Calculators / 12
Microstrip and Stripline Impedance Calculator
A trace's impedance is set by how much capacitance and inductance it has per unit length, and both come from geometry: wider traces and thinner dielectrics add capacitance and lower the impedance. These are the closed forms that field solvers are checked against. Use them to get close, then confirm with your fabricator's solver, which knows the real stackup.
Reading the numbers
- Impedance falls with width roughly logarithmically: doubling the width takes off far less than half the ohms.
- εeff is the Dk the wave actually sees. For microstrip it is lower than the laminate's, because part of the field is in air.
- Fabricators etch trapezoids, not rectangles, and plating adds copper. Expect their solver to differ by a few ohms.
Every value is in the address bar as you change it, so a link reproduces the calculation exactly.
Type values with SI prefixes — 100n, 2.2p, 16G — or
drag the sliders; a bare number keeps the prefix already shown.
The formulas
Go deeper — assumptions, how it is checked, and sources
Assumptions. Quasi-static: no dispersion, which raises microstrip's εeff a little at high frequency. Rectangular copper, a homogeneous dielectric, and no solder mask — mask lowers a surface trace's impedance by a few ohms. Stripline is centred and zero-thickness; real copper reads a few ohms lower.
How it is checked. For microstrip, Hammerstad–Jensen and Wheeler's independently derived 1977 formula agree within 1% across widths and Dk values, εeff is exactly 1 in air and lies between (εr+1)/2 and εr, and a 50 Ω line on εr 4.4 lands on the widely published w/h. For stripline, a very wide strip approaches the parallel-plate limit, and the result agrees with the IPC-2141 approximation within its stated range.
Sources. E. Hammerstad and O. Jensen, “Accurate models for microstrip computer-aided design,” IEEE MTT-S Int. Microwave Symp. Digest, 1980; H. A. Wheeler, “Transmission-line properties of a strip on a dielectric sheet on a plane,” IEEE Trans. Microwave Theory Tech., vol. 25, no. 8, 1977; S. B. Cohn, “Characteristic impedance of the shielded-strip transmission line,” IRE Trans. Microwave Theory Tech., vol. 2, no. 2, 1954.