What this calculator does
Ohm’s law links voltage, current and resistance in any simple circuit: V = IR. Knowing any two of the three lets you find the third, and once all three are known, power follows directly as well, since power is voltage times current. This ohms law calculator solves for whichever pair of values are missing, picking the correct rearrangement automatically based on which two quantities were supplied.
The four quantities involved, voltage, current, resistance and power, are connected by more than one formula: P = VI, but also P = I²R and P = V²/R, both of which follow from substituting Ohm’s law into the basic power formula. This calculator reports all four figures together, so the full picture of a simple circuit is available from just two starting numbers.
The formula
Pick which two of voltage, current, resistance and power are already known and enter their values. The calculator applies Ohm’s law, V = IR, together with the power relationships P = VI, P = I²R and P = V²/R, to work out the other two.
| Term | Meaning |
|---|---|
| V (voltage) | Electrical potential difference driving current through the circuit, in volts. |
| I (current) | The rate of charge flow through the circuit, in amps. |
| R (resistance) | How strongly the circuit resists current flow, in ohms. |
| P (power) | The rate of energy use or dissipation, in watts: P = VI. |
The inputs explained
| Field | What to enter |
|---|---|
| Known quantity 1 | The value of the first known quantity. |
| is | What the first known quantity is: voltage, current, resistance or power. |
| Known quantity 2 | The value of the second known quantity. |
| is | What the second known quantity is, which must be different from the first. |
When to use it
Sizing a resistor for an LED or component
Knowing the supply voltage and the current a component needs to draw gives the resistance required to protect it, along with the power the resistor will need to dissipate.
Checking a circuit against a power rating
Working out power from a known voltage and resistance shows whether a resistor, fuse or component is operating within its rated limits before it overheats or fails.
Working backwards from a power rating
A device’s power rating and its supply voltage together reveal the current it draws, which matters for choosing wiring, fuses or circuit breakers of an adequate rating.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How do current and power change with voltage, at a fixed resistance?
A fixed resistance, across a range of applied voltages.
| Voltage | Current | Power |
|---|---|---|
| 3 V | 0.0300 A | 0.0900 W |
| 5 V | 0.0500 A | 0.2500 W |
| 9 V | 0.0900 A | 0.8100 W |
| 12 V | 0.1200 A | 1.440 W |
| 24 V | 0.2400 A | 5.760 W |
| 48 V | 0.4800 A | 23.040 W |
How do current and power change with resistance, at a fixed voltage?
A fixed voltage, across a range of resistances.
| Resistance | Current | Power |
|---|---|---|
| 10 Ω | 1.200 A | 14.400 W |
| 47 Ω | 0.2553 A | 3.064 W |
| 100 Ω | 0.1200 A | 1.440 W |
| 220 Ω | 0.0545 A | 0.6545 W |
| 470 Ω | 0.0255 A | 0.3064 W |
| 1000 Ω | 0.0120 A | 0.1440 W |
Questions
What if I only know one quantity?
Ohm’s law needs two independent quantities to solve for the rest; with only one figure, there are infinitely many combinations of the other three that would fit, so a second value is always required.
Does Ohm’s law apply to all components?
It applies exactly to "ohmic" components, mainly resistors, where resistance stays constant regardless of voltage or current. Components like diodes, transistors and light bulbs are non-ohmic: their effective resistance changes with the conditions, so V = IR only holds approximately or at a single operating point for those.
Why does power depend on the square of voltage or current?
Because power itself is voltage times current, and Ohm’s law lets you substitute one for the other in terms of resistance. Replacing current with V/R in P = VI gives P = V²/R; replacing voltage with IR gives P = I²R. Both are exact restatements of the same P = VI relationship.
What is "energy over 1 hour" showing?
It converts the calculated power into kilowatt-hours for a full hour of continuous operation at that power level, which is the unit electricity is typically billed in, useful for a rough sense of running cost.
For the combined resistance of multiple resistors in series or parallel, see the resistors calculator.