Wire Resistance Calculator

Every practical conductor resists the motion of electric charge. For a uniform round wire, resistance is calculated with R = ρL/A. In this expression, R is resistance in ohms, ρ is the material resistivity in ohm-metres, L is conductor length in metres, and A is cross-sectional area in square metres. The calculator derives area from the entered diameter using A = π(d/2)², taking care to convert millimetres to metres before applying the resistance formula. The default resistivity, 1.724 × 10⁻⁸ Ω·m, represents annealed copper near 20°C. The field is expressed in units of 10⁻⁸ Ω·m so a copper value can be entered as 1.724 rather than a long decimal. Aluminum is commonly approximated near 2.82, silver near 1.59, and other alloys may be much higher. Material purity, manufacturing method, and temperature affect actual values, so use a supplier specification when precision matters. Once resistance is known, the calculator applies Ohm's law to estimate voltage drop as V = IR. It also calculates heating loss with P = I²R. Both values use the current entered by the user. The entered length is used exactly once, making this tool suitable for evaluating a single conductor or component. For a two-wire supply-and-return circuit, enter the combined conductor length or calculate each conductor separately. This distinction is important because omitting the return path can understate circuit voltage drop by half. Resistance generally increases as a metal conductor gets hotter. Connections, splices, terminals, strand geometry, alternating-current skin effect, and installation conditions can add effects not represented by this simple direct-current model. The result is therefore most useful for comparisons, preliminary designs, laboratory exercises, and checking measured values. Electrical installations must still meet applicable ampacity, insulation, temperature, and voltage-drop requirements. Consult code tables and a qualified professional for safety-critical wiring.