Cable Impedance Calculator
Calculate the characteristic impedance of a coaxial or twisted-pair cable from its physical dimensions, or find the AC impedance of any power or signal cable from its resistance and reactance components. Results include capacitance per metre, inductance per metre, propagation delay, inductive and capacitive reactance, and voltage drop. Switch between cable types and metric or imperial units - everything updates instantly.
What is cable impedance?
Cable impedance is the opposition a cable presents to an alternating or high-frequency signal. For transmission lines such as coaxial and twisted-pair cables, the key figure is the characteristic impedance (Z0) - the ratio of voltage to current for a travelling wave, determined entirely by the cable geometry and the dielectric material between the conductors, not by the cable length. For power cables, the relevant quantity is the total AC impedance, which combines the resistive voltage drop with the inductive and capacitive reactances at the operating frequency. Matching source, cable, and load impedance minimises signal reflections and maximises power transfer.
Coaxial cable: how characteristic impedance is calculated
A coaxial cable has a centre conductor of diameter d surrounded by a dielectric, and an outer shield whose inner diameter is D. The characteristic impedance is Z0 = (60 / sqrt(er)) x ln(D/d), where er is the relative permittivity of the dielectric and ln denotes the natural logarithm. The distributed capacitance is C = 2*pi*eps0*er / ln(D/d) farads per metre, and the distributed inductance is L = (mu0 / (2*pi)) x ln(D/d) henries per metre. Propagation delay is td = sqrt(er) / c0 seconds per metre, where c0 is the speed of light in vacuum. Standard coaxial impedances are 50 Ohm (RF/microwave) and 75 Ohm (video/CATV), achieved by choosing appropriate D/d ratios and dielectric materials.
Twisted-pair cable: characteristic impedance formula
In a balanced twisted pair, two conductors of diameter d are arranged with a centre-to-centre spacing s. The characteristic impedance is Z0 = (120 / sqrt(er)) x ln(2s/d). Capacitance per metre is C = pi*eps0*er / ln(2s/d) and inductance per metre is L = (mu0 / pi) x ln(2s/d). The standard impedance for Ethernet Cat 5e/6/6A cable is 100 Ohm, achieved with a specific ratio of conductor diameter to spacing and a polyethylene or FEP dielectric. RS-485 industrial bus cables are nominally 120 Ohm. Twisting the conductors tightly together cancels electromagnetic interference (EMI) because both wires receive virtually identical noise, which the differential receiver then rejects.
AC power cable: resistance, reactance, and voltage drop
For power and low-frequency signal cables, total AC impedance is Z = sqrt(R^2 + (XL - XC)^2), where R = r x length is the total resistance, XL = 2*pi*f*L*length is the inductive reactance, and XC = 1 / (2*pi*f*C*length) is the capacitive reactance. At power frequencies (50/60 Hz) the capacitive reactance of typical cables is enormous compared with the inductive reactance, so it can often be ignored - leaving Z dominated by R and XL. The voltage drop across a cable carrying current I is V = I x Z, and the resistive power loss is P = I^2 x R. Most electrical codes limit voltage drop to 3 to 5 percent of the supply voltage; oversizing the conductor cross-section is the standard remedy.
Common cable impedance standards
| Impedance (Ohm) | Cable / standard | Typical application |
|---|---|---|
| 50 | RG-58, RG-8, LMR-400 | RF test equipment, antenna feeders, GPS, Wi-Fi |
| 75 | RG-6, RG-59 | Cable TV (CATV), video distribution, satellite IF |
| 93 | RG-62 | ARCNET, IBM 3270 terminals |
| 100 | Cat 5e, Cat 6, Cat 6A | Ethernet 100BASE-TX, 1000BASE-T, 10GBASE-T |
| 110 | Cat 3 UTP, Cat 5 UTP | Voice and older data networks |
| 120 | STP twisted pair | RS-485, RS-422, CAN bus, industrial fieldbus |
| 150 | IBM Type 1 STP | Token Ring, STP-A applications |
| 300 | Twin-lead | TV/FM antenna lead-in (legacy) |
Standard characteristic impedances used in RF, data, and broadcast cabling applications.
Frequently asked questions
What is characteristic impedance and why does it matter?
Characteristic impedance (Z0) is the impedance a transmission line presents to a travelling electromagnetic wave. When the impedance of the source, the cable, and the load all match, the signal travels without reflections. Any mismatch creates reflected waves that cause signal degradation, standing-wave patterns, and reduced power transfer. In RF systems even a small mismatch can measurably increase the voltage standing wave ratio (VSWR) and reduce efficiency.
What dielectric constant should I use?
Common values: solid polyethylene (PE) - 2.25 to 2.35; foam PE - 1.40 to 1.55; solid PTFE (Teflon) - 2.05 to 2.10; PVC - 3.0 to 4.5; air (ideal) - 1.0. The cable datasheet will always list the exact value or the velocity of propagation (VoP), from which er = (1 / VoP)^2.
Why is 50 Ohm the standard for RF?
Air-dielectric coaxial cable has minimum signal loss at around 77 Ohm and maximum power handling at around 30 Ohm. The 50 Ohm standard was adopted as a practical compromise between these two, close enough to both to serve the widest range of RF applications. 75 Ohm was chosen for broadcast and CATV distribution where long cable runs favour lower loss over maximum power handling.
How do I convert velocity of propagation to dielectric constant?
Velocity of propagation (VoP) is usually given as a percentage of the speed of light. Convert it to a decimal (e.g. 66% becomes 0.66) then use er = 1 / VoP^2. For example, a VoP of 0.66 gives er = 1 / 0.66^2 = 2.30.
What limits cable length in RF applications?
At higher frequencies, conductor skin-effect losses and dielectric losses both increase, so the maximum practical cable length shrinks as frequency rises. The cutoff frequency shown by this calculator is a separate upper limit: above it, higher-order waveguide modes can propagate inside the coaxial cable alongside the TEM mode, distorting the signal. Both effects set an upper frequency limit for any given cable diameter.
How do I reduce voltage drop in a power cable?
Voltage drop is proportional to cable resistance, which decreases with a larger conductor cross-section. Increasing the cross-section from 2.5 mm^2 to 6 mm^2 reduces resistance (and voltage drop) by roughly 2.5 times. Shortening the cable run, reducing the load current, or raising the supply voltage are other levers. For three-phase systems the drop is lower by a factor of sqrt(3) compared with single-phase for the same power.