Estimate first-pass characteristic impedance for single-ended and differential PCB traces across microstrip and stripline geometry modes.
Stackup and trace geometry
Estimate first-pass PCB controlled impedance from trace geometry and laminate Dk (Er).
Single-ended mode calculates characteristic impedance, effective dielectric constant (Er or Dk), velocity factor, and delay per length. Microstrip mode treats an external trace over one reference plane and estimates effective dielectric constant (Er or Dk) from W/H.
Characteristic impedance (Ω)Output
ΩUnit: Ω
Tolerance range: 59.28 to 75.26 Ω
Use metres as the natural unit for dimensions. Values such as 150µ, 0.15m, and 35µ are accepted where supported by the engineering parser.
Controlled-impedance results
Single-ended microstrip estimate.
Impedance and propagation context
Characteristic impedance66.89ΩRange: 59.28 to 75.26 Ω
Trace width150µmRange: 135µ to 165µ m
Effective dielectric constant (Er or Dk)3.078Range: 2.9039 to 3.2603
Velocity factor0.57×cRange: 553.83m to 586.83m ×c
Delay per millimetre5.852ps/mmRange: 5.684p to 6.023p s/mm
Delay per inch148.6ps/inRange: 144.4p to 153p s/in
W/H ratio1Range: 818.18m to 1.2222
Transmission-line parameters per mm
Trace resistance per mm3.284mΩ/mmRange: 2.714m to 4.054m Ω/mm
Trace capacitance per mm87.49fF/mmRange: 75.53f to 101.6f F/mm
Trace inductance per mm391.4pH/mmRange: 356.6p to 428.3p H/mm
PCB signal integrity
Choose structure and geometry before trusting the number
Controlled impedance depends on both the signal structure and the reference-plane geometry. This calculator separates single-ended versus differential traces from microstrip versus stripline construction so those decisions stay visible.
Single-ended
Estimate Z₀, effective dielectric constant (Er or Dk), velocity factor, propagation velocity, and delay per length for one trace over or between reference planes.
Differential / coupled pair
Add pair spacing to estimate differential impedance, odd-mode impedance, even-mode impedance, common-mode impedance, and coupling coefficient.
Microstrip and stripline
Use microstrip for external traces over one plane. Use stripline for internal traces between two planes under a homogeneous dielectric assumption.
Calculation model and solved forms
The calculator uses closed-form quasi-static PCB transmission-line estimates. It can solve impedance, trace width, or differential pair spacing where the selected mode supports it.
Core relationships
First-pass impedance relationships
Microstrip effective dielectric constant
External traces use an effective dielectric constant because fields travel partly in air and partly in laminate.
Differential impedance
Differential mode applies an empirical spacing correction to the single-ended estimate.
Velocity from effective dielectric constant
Effective dielectric constant also gives first-pass propagation velocity and delay per length.
Variables and outputs
Z0: Characteristic impedance
Unit: Ω
Single-ended transmission-line impedance estimated from the selected geometry.
Zdiff: Differential impedance
Unit: Ω
Estimated differential impedance for an edge-coupled trace pair.
Zodd: Odd-mode impedance
Unit: Ω
Mode impedance used by differential signalling and crosstalk workflows.
Zeven: Even-mode impedance
Unit: Ω
Mode impedance used together with odd-mode impedance to estimate coupling and common-mode context.
Ereff: Effective dielectric constant (Er or Dk)
Unit: ratio
Equivalent dielectric constant used to estimate velocity for microstrip or stripline propagation.
R': Trace resistance per millimetre
Unit: Ω/mm
Estimated DC conductor resistance per millimetre from copper resistivity, trace width, and copper thickness.
C': Trace capacitance per millimetre
Unit: F/mm
Derived capacitance per millimetre from the solved characteristic impedance and propagation velocity.
L': Trace inductance per millimetre
Unit: H/mm
Derived inductance per millimetre from the solved characteristic impedance and propagation velocity.
W/H: Width-to-height ratio
Unit: ratio
Trace width relative to dielectric height for the selected geometry.
Model boundary
This is a screening calculator. It does not model solder mask, copper roughness, frequency dispersion, glass weave, etch compensation, resin-rich layers, vias, pads, connectors, manufacturing coupons, or impedance-test tolerances.
Use it to start layout and stackup conversations, then confirm with the fabricator and solver-backed data.
Worked examples
These examples show how the same calculator moves between single-ended and differential controlled-impedance checks.
Worked example
Single-ended microstrip
A 150µm external trace over 150µm dielectric on Dk 4.2 laminate gives a first-pass Z₀ estimate and velocity context.
Inputs
Geometry
W = 150µm, H = 150µm, T = 35µm, Er = 4.2
Mode
Single-ended microstrip
Equation and substitution
Use result as
A first-pass trace-width and stackup estimate.
Next check
Confirm with fabricator stackup and impedance solver data.
Worked example
Differential microstrip
The same geometry with 150µm edge-to-edge spacing estimates Zdiff plus Zodd and Zeven for coupled-line workflows.
Inputs
Geometry
W = 150µm, S = 150µm, H = 150µm, T = 35µm, Er = 4.2
Mode
Differential microstrip
Equation and substitution
Use result as
Early differential-pair width and spacing guidance.
Next check
Use solver-backed Zodd and Zeven for critical pairs.
Design follow-up
Next engineering checks
Use these follow-up checks before turning the calculated value into a component choice, layout decision, or production limit.
Confirm the fabricator stackup
Use actual dielectric thickness, resin content, copper thickness, solder mask, and Dk from the PCB manufacturer before release.
Use a field solver for signoff
Closed-form equations are useful for first-pass layout work. Controlled-impedance coupons, solver data, or measurement should drive final release values.
Check discontinuities
Vias, pads, neck-downs, layer changes, connectors, packages, and return-path interruptions can dominate real impedance even when the straight trace is close.
Feed critical coupled-line data forward
Use solver-backed Zodd and Zeven values for critical differential pairs and for the engineering mode in the crosstalk calculator.
Related calculators and next checks
Follow the next check based on whether the controlled-impedance result affects timing, crosstalk, edge-rate bandwidth, wavelength, or unit conversion.
Does this controlled impedance calculator replace a field solver?
No. It is a first-pass screening calculator for early stackup and routing decisions. Use the PCB fabricator stackup, solver-backed impedance data, coupons, or measurement for release decisions.
When should I use microstrip instead of stripline?
Use microstrip for an external PCB trace over a reference plane. Use stripline for an internal trace between reference planes under a mostly homogeneous dielectric assumption.
What is the difference between single-ended and differential mode?
Single-ended mode estimates characteristic impedance for one trace. Differential mode also uses pair spacing to estimate differential impedance, odd-mode impedance, even-mode impedance, common-mode impedance, and coupling context.
Can I use this to choose final PCB trace widths?
Use it to start the conversation and sanity-check geometry. Final widths should come from the actual manufacturer stackup, etch compensation, solder mask assumptions, copper thickness, impedance tolerance, and solver or coupon data.
Is PDN target impedance the same as controlled trace impedance?
No. PDN target impedance is a power-integrity rail target from allowed voltage droop and transient current. Controlled trace impedance is a signal-integrity transmission-line property from stackup and trace geometry.
