Ohm’s Law Calculator
Enter any two of voltage, current, resistance or power — the other two are solved instantly.
Ohm’s-law wheel
Enter two values
How Ohm’s Law Calculator works
Ohm’s law states that the current through a conductor is proportional to the voltage across it and inversely proportional to its resistance. Written as V = I × R, it is the single most-used relationship in electronics: know any two of voltage, current, and resistance and the third follows.
Power is the natural fourth quantity. Since power is voltage times current, substituting Ohm’s law gives three equivalent expressions — P = V × I, P = I² × R, and P = V² / R — which is why entering any two of the four quantities determines the other two. The choice of which power formula to use is a matter of which values you already have, not of correctness.
The I²R form is the one worth internalising, because it explains why current, not voltage, drives heating. Doubling the current through a resistor quadruples the power it dissipates. That is why a component runs cool at one operating point and destroys itself at another that looks only slightly more demanding.
Ohm’s law applies to ohmic components — resistors, wires, heating elements — where resistance is constant. It does not describe diodes, LEDs, transistors, or any semiconductor junction, whose current rises exponentially with voltage. For those you use the device’s own characteristic curve, and apply Ohm’s law only to the resistors around them.
Reference
- V = I × R
- I = V / R
- R = V / I
- P = V × I = I² × R = V² / R
How to use this calculator
Enter any two quantities
Fill in two of voltage, current, resistance, or power. The two you edited most recently drive the calculation.
Use engineering units
Enter values with their natural prefixes — mA, kΩ, mW — rather than converting to base units by hand.
Read the derived pair
The remaining two quantities are solved instantly and update as you type.
Check the power figure
Compare the dissipated power against your resistor’s rating, and derate to roughly half of it for a comfortable margin.
Worked examples
Current through a pull-up
- Given
- 5 V across a 10 kΩ resistor
- Result
- I = 0.5 mA, P = 2.5 mW
A quarter-watt resistor is 100 times oversized here. Pull-ups are chosen for noise immunity and speed, not power.
Sizing a load resistor
- Given
- A 12 V supply and a wanted current of 250 mA
- Result
- R = 48 Ω, P = 3 W
Three watts in a resistor is a real thermal design problem — you need a 5 W part with airflow, not a standard axial resistor.
Why current dominates heating
- Given
- The same 10 Ω resistor at 1 A, then at 2 A
- Result
- 10 W, then 40 W
The I² term means a 2× current increase is a 4× power increase. Most thermal failures come from underestimating exactly this.
When to use it
- Sizing a current-limiting or load resistor for a known supply voltage.
- Checking whether a resistor’s power rating is adequate before committing to a design.
- Working out the current a circuit will draw from a supply to size the supply itself.
- Measuring an unknown resistance indirectly from a voltage and current reading.
- Sanity-checking a bench measurement that seems wrong.
Things to watch out for
- Ohm’s law holds only for ohmic components. Diodes, LEDs, and transistors have exponential characteristics and cannot be modelled with a single resistance value.
- Resistance rises with temperature in most conductors, so a part running hot draws less current than the cold-state calculation predicts. Copper changes roughly 0.4% per °C.
- Derate resistors to about half their rated power in still air. Ratings assume ideal conditions that a crowded board does not provide.
- Real supplies have output impedance and real wires have resistance. At high currents the voltage at the load is measurably lower than at the supply terminals.
Frequently asked questions
What are the four Ohm’s law formulas?
V = I × R, I = V / R, and R = V / I are the three rearrangements of Ohm’s law itself. Adding power gives P = V × I, P = I² × R, and P = V² / R, which is why knowing any two of the four quantities determines the other two.
Does Ohm’s law work for LEDs?
No. An LED is a diode whose current rises exponentially with voltage above its forward threshold, so it has no fixed resistance. Ohm’s law applies to the series resistor you place with it — which is exactly what the LED resistor calculator sizes.
How do I choose a resistor power rating?
Calculate the dissipated power with P = I² × R, then pick a part rated at roughly twice that. Manufacturer ratings assume free air at 25 °C, and a resistor packed among other components on a board will run considerably hotter.
Why is my measured current lower than calculated?
Real circuits include resistance the calculation ignored: wire and trace resistance, connector contact resistance, the supply’s own output impedance, and the burden resistance of the ammeter itself. Each drops a little voltage that never reaches the load.
Why does current matter more than voltage for heating?
Because dissipation follows I² × R. Current appears squared, so doubling it quadruples the heat, while the resistance term is fixed. This is also why power transmission uses high voltage and low current over long distances.
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