Cable Impedance Calculator
Calculate characteristic impedance, capacitance, and inductance for coaxial and twisted pair cables
Calculate Cable Impedance
Select the type of cable for impedance calculation
Diameter of the inner conductor
Inner diameter of the outer shielding
Relative permittivity of the insulation material (typical: 2.0-4.0)
Input Error
Inner conductor diameter must be greater than 0
Common Cable Examples
RG-58 Coaxial
Type: Coaxial
Inner: 0.9 mm
Outer: 2.95 mm
~50Ω
RG-59 Coaxial
Type: Coaxial
Inner: 0.58 mm
Outer: 3.7 mm
~75Ω
CAT5e UTP
Type: Twisted Pair
Inner: 0.51 mm
Spacing: 1.02 mm
~100Ω
CAT6 UTP
Type: Twisted Pair
Inner: 0.57 mm
Spacing: 1.14 mm
~100Ω
RG-6 Coaxial
Type: Coaxial
Inner: 1.02 mm
Outer: 4.57 mm
~75Ω
RG-213 Coaxial
Type: Coaxial
Inner: 2.17 mm
Outer: 7.24 mm
~50Ω
Common Dielectric Materials
Standard Cable Impedances
50Ω Coaxial
RF applications, test equipment
75Ω Coaxial
Video, cable TV, CATV
100Ω Twisted Pair
Ethernet, data communications
120Ω Twisted Pair
Differential signals, RS-485
Measurement Tips
Use calipers for accurate diameter measurements
Check manufacturer specifications for dielectric values
Account for conductor plating in measurements
Temperature affects dielectric properties
Understanding Cable Impedance
What is Impedance?
Impedance is the effective resistance that a material presents to the flow of alternating current (AC). For cables, characteristic impedance determines how electromagnetic waves propagate through the transmission line.
Why It Matters
- •Signal integrity in high-frequency applications
- •Impedance matching prevents reflections
- •Maximum power transfer efficiency
- •Reduced crosstalk and interference
Cable Types
Coaxial Cable
Central conductor surrounded by dielectric and outer shield. Used for RF and video signals.
Twisted Pair
Two insulated wires twisted together. Common in data communications and telephone systems.
Key Parameters
d: Inner conductor/wire diameter
D: Outer shield inner diameter (coaxial)
s: Wire center-to-center spacing (twisted pair)
εᵣ: Relative permittivity of dielectric
Formula Derivation
Coaxial Cable:
Z₀ = (60/√εᵣ) × ln(D/d)
Based on distributed LC parameters
L = (μ₀/2π) × ln(D/d)
C = (2πε₀εᵣ) / ln(D/d)
Z₀ = √(L/C)
Twisted Pair:
Z₀ = (120/√εᵣ) × ln(2s/d)
Approximation for parallel wires
L = (μ₀/π) × ln(2s/d)
C = (πε₀εᵣ) / ln(2s/d)
Valid when s >> d