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.

How does Ohm's law apply to speakers?

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.

What is the difference between impedance and resistance?

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.

Why is power proportional to voltage squared?

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.

AudioCalcs

Ohm's Law Calculator — Voltage, Current, Resistance & Power

Calculate voltage, current, resistance and power using Ohm's law for audio circuits.

Known ValuesPick which two quantities you already know — the calculator solves for the other two

Voltage

Amplifier output voltage in volts RMS (e.g. 28.28 V = 100 W into 8 Ω)

Resistance

Speaker nominal impedance in ohms (typically 4, 8 or 16 Ω)

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

Related Calculators

dB Conversion Calculator

Convert between dBu, dBV, dBFS, dBm and voltage or power values instantly. An essential daily reference calculator for audio engineers.

Frequency to Note Calculator

Find the closest musical note to any frequency with cents deviation from equal temperament. Free online audio reference calculator.

Wavelength Calculator

Calculate the wavelength of any sound wave at a given frequency and air temperature. Useful for speaker placement and room acoustics.

Speed of Sound Calculator

Calculate the speed of sound in air at any given temperature and humidity level. Useful for delay time alignment and acoustic design.

Audio Power Calculator

Calculate audio power from voltage and impedance, or find voltage from power and impedance. Essential for amplifier and speaker work.

Last updated: February 2026

All calculations are estimates based on standard formulas. Always verify results for critical applications.