Coaxial Cable Capacitance Calculator

Treating a coax run as one long cylindrical capacitor

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 epsilon_r. 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.

The ln(b/a) relation and why it looks that way

Because the field between the conductors spreads radially rather than staying parallel, the two radii enter through their ratio inside a logarithm rather than a simple gap distance. 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.

Working a one-meter RG-style run by hand

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

The same ln(b/a) sets impedance, so C per meter is predictable

The logarithm that fixes capacitance also fixes the cable's characteristic impedance. For a lossless line, Z₀ = (1/2π)√(μ/ε) × ln(b/a), while capacitance per length is 2πε / ln(b/a). The same ratio appears in both, once on top and once on the bottom, so for a fixed dielectric a cable with higher impedance always carries lower capacitance per meter. That is why 50-ohm cables run near 100 pF/m and 75-ohm cables run closer to 67 pF/m: the wider radius ratio that raises the impedance is the same ratio that thins out the capacitance. If a datasheet gives you impedance and dielectric but not capacitance, you can still sanity-check this tool's answer, because the two numbers are locked together by geometry rather than being free to vary independently.

This coupling is also a useful debugging trick. If you solve for a radius here and the implied impedance drifts far from the cable's rated 50 or 75 ohms, one of your inputs is almost certainly a jacket radius, a diameter entered as a radius, or a permittivity for the wrong dielectric.

Typical capacitance for common cable types

The values below are representative solid-dielectric figures. Foamed or air-spaced dielectrics lower the effective permittivity and therefore the capacitance, so always defer to the actual datasheet for a specific part.

Notice that both 50-ohm entries land near the same capacitance even though their overall diameters differ. Capacitance per meter follows the radius ratio and the dielectric, not the absolute size of the cable, so a thin RG-174 and a fatter RG-58 that share an impedance and a dielectric come out close together. When your computed result lands well outside these bands, treat it as a prompt to recheck which radius you measured and which dielectric constant you assumed.

Where the ideal model stops matching real cable

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.

Filling in the fields

1. Decide which quantity you want back — capacitance, length, either radius, or permittivity — and leave that one field empty.
2. Enter the four values you know. Put capacitance in farads (a 100 pF reading is 1e-10), length and both radii in meters, and permittivity as a plain ratio such as 2.25 for solid polyethylene.
3. Convert millimeter dimensions before typing them: 0.45 mm becomes 0.00045 m. Use the shield's inner radius, not the outside jacket radius.
4. Press Compute Missing Value. If you are solving for a radius or permittivity, check that the answer is physically buildable, then try a second geometry to see how sensitive the result is to the b/a ratio.