PCB impedance calculator

Microstrip     Hammerstad and Jensen (1980), u = w/h widened for copper thickness t
               Z0 = Z01(u) / √εe(u),  εeff from the same model (see Microstrip note)
Pair Z0        Z0 = 87 / √(εr + 1.41) · ln( 5.98h / (0.8w + t) )   (IPC-2141)
Embedded       εr′ = εr · (1 − e^(−1.55 h1 / h))
               Z0 = 60 / √εr′ · ln( 5.98h / (0.8w + t) )
Stripline      Z0 = 60 / √εr · ln( 4b / (0.67π (0.8w + t)) ),  b = plane to plane
Diff. micro.   Zdiff = 2 Z0 · (1 − 0.48 e^(−0.96 s/h))
Diff. strip.   Zdiff = 2 Z0 · (1 − 0.347 e^(−2.9 s/b))
CPWG           k = w/(w + 2g),  k3 = tanh(πw/4h) / tanh(π(w + 2g)/4h)
               εeff = (R(k) + εr R(k3)) / (R(k) + R(k3)),  R(k) = K(k)/K(k′)
               Z0 = 60π / ( √εeff · (R(k) + R(k3)) )
Delay = √εeff / c

Valid range

Surface microstrip: the Hammerstad and Jensen model covers the usual PCB range of w/h and εr without a stated cut-off here. Microstrip pairs and embedded: 0.1 < w/h < 3.0 and 1 < εr < 15. Stripline: t/h < 0.25 (h trace to plane) and w/(b − t) < 0.35 (b plane to plane). Outside these the result is marked.

Microstrip

Surface microstrip uses Hammerstad and Jensen (1980) with the copper-thickness correction. Its effective εr matches TI SCAA082A Table 2 within 0.05 %. Its Z0 is within about 1 % of a 2D field solver on 0.1 to 0.35 mm lines over 0.2 mm FR-4. The IPC-2141 form used before read 3.5 to 6 % low there.

Accuracy

Stripline (IPC-2141) is within 1 % of a 2D field solver over most of its valid range, w/(b − t) below 0.35, and more than 30 % off below 0.05 (LearnEMC). Pairs: see below.

Pairs, coupling

Differential stripline uses the IPC-2141 coupling constant 0.347; some tools print 0.374, which reads 0.7 to 1.2 % lower on typical pairs.

Embedded

The εr′ form above (IPC-D-317A) is the one most PCB tools adopted. A primary copy of it could not be checked, and no field-solver accuracy figures are published for it here. LearnEMC uses a different IPC-2251 draft form for embedded microstrip, rated within 20 % of a field solver for w/h below 1.0. Treat the embedded result as a rough estimate.

Pairs

Microstrip pairs use the IPC-2141 single-ended Z0 with its coupling term, the form the coupling constants were fitted with. The coupling terms are empirical fits with no published validity range: against a 2D field solver the microstrip pair read 2.5 to 4 % low. Effective εr and delay are shown for the single-ended line (Hammerstad and Jensen).

CPWG

Conformal mapping with a lower ground plane, zero copper thickness, side grounds wide, air above. K is evaluated exactly by the arithmetic-geometric mean.

Presets

JLC04161H-7628: 7628 prepreg 0.2104 mm, εr 4.4, 0.035 mm outer copper. OSH Park 4-layer: FR408HR 2113 prepreg 0.1999 mm, Dk 3.61 at 1 GHz, 0.0432 mm outer copper. Both read from the fab pages on 13 October 2026.

Assumes

Rectangular trace, no solder mask on surface lines, a single homogeneous εr at one frequency, smooth copper. The fab builds to its own field solver; send them the target impedance, not just a width.