How this calculator works
Resistance grows with conductor length and decreases with cross-sectional area. Resistivity is the material property in ohm metres, while conductivity is its reciprocal in siemens per metre. Conductance is the reciprocal of the actual wire resistance. These are bulk DC estimates for uniform material. Temperature, alloy composition, contact resistance and AC skin effect can substantially alter measured results. Approximate glass, carbon, silicon and polymer values vary especially widely.
Formula
R = rho l/A; A = pi d²/4; conductivity = 1/rho; conductance = 1/R.
Worked example
A material with resistivity 2 × 10^-8 ohm m, length 10 m and area 1 mm² has resistance 0.2 ohm and conductance 5 S. At the same resistivity, 0.5 ohm over 2 mm² corresponds to 50 m.
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Frequently asked questions
Should I double the cable run length?
If current travels out and back through two identical wires, use the combined conductor length to find loop resistance.
Does this include connection resistance?
No. Add terminal, splice and contact resistances separately.
Are conductivity and conductance the same?
No. Conductivity describes a material; conductance describes a particular component including its geometry.