Coaxial Cable Capacitance Calculator
Introduction: What the Coaxial Capacitance Calculator Does
A coaxial cable behaves like a long cylindrical capacitor: the center conductor is one plate, the shield is the other plate, and the dielectric between them stores electric field energy. This calculator uses the ideal coaxial-capacitor equation to solve for one missing value. Enter the four quantities you know and leave exactly one field blank.
The tool can solve for total capacitance C, cable length L, inner conductor radius a, shield inner radius b, or relative permittivity epsilonr. Use meters for all dimensions and farads for capacitance. If you are working from a cable datasheet that lists capacitance per meter or per foot, multiply by length first before entering total capacitance, or leave length blank to infer the equivalent run length.
Formula
For a uniform coaxial geometry, capacitance is:
Here, epsilon0 is the vacuum permittivity, approximately 8.854 x 10-12 F/m. The radius a is the radius of the center conductor. The radius b is the inside radius of the outer conductor or shield, not the outside jacket radius. The formula requires b > a > 0.
Worked Example
Suppose a 1 m cable has an inner conductor radius of 0.45 mm, a shield inner radius of 1.50 mm, and a polyethylene-like dielectric with relative permittivity 2.25. Convert the radii to meters: a = 0.00045 m and b = 0.00150 m. The calculator evaluates:
C = 2π(8.854 x 10-12)(2.25)(1) / ln(0.00150 / 0.00045)
The result is about 1.04 x 10-10 F, or 104 pF per meter. That is in the range of common 50-ohm coaxial cables, though actual datasheet values also depend on dielectric construction and manufacturing tolerances.
How Geometry Changes Capacitance
| Change | Effect | Reason |
|---|---|---|
| Increase cable length | Capacitance increases linearly | More length means more electric field volume. |
| Increase dielectric permittivity | Capacitance increases linearly | A higher-permittivity material stores more electric field energy. |
| Move shield farther from center conductor | Capacitance decreases | The logarithmic term ln(b/a) gets larger. |
| Increase center conductor radius while shield is fixed | Capacitance increases | The logarithmic spacing ratio gets smaller. |
Limitations and assumptions: Assumptions and Limits
This is an electrostatic idealization. It assumes a perfectly concentric cable, a uniform dielectric, negligible end effects, and no frequency-dependent loss. Real cables include braided or foil shields, foamed dielectrics, conductor surface roughness, dielectric loss tangent, and tolerances that shift capacitance from the ideal value. For RF design, also check characteristic impedance, velocity factor, attenuation, and manufacturer datasheets.
Input sanity checks
Leave exactly one field blank. If you are solving for capacitance, the known length, radii, and dielectric constant must all be positive and the shield inner radius must be larger than the center conductor radius. If you are solving for geometry, confirm the result is physically buildable before using it as a design dimension.
When converting datasheet values, keep track of units. Many coaxial cables list capacitance in pF/ft or pF/m. To enter total capacitance, multiply the per-length value by the cable length and convert picofarads to farads. To solve for length from a per-meter datasheet value, enter the target total capacitance and the cable geometry from the datasheet.
Practical interpretation
Higher capacitance is not automatically better or worse. In timing circuits, sensor cables, and high-impedance measurements, extra capacitance can slow edges or load the signal. In RF transmission lines, capacitance is only one part of the cable behavior and must be considered with inductance, impedance, dielectric loss, and shielding.
Document whether dimensions came from a datasheet, a drawing, or physical measurement. A small error in the radius ratio changes the logarithmic denominator, and using jacket diameter instead of shield inner diameter can make the result misleading.
When comparing cables, normalize to capacitance per unit length. Total capacitance depends on length, so two cables with different runs cannot be compared fairly until they are converted to the same length basis.
How to use this calculator
- Enter Capacitance C (F) using the unit or time period shown by the field.
- Enter Length L (m) using the unit or time period shown by the field.
- Enter Inner Radius a (m) using the unit or time period shown by the field.
- Run the calculation and compare the output with a second scenario before acting on it.
Arcade Mini-Game: Coaxial Cable Capacitance Calculator Calibration Run
Use this quick arcade run to practice separating useful scenario inputs from common planning mistakes before you rely on the calculator output.
Start the game, then use your pointer or arrow keys to catch useful inputs and avoid bad assumptions.