How this calculator works
Choose the cross section before entering dimensions. The microstrip and symmetric stripline equations are IPC-style logarithmic estimates; coupled edge models apply exponential spacing corrections. Wire models use homogeneous dielectric image formulas. Embedded mode includes dielectric cover height. Asymmetric mode uses the nearer and farther plane clearances in the offset-stripline fit. Broadside mode deliberately restricts itself to wide plates, using ground and mutual parallel-plate capacitances, so it is unsuitable for ordinary narrow broadside routing. These models omit solder mask, dispersion, copper roughness and detailed fringing. A fabrication stackup field solver is needed for controlled-impedance sign-off.
Formula
Microstrip Z = 87/sqrt(er+1.41) ln[5.98H/(0.8W+T)]. Symmetric stripline Z = 60/sqrt(er) ln[1.9B/(0.8W+T)]. Edge estimates: Zodd = Z(1-k), Zeven = Z(1+k), Zdiff = 2Zodd, Zcommon = Zeven/2. Broadside: Zeven = (377/sqrt(er))H/W; Zodd = (377/sqrt(er))/[W(1/H+2/S)].
Worked example
For W = 20 mil, H = 10 mil, T = 1.4 mil and er = 4, the logarithmic microstrip estimate is about 46.2 ohms. At equal stripline clearances, the offset formula reduces to B = 2H + T and the symmetric-stripline coefficient.
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Frequently asked questions
Are these fabrication-ready impedances?
No. The formulas are estimates with limited geometry ranges. Use the board supplier stackup and a validated field solver for final dimensions.
Why does broadside mode reject a narrow trace?
Its parallel-plate model neglects edge fringing and requires trace width much larger than the dielectric gaps.
Why are there four coupled impedances?
Odd and even modes describe one conductor under opposite or equal drive. Differential impedance is twice odd mode; common-mode pair impedance is half even mode.