AudioCalcs
Ohm's Law Calculator — Voltage, Current, Resistance & Power
Calculate voltage, current, resistance and power using Ohm's law for audio circuits.
How We Calculate This
Ohm's Law
V = I × R
Power Equations
P = V × I = V² / R = I² × R
Given any two of V, I, R, P, the other two can be calculated.
Cable Resistance & Power Loss
When you enter a non-zero Cable Resistance in Advanced Options it is added in series with the load, so the current drops and the reported Resistance (R) is the total circuit resistance (load + cable). The headline Total power from amp (V × I) is the power the amplifier delivers into the whole circuit. The calculator then splits this into Power to load (I² × R_load) — the watts reaching the speaker — and Cable loss (I² × R_cable), the watts dissipated in the cable. With zero cable resistance all three figures coincide and only a single Power (P) line is shown.
Frequently Asked Questions
Ohm's law states that the current through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance: V = I × R. Combined with the power equation (P = V × I), you can calculate any two unknowns from any two knowns.
A speaker's nominal impedance (e.g. 8 ohms) determines how much current it draws from an amplifier at a given voltage. A 100W amplifier into 8 ohms produces 28.28V RMS and draws 3.54A. The same amplifier into 4 ohms would attempt to deliver 200W.
Resistance is opposition to DC current (measured in ohms). Impedance includes resistance plus reactance from capacitors and inductors, and varies with frequency. A speaker's nominal impedance is its average impedance across the audio range.
P = V²/R means that doubling the voltage quadruples the power. In decibels these reconcile to the same number: doubling the voltage is +6 dB (20 × log₁₀(2) = 6.02 dB) and quadrupling the power is also +6 dB (10 × log₁₀(4) = 6.02 dB). The voltage rule uses 20 × log and the power rule uses 10 × log precisely so that the same physical change reads as the same dB.
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Last updated: February 2026
All calculations are estimates based on standard formulas. Always verify results for critical applications.