Quick answer
Ohm's law is V = I × R: voltage equals current times resistance. With 12 V across 6 Ω the current is 12 ÷ 6 = 2 A, and the power is V × I = 24 W. Leave one of the fields blank and the calculator solves for it.
Enter any two of voltage, current and resistance. The calculator returns the third.
Related tools: Wattage Calculator: Solve power, voltage or current for a DC or resistive load.
How to use the Ohm's Law Calculator
- Optionally, enter voltage (V) (V).
- Optionally, enter current (I) (A).
- Optionally, enter resistance (R) (Ω).
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Voltage (V) (optional) | A number, in V. Leave blank to solve for this value | — |
| Current (I) (optional) | A number, in A. Leave blank to solve for this value | — |
| Resistance (R) (optional) | A number, in Ω. Leave blank to solve for this value | — |
Formula and method
Formula
V = I × R
IEEE-754 double precision; shown to at most 6 decimal places.
Fill in any two values and leave the third blank; the blank one is calculated. Use consistent units.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Voltage (V) = 12; Resistance (R) = 6.
| Result | Value |
|---|---|
| Current | 2 A |
Example 2
Inputs: Current (I) = 2; Resistance (R) = 6.
| Result | Value |
|---|---|
| Voltage | 12 V |
Example 3
Inputs: Voltage (V) = 12; Current (I) = 2.
| Result | Value |
|---|---|
| Resistance | 6 Ω |
The first line is the headline result (current). IEEE-754 double precision; shown to at most 6 decimal places.
The triangle of formulas
V = I × R, I = V ÷ R and R = V ÷ I. Power follows from P = V × I = I² × R = V² ÷ R, so a 6 Ω resistor on 12 V dissipates 24 W; it needs a part rated well above that.
When it applies
Ohm's law describes ohmic components such as resistors at a constant temperature. Diodes, LEDs and bulbs do not have a constant resistance. Exactly one field must be blank, and dividing by zero is rejected.
How the result changes with voltage
Holding every other input at the first worked example, this table changes only voltage. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| Voltage | Current |
|---|---|
| 6 | 1 A |
| 12 | 2 A |
| 24 | 4 A |
| 48 | 8 A |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Voltage (V): entering "1" shows "Leave exactly one field blank and fill in the other two." beside that field and no result.
Choosing between the Ohm's Law Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Wattage Calculator](/engineering/wattage/) | Power | |
| [Voltage Drop Calculator](/engineering/voltage-drop-calculator/) | current, one-way length and wire gauge | Voltage drop |
| [Wire Size Calculator](/engineering/wire-size/) | current, one-way length and supply voltage | Smallest AWG for the voltage drop |
| [Three-Phase Power Calculator](/engineering/three-phase-power-calculator/) | line-to-line voltage, line current and power factor | Real power |
| [Parallel Resistor Calculator](/engineering/parallel-resistor-calculator/) | resistances | Parallel resistance |
Background: electrical circuits
Ohm's law links voltage, current and resistance: V = I × R, so I = V ÷ R and R = V ÷ I. Power is P = V × I, which also equals I² × R and V² ÷ R.
These relations hold for ohmic components, where resistance is constant. Diodes and transistors are non-linear, and in AC circuits with reactive parts impedance (Z) takes the place of resistance. Check component ratings and local electrical code before relying on any calculation.
Assumptions and limitations
- Exactly one field must be left blank
- Division by zero is rejected
Use results as engineering estimates and check them against the governing standards, safety factors and a qualified engineer's judgement before relying on them.
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for voltage (V) to confirm this.
Frequently asked questions
What are the units?
Volts, amperes and ohms.
Can I find power?
Multiply voltage and current.
Why does a bulb not follow it?
Its resistance rises as the filament heats.
What does the Ohm's Law Calculator calculate?
Solve voltage, current or resistance. It returns Current.
What formula does the Ohm's Law Calculator use?
V = I × R. Fill in any two values and leave the third blank; the blank one is calculated.
What do I need to enter in the ohm's Law Calculator?
You enter . Optional: Voltage (V) (V), Current (I) (A) and Resistance (R) (Ω).
How is the result rounded or limited?
IEEE-754 double precision; shown to at most 6 decimal places. Exactly one field must be left blank.
What are the limits of this calculator?
Exactly one field must be left blank. Division by zero is rejected.
What is an example of a ohm's Law result?
With voltage (V) 12 and resistance (R) 6, the current is 2 A.
Where does the method come from?
The method follows: Ohm GS, Die galvanische Kette, mathematisch bearbeitet (1827).
Can I rely on this calculator for a decision?
Use results as engineering estimates and check them against the governing standards, safety factors and a qualified engineer's judgement before relying on them. For this calculator in particular: Exactly one field must be left blank.
Related calculators
- Wattage Calculator — Solve power, voltage or current for a DC or resistive load.
- Three-Phase Power Calculator — Real, apparent and reactive power of a balanced three-phase load.
- Parallel Resistor Calculator — Equivalent resistance of resistors in parallel and in series.
- Watt-Hours Calculator — Energy in watt-hours from power and time, or from amp-hours and voltage.
- LC Resonant Frequency Calculator — Resonant frequency of an LC circuit.
- Battery Runtime Calculator — Runtime of a battery under a constant load.
- kW to VA Converter — Apparent power in VA from real power and power factor.
Sources
- Ohm GS, Die galvanische Kette, mathematisch bearbeitet (1827)
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.