Formula Used
The calculator treats the microstrip as a quasi TEM line. It first applies a practical finite thickness width correction.
W effective = W + ΔW
For narrow lines, W/H ≤ 1:
εeff = (εr + 1) / 2 + (εr - 1) / 2 × [1 / √(1 + 12H/W) + 0.04(1 - W/H)²]
Z0 = 60 / √εeff × ln(8H/W + W / 4H)
For wider lines, W/H > 1:
Z0 = 120π / [√εeff × (W/H + 1.393 + 0.667ln(W/H + 1.444))]
The characteristic capacitance is:
C′ = √εeff / (c × Z0)
C total = C′ × length × number of parallel lines
How to Use This Calculator
- Enter trace width, substrate height, copper thickness, and units.
- Add trace length and material dielectric constant.
- Enter frequency, loss tangent, conductivity, and roughness factor.
- Set tolerance values for width, height, and dielectric spread.
- Press calculate to show capacitance, impedance, delay, and loss.
- Use CSV or PDF export after the result appears.
Example Data Table
| Board Case |
W |
H |
εr |
Length |
Approx Z0 |
Approx C′ |
Total C |
| FR4 single trace |
3.0 mm |
1.6 mm |
4.4 |
100 mm |
49.94 Ω |
121.93 pF/m |
12.19 pF |
| Low loss digital line |
1.2 mm |
0.8 mm |
3.48 |
50 mm |
61.85 Ω |
88.04 pF/m |
4.40 pF |
| Thin RF substrate |
0.45 mm |
0.254 mm |
2.2 |
25 mm |
67.66 Ω |
66.57 pF/m |
1.66 pF |
Microstrip Capacitance Design Notes
Why microstrip capacitance matters
A microstrip line stores electric energy between a copper trace and its reference plane. That stored energy appears as characteristic capacitance. It works with inductance to set impedance, velocity, and edge timing. A small geometry change can move capacitance enough to affect RF filters, clock routes, and sensor front ends.
How the estimate is made
This calculator uses common closed form equations for practical board estimates. It first converts all dimensions to meters. Then it applies an optional trace thickness correction. The corrected width is compared with substrate height. That ratio selects the narrow or wide microstrip impedance equation. Effective permittivity is then found from dielectric constant and field fringing.
Understanding the outputs
The key output is capacitance per unit length. It is useful when you need lumped loading, delay, or energy estimates. Total capacitance is found by multiplying by trace length and line count. The tool also reports phase delay, wavelength, electrical length, and approximate loss. These outputs help compare stackups before field solving or layout review.
Using target and tolerance options
Use the target impedance option when you know the required line impedance. The solver searches for a width that meets the target. It also returns the matching capacitance. Tolerance inputs show how copper width, dielectric height, and dielectric constant may shift results. This is important for high speed boards, because fabrication spread can move timing and matching.
Practical limits
The loss values are planning estimates. Real boards include solder mask, copper roughness shape, weave, surface finish, and discontinuities. Use the roughness multiplier to add margin. For final RF signoff, verify the route with a field solver or measured coupon data.
Design interpretation
A lower impedance line is usually wider. It has higher capacitance per meter. A thinner substrate also increases capacitance. A higher dielectric constant slows the wave and raises capacitance. Short routes may still have small total capacitance, but they can matter on fast edges. Check both capacitance and electrical length before deciding whether the line behaves as a lumped load or a transmission line.
Documentation tips
The example table gives starting points only. Use your real stackup, copper thickness, and frequency for final estimates. Keep units consistent. Round results for communication, not for design storage. Save the exported report with the board revision so later reviews can trace each assumption and production lot data.
FAQs
What is microstrip characteristic capacitance?
It is the capacitance per unit length of a microstrip transmission line. It depends on trace width, substrate height, dielectric constant, and field fringing around the trace.
Why is effective permittivity lower than dielectric constant?
Some electric field travels through air above the trace. The line therefore sees a mixed medium. Effective permittivity represents that combined field path.
Does copper thickness change the capacitance?
Yes. Thicker copper slightly increases effective width. That can raise capacitance and lower impedance, especially on thin substrates or narrow traces.
Can this replace a field solver?
No. It is useful for planning and early design checks. Final RF work should use a field solver, stackup data, and measured coupons when accuracy is critical.
What does the target impedance option do?
It searches for a trace width that gives the selected impedance. The result also shows the matching capacitance for that width.
Why enter frequency?
Frequency is used for wavelength, electrical length, surface resistance, and loss estimates. The basic capacitance formula itself is mostly geometry based.
How should I use tolerance results?
Use them as a quick fabrication spread check. They show how width, height, and dielectric variation may move capacitance from the nominal design.
What conductivity should I use for copper?
A common copper value is 58,000,000 S/m. Use your material supplier value when plating, finish, temperature, or roughness makes the conductor less ideal.