Enter any two of voltage, current and resistance — leave the third blank to solve for it.
V = I × R — voltage equals current times resistance. Power P = V × I.
Fill in any two fields and leave the one you want to solve for blank.
Ohm's law — V = I × R (voltage equals current times resistance) — describes the fundamental relationship between the three basic electrical quantities in a circuit: voltage (the electrical "push" measured in volts), current (the flow of charge measured in amps), and resistance (opposition to that flow, measured in ohms). Because it's a simple three-variable relationship, knowing any two of the three lets you solve directly for the third — rearranged as I = V/R or R = V/I depending on which value is unknown — which is exactly why it's the single most-used formula in basic circuit analysis and troubleshooting.
Power — measured in watts — extends this same relationship one step further: P = V × I (power equals voltage times current), and because Ohm's law already links V, I and R together, power can also be expressed as P = I² × R or P = V²/R by substituting one relationship into the other. This combined toolkit is what lets someone calculate any one of five related quantities (voltage, current, resistance, power, and by extension energy over time) from just two known values, which is exactly why Ohm's law and the power formula are taught together rather than separately — in practice, a real circuit problem rarely hands you the exact two variables you'd prefer, so being able to combine both formulas and substitute between them is what actually solves most basic electrical calculations.
Voltage equals current multiplied by resistance (V = I × R), describing the fundamental relationship between the electrical 'push' (voltage, in volts), the flow of charge (current, in amps), and the opposition to that flow (resistance, in ohms) in a circuit.
Rearrange the formula for the unknown quantity: if you know current and resistance, voltage is V = I × R. If you know voltage and resistance, current is I = V ÷ R. If you know voltage and current, resistance is R = V ÷ I. Any two known values let you solve directly for the third.
Power (in watts) is P = V × I (voltage times current). Because Ohm's law links V, I and R together, power can also be expressed as P = I² × R or P = V² ÷ R by substituting Ohm's law into the power formula — useful when you know resistance and only one other value.
Because a real circuit problem rarely provides exactly the two values needed for a single formula. Combining Ohm's law (V, I, R) with the power formula (P, V, I) creates a fuller toolkit — five related quantities (voltage, current, resistance, power, and by extension energy) that can be solved from whichever two values happen to be known, by substituting between the formulas as needed.