Long extension cords make power tools sluggish for a reason you can calculate: wire has resistance. A conductor's resistance equals its material resistivity times its length divided by its cross-sectional area, R = ρ × (L/A). Double the length and you double the resistance; double the wire's thickness and you quarter it — which is why the fat cable feeding your house dwarfs the hair-thin wire in your earbuds.

This calculator runs the geometry half of that formula. Type the metal's resistivity ρ in ohm-meters into Variable A — copper is 1.68 × 10^-8, aluminum 2.82 × 10^-8 — and the length-over-area ratio L/A into Variable B, with length in meters and area in square meters; the Result box gives the resistance in ohms. An electrician in Texas running 150 feet of 12-gauge copper to a backyard shed, for instance, computes L/A from the wire tables, multiplies here, and checks the voltage drop stays under the 3% rule. Swap copper for aluminum in Variable A and the same run's resistance jumps by two-thirds — the reason aluminum feeders must be upsized.

With the pre-filled defaults — Variable A 10, Variable B 5 — the Resistance Calculator returns Result 50.0000. Change any input and those results update instantly, so you can see exactly how each number you enter moves the answer.