Ohm's Law Calculator - Solve for V, I & R

Parameters

Solve Ohm's Law (V=IR) for voltage, current, resistance, or power. Built-in unit conversions (mV, µA, kΩ) and power formulas for DC circuits.

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Enter known values

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Try a preset example (optional)

This solves V = I × R exactly for the values entered. Real components differ from their marked values by their tolerance and change with temperature, which this does not model, so measure a circuit before relying on a calculated value. Mains voltage can kill: leave work on mains wiring to a licensed electrician.

Formulas and sources

Each formula this calculator uses, where it comes from, and what it leaves out. "How it works" shows the same formula with your own numbers.

Ohm's law

Voltage (V), Current (I), Resistance (R) and Power (P) modes

V = I × R
V
voltage, in volts
I
current, in amperes
R
resistance, in ohms

I = V ÷ R and R = V ÷ I are the same relation rearranged. Every value is a magnitude: DC, or AC RMS values on a purely resistive load.

Source: Uses the SI's definition of the ohm as one volt per ampere (BIPM) [1]. The IEC defines resistance as the same quotient and says it cannot be negative [2]. IEC 80000-6:2022 writes voltage as U [3]. The Human Body example takes its current ranges from OSHA 3075 and assumes the skin resistances, which OSHA does not give [4].

Power

Voltage (V), Current (I), Resistance (R) and Power (P) modes

P = I² × R = V × I
P
power, in watts
I
current, in amperes
R
resistance, in ohms
V
voltage, in volts

Source: Uses the SI's definition of the volt as one watt per ampere [1], with Ohm's law above.

Power from voltage and resistance

Voltage (V), Current (I), Resistance (R) and Power (P) modes

P = V² ÷ R
P
power, in watts
V
voltage, in volts
R
resistance, in ohms

The third form, used when voltage and resistance are the values you know. The wheel shows all twelve forms of the four values.

Source: Ohm's law substituted into P = V × I, with the same SI units [1].

Sources

  1. Bureau International des Poids et Mesures (BIPM). The International System of Units (SI), section 2.3.4, Table 4, p. 133. 9th edition (2019), version 4.01, June 2026.
  2. International Electrotechnical Commission (IEC). IEC 60050-131, International Electrotechnical Vocabulary, Circuit theory, IEV 131-12-04, resistance. Electropedia entry, read October 5, 2026.
  3. International Electrotechnical Commission (IEC). IEC 80000-6, Quantities and units, Electromagnetism, item 6-11.3, voltage, electric tension, p. 13. Edition 2.0 (2022).
  4. Occupational Safety and Health Administration (OSHA). Controlling Electrical Hazards, OSHA 3075, "Effects of Electric Current in the Human Body", p. 7. 2002 (revised).

What this calculator does not cover

  • This applies V = I × R to one resistance. It does not model reactance, parts whose resistance changes with current (LEDs, diodes, lamp filaments) or the rise of resistance with temperature.
  • For AC circuits with capacitors or inductors the relation uses impedance (Z). The LC Circuit calculator covers resonance and impedance.

Sources checked on .

Help

Quick Start Guide

  1. Choose what to calculate: Select Voltage, Current, Resistance or Power: the one you want to solve for.
  2. Enter known values: Input any two known values, power included, such as 1,200 W and 120 V. All SI units (V, A, Ω, W).
  3. View results: See all four electrical values: voltage, current, resistance, and power, plus the interactive Ohm's Law wheel.
  4. Try a preset example (optional): Load a real-world scenario like an LED series resistor or car headlight to see the formulas applied.
  5. Switch calculation target: Click a quadrant of the wheel above the result to solve for another value, or open How it works for the equation with your own numbers in it.

Frequently Asked Questions

Ohm's Law states that V = I × R (Voltage = Current × Resistance). It describes how current flows through a conductor proportional to voltage and inversely proportional to resistance.

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