Electrical Engineering

Ohm's Law Calculator

Solve for voltage, current, or resistance using Ohm's Law, with power and wattage checks called out as the next step.

Formula V = I × RReviewed Sep 9, 2026

Ohm's Law describes the relationship between voltage, current and resistance in a simple electrical circuit: voltage equals current multiplied by resistance. Enter any two of the three values to solve for the third. This calculator is intended for basic DC circuit analysis and general reference - it assumes an ideal resistive load.

Calculation Bench

Solve for

01Current

I · Rate of electric charge flow through the circuit.

02Resistance

R · Opposition to current flow presented by the load.

Solution

Enter the required values to calculate voltage.

V = I × R

Formula Sheet

V = I \times R

I = \dfrac{V}{R}

R = \dfrac{V}{I}

VVoltage
ICurrent
RResistance

Variables & Units

Symbol	Variable	Description	Common Units
V	Voltage	Electrical potential difference across the circuit.	V, mV, kV
I	Current	Rate of electric charge flow through the circuit.	A, mA
R	Resistance	Opposition to current flow presented by the load.	Ω, kΩ, MΩ

How to Use This Calculator

01Choose which quantity to solve for - Voltage, Current, or Resistance - using the solve-for selector.
02Enter the two known values in their fields, picking the correct unit for each (for example mA vs A for current).
03After solving, check the related power result with the Electrical Power Calculator or use a resistor wattage margin before choosing a real part.
04Select Calculate to see the result, along with the formula substitution showing exactly how it was computed.
05Change any input and recalculate to explore how the result responds - useful for sizing a resistor or checking an expected current draw before wiring a circuit.

How the Formula Works

Ohm's Law states that the current through a conductor between two points is directly proportional to the voltage across those points, and inversely proportional to the resistance between them: V = I × R. Rearranging that single equation gives the other two forms used above - I = V / R and R = V / I - so any two known quantities are enough to find the third.

Physically, resistance acts as a restriction on current flow for a given voltage: doubling the resistance halves the current for the same voltage, while doubling the voltage doubles the current for the same resistance. This linear relationship holds for ohmic materials - resistors and simple conductors - but not for components like diodes or transistors, whose effective resistance changes with the applied voltage or current.

Worked Example 01

Solving for voltage

Known

Current (I): 2 A
Resistance (R): 6 Ω

Formula

V = I × R

Substitution

V = 2 × 6

Result

V = 12 V

A 2 A current through a 6 Ω resistor produces a 12 V potential difference across it.

Worked Example 02

Solving for current

Known

Voltage (V): 120 V
Resistance (R): 15 Ω

Formula

I = V / R

Substitution

I = 120 / 15

Result

I = 8 A

A 120 V supply connected across a 15 Ω resistance draws 8 A of current.

Worked Example 03

Solving for resistance

Known

Voltage (V): 9 V
Current (I): 0.5 A

Formula

R = V / I

Substitution

R = 9 / 0.5

Result

R = 18 Ω

A 9 V battery driving a 0.5 A current through a load implies an 18 Ω resistance.

Applications

01Basic circuit analysis and troubleshooting
02Verifying resistor, wire, or component ratings
03Estimating current draw for a known load

Assumptions

01The load behaves as an ideal, linear (ohmic) resistor.
02The circuit is a steady-state DC circuit.
03Temperature-dependent resistance changes are not modeled.

Where This Model Stops

01Does not apply to AC circuits with reactance (inductors, capacitors) - use impedance instead of plain resistance.
02Not valid for non-ohmic components such as diodes or transistors, whose resistance varies with voltage/current.
03Does not check resistor wattage, wire ampacity, circuit breaker sizing, heat rise, tolerance, or supply current limits.

References

[1]
Ohm's Law
Standard electrical engineering and physics fundamentals
V = IR is a foundational relationship in circuit theory.

Frequently Asked Questions

Does Ohm's Law apply to AC circuits?

Only for purely resistive AC loads. Circuits with inductance or capacitance require impedance (a complex quantity) instead of plain resistance.

Why do I only enter two values?

Ohm's Law relates three quantities with one equation - any two values are enough to solve for the third.

Why do many Ohm's Law calculators include power too?

Power comes from the companion equation P = V × I. This page focuses on the V-I-R relationship, then links to the Electrical Power Calculator for wattage checks so the sizing step is not hidden.

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