frequency (RF) design. If the impedance is not matched between a source, the trace, and the load, signals can bounce back and forth along the trace. These reflections cause ringing, signal distortion, and data errors. Common target impedances are 50 Ω for single-ended traces and 100 Ω for differential pairs, though other values are used depending on the design standard. When sizing traces for current-carrying capacity rather than impedance matching, you may also want to check our Wire Size Calculator and Voltage Drop Calculator for related conductor sizing tasks.
Common PCB Trace Models
- Microstrip – A trace on the outer layer of a board with a ground plane below it. This is the most common type.
- Stripline – A trace sandwiched between two ground planes inside the board. It offers better shielding from noise.
- Coplanar Waveguide (CPW) – A trace with ground copper on either side of it on the same layer, sometimes with a ground plane below.
- Differential Pair – Two traces that carry opposite signals. The spacing between them controls the differential impedance.
RLC Circuit Impedance
An RLC circuit contains up to three types of components: a resistor (R), an inductor (L), and a capacitor (C). Each one affects impedance differently. A resistor provides a fixed opposition to current. An inductor's reactance increases with frequency, meaning it blocks high-frequency signals more. A capacitor's reactance decreases with frequency, meaning it blocks low-frequency signals more. For circuits that combine multiple resistors, our Parallel Resistor Calculator can help you find equivalent resistance values before entering them here.
When these components are connected in series, you add their impedances together. The total reactance is the difference between the inductive reactance (XL = 2πfL) and the capacitive reactance (XC = 1/(2πfC)). When they are connected in parallel, the math uses reciprocals, and the total impedance is generally lower than any single branch.
Resonance and Quality Factor
At a specific frequency called the resonant frequency (f₀ = 1 / (2π√(LC))), the inductive and capacitive reactances cancel each other out. In a series circuit, this makes the impedance drop to its lowest point (equal to R alone). In a parallel circuit, the impedance rises to its highest point at resonance. The quality factor (Q) tells you how sharp the resonance peak is. A high Q means a narrow, sharp peak, which is useful for filters and tuning circuits. The bandwidth is the range of frequencies around resonance where the circuit responds strongly, and it equals f₀ divided by Q. For audio applications where impedance matching and signal levels are important, you might also find our dB Calculator useful for working with decibel values.
Phase Angle
The phase angle describes whether the current leads or lags behind the voltage. When the circuit is mostly inductive, current lags voltage and the phase angle is positive. When it is mostly capacitive, current leads voltage and the phase angle is negative. At resonance, the phase angle is zero, meaning voltage and current are perfectly in sync. Understanding phase relationships is also important when working with voltage divider circuits, where impedance ratios determine output voltage levels at a given frequency. For broader electrical power calculations involving voltage and current, our Power Calculator and Amp Calculator are also helpful companion tools.