Ohm's Law Calculator
Calculate voltage, current, resistance and power from any two known values with V = I × R and P = V × I, with mA, kΩ and kW units.
🔒 Runs entirely in your browser — nothing is uploadedHow Ohm's law and the power formula fit together
Ohm's law describes how voltage, current and resistance relate in an electrical circuit. Voltage (V, measured in volts) is the electrical pressure that pushes charge, current (I, in amperes) is the rate at which charge flows, and resistance (R, in ohms) is how strongly a component opposes that flow. The relationship is V = I × R, which can be rearranged to I = V ÷ R and R = V ÷ I. Adding the power formula P = V × I, where power is measured in watts, gives four quantities linked by two equations.
Because there are two equations, knowing any two quantities is enough to find the other two. Substituting one formula into the other gives the full set used by this calculator: P = I² × R, P = V² ÷ R, I = √(P ÷ R) and V = √(P × R). These twelve combinations are often drawn as the "Ohm's law wheel". Instead of memorising the wheel, enter the two values you know, choose the units, and the calculator picks the right pair of formulas and shows them under the result.
Practical examples
An LED current-limiting resistor is a classic use. If a 5 V supply drives an LED with a 2 V forward voltage at 20 mA, the resistor must drop 3 V, so enter 3 V and 20 mA to get 150 Ω and 60 mW — a standard 1/4 W resistor is plenty. For a 12 V car accessory drawing 4 A, enter both values to find a 3 Ω equivalent load and 48 W of power, which tells you the fuse and wire need to handle at least that current. A 2000 W kettle on a 230 V supply draws about 8.7 A and has a heating element of roughly 26 Ω.
Results are shown with engineering prefixes such as mA, kΩ and kW, so very small and very large values stay readable. Keep in mind that Ohm's law in this simple form applies to resistive loads. Real components change resistance with temperature, and inductive or capacitive loads need impedance and power factor. Everything is calculated in your browser and no values are sent anywhere.
How to use
- Enter two known valuesType any two of voltage, current, resistance and power, and leave the other two fields empty.
- Pick the unitsChoose mV, V or kV, µA, mA or A, mΩ to MΩ, and mW, W or kW next to each field.
- Read all four valuesThe calculator fills in the missing two quantities instantly and shows the formulas it used.