Calculator
Example Data Table
Use these sample values to test different models quickly.
| Scenario | Model | Main Inputs | Typical Range |
|---|---|---|---|
| Overlapping pads | Parallel Plate | Area 100 mm², gap 0.2 mm, εr 4.2 | 0.01–5 pF |
| Shielded cable | Coaxial | a 0.5 mm, b 1.5 mm, length 50 mm, εr 2.1 | 1–200 pF |
| Adjacent leads | Parallel Wires | r 0.25 mm, D 2.0 mm, length 100 mm | 0.1–50 pF |
| Trace over reference | Trace Over Plane | h 0.8 mm, r 0.2 mm, length 50 mm, εr 4.2 | 0.2–30 pF |
| Known parts list | Lumped Sum | Pin 1.2 pF, trace 0.8 pF, via 0.5 pF | 0.5–100 pF |
Formula Used
- Parallel plate: C = ε0·εr·A / d
- Coaxial: C = (2π·ε0·εr·L) / ln(b/a)
- Parallel wires: C′ = (π·ε0·εr) / acosh(D / (2r)), then C = C′·L
- Trace over plane: C′ = (2π·ε0·εr) / acosh(h / r), then C = C′·L
- Lumped sum: Ctotal = ΣCi
These are fast engineering approximations. Real layouts also include fringing fields, nearby conductors, solder mask, and frequency-dependent effects.
How to Use This Calculator
- Select the geometry model that matches your structure.
- Enter εr for the dielectric material in your field region.
- Fill in dimensions with the correct units.
- Click Calculate to view the result above the form.
- Use Download CSV or Download PDF for reporting.
FAQs
1) What is parasitic capacitance?
It is unintended capacitance formed by nearby conductors and dielectrics. It can slow signals, shift resonant frequencies, and increase coupling noise.
2) Which model should I choose?
Use Parallel Plate for overlapping areas, Coaxial for shielded cylinders, Parallel Wires for adjacent leads, Trace Over Plane for a conductor above a reference, and Lumped Sum when you already know individual parasitics.
3) What εr value should I enter for FR‑4?
A common quick choice is 4.2. In practice εr varies with resin/glass mix, frequency, and manufacturer. If you have a datasheet value, prefer that.
4) Why does the calculator warn about geometry limits?
Some equations assume certain spacing relationships. When inputs violate them, the math becomes unstable. The tool slightly adjusts values so you still get a usable estimate.
5) How accurate are these results?
They are first-order estimates meant for design decisions and comparisons. For tight tolerance designs, validate with field solvers, measurement, or vendor models.
6) Can I combine geometry results with lumped items?
Yes. Calculate the geometry capacitance first, then add it as an item in the Lumped Sum list to combine with package, pin, and via parasitics.
7) What units does the result use?
The output is shown in engineering form, such as pF, nF, or µF depending on magnitude. The details table also shows the value in farads.
8) Why is “equivalent radius” used for a trace?
The trace-over-plane model uses a cylindrical approximation. Converting a flat trace to an “equivalent” radius gives a practical estimate. A rough starting point is width divided by four.