Ohm’s law calculator (voltage, current, resistance and power)
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Result
A small through-hole resistor is usually rated for 0.25 W. Choose a resistor rated for at least about twice the power it dissipates.
Find the voltage, current, resistance and power of a resistor from any two of them. Choose which two values you know and the calculator applies Ohm’s law, V = I·R, together with the power formula P = V·I = I²·R = V²/R. Results are given with sensible unit prefixes such as mA and kΩ, with every step written out and checked.
Step-by-step solution
Ohm’s law and the power formula
Calculate the two unknowns
Check
Key formulas
- Ohm's law
- V = I·R
- Power
- P = V·I = I²R = V²/R
How to use Ohm’s law
- Write down the two quantities you know, in volts, amperes, ohms and watts. Convert prefixes first: 4.7 kΩ = 4700 Ω and 20 mA = 0.02 A.
- If you know two of V, I and R, use V = I·R, rearranged as I = V/R or R = V/I, to find the third.
- For the power use P = V·I. Substituting Ohm’s law gives P = I²·R and P = V²/R, which save a step when V or I is unknown.
- If you know the power and one other quantity, rearrange a power formula: I = P/V, V = √(P·R) or I = √(P/R).
- Check the answer: I·R must give back V and V·I must give back P.
Questions and answers
What is Ohm’s law?
For many conductors, such as metal-film resistors at constant temperature, the current is proportional to the voltage: V = I·R. The constant R is the resistance in ohms. Components such as diodes, LEDs and lamps do not follow Ohm’s law, because their resistance changes with the current.
How do I remember the formulas?
Use the triangle with V at the top and I and R at the bottom: cover the quantity you want. Covering V leaves I·R, covering I leaves V/R and covering R leaves V/I. The power triangle works the same way with P at the top and V and I at the bottom.
Why does the calculator mention the power rating?
A resistor turns the power P = I²·R into heat. Small through-hole resistors are usually rated for 0.25 W. Choosing one rated for about twice the calculated power keeps it from running hot.
Does Ohm’s law work for AC circuits?
Yes, for resistors, with RMS values of voltage and current. With capacitors and inductors the resistance is replaced by the impedance Z, and the power also depends on the power factor.
Practise it yourself
Practise Series & parallel resistors
A calculator gives the answer. In the exam you need the method: practise it with new problems, hints and worked solutions.