Enter any two of voltage, current, resistance and power and get the other two with Ohm’s law and the power law.
V
A
Ω
W
Enter the values to see the result here.
Formula
V=IRP=VI=I2R=RV2
with voltage V in volts, current I in amperes, resistance R in ohms and power P in watts. Each pair of known quantities gives the other two directly, for example R=V2/P from power and voltage.
Worked example
A lamp is rated 60 W at 120 V. What current does it draw and what is its hot resistance?
Current: I=P/V=60/120=0.5 A.
Resistance: R=V2/P=1202/60=240Ω.
Check: V=IR=0.5×240=120 V. On a 230 V supply the same lamp would dissipate 2302/240=220 W.
Common mistakes
Using millivolts or kilohms without converting. Convert to volts and ohms first, or type 4.7e3 for 4.7 kΩ.
Mixing peak and RMS values for AC.
Applying a constant resistance to a lamp or diode, whose resistance changes with temperature and voltage.
Forgetting that power depends on the square of voltage or current.
Frequently asked questions
What is Ohm's law?
It says the current through a conductor is proportional to the voltage across it: V = I × R. A 12 V supply across 4 ohms drives 3 amperes. It holds for resistors at constant temperature but not for diodes or lamp filaments, whose resistance changes.
How is power related to voltage and current?
Electrical power is P = V × I. Combined with Ohm's law this gives P = I²R and P = V²/R, which is why doubling the voltage across a fixed resistance quadruples the power.
Which two quantities should I enter?
Any two. Choose the pair you know from the selector and the tool computes the other two, for example the current and resistance of a 60 W, 120 V lamp.