Ohm's Law Calculator
Compute V, I, R and P from any two values, size a voltage divider, or find the right series resistor for an LED
Enter any two of voltage, current, resistance and power. The other two are calculated for you.
About this tool
This Ohm's law calculator works out voltage, current, resistance and power from any two known values, using V = I·R together with P = V·I = I²·R = V²/R. It also includes two everyday circuit calculators: a voltage divider that finds the output of two series resistors, and an LED series resistor calculator that sizes the resistor needed to drive an LED safely and suggests the nearest standard E12 value. SI prefixes are accepted in every field, so you can type 4.7k, 20m or 1M directly. It is built for makers, electronics students and hobbyists who want quick, reliable answers without reaching for a datasheet.
How to use
- 1 Choose a mode: Ohm's law & power, voltage divider, or LED resistor.
- 2 In Ohm's law mode, fill in exactly two of voltage, current, resistance and power; the other two boxes lock automatically.
- 3 Type values with optional SI prefixes such as 4.7k, 20m or 1M; results update instantly.
- 4 Read the calculated quantities and the formulas used, and use Reset to start over.
How it works
Ohm's law states that the voltage across a resistor equals the current through it times its resistance: V = I·R. Power is the rate of energy use and can be written three equivalent ways: P = V·I = I²·R = V²/R. Given any two of the four quantities, the calculator picks the correct pair of formulas to derive the other two. For example, with V = 12 V and R = 4 Ω it finds I = V/R = 3 A and P = V²/R = 36 W. A voltage divider uses two resistors in series so the output across R2 is Vout = Vin·R2/(R1+R2); with Vin = 9 V, R1 = 1 kΩ and R2 = 2 kΩ the output is 6 V. For an LED, the series resistor drops the difference between the supply and the LED forward voltage at the chosen current: R = (Vs−Vf)/If; driving a 2 V LED at 20 mA from a 5 V supply needs (5−2)/0.02 = 150 Ω, dissipating 0.06 W.
Frequently asked questions
How many values do I need to enter for Ohm's law?
Exactly two of the four quantities — voltage, current, resistance and power. Once two boxes are filled the rest lock, and the calculator derives the remaining two using V = I·R and P = V·I. Clear a box to change which two you provide.
Can I use prefixes like k, m or M?
Yes. Every field accepts SI prefixes, so 4.7k means 4700, 20m means 0.02, and 1M means 1,000,000. This lets you enter kilo-ohms, milliamps or mega-ohms without converting by hand.
How do I size a resistor for an LED?
Subtract the LED forward voltage from the supply voltage and divide by the desired current: R = (Vs−Vf)/If. For example a 2 V LED at 20 mA from 5 V needs about 150 Ω. The tool also suggests the nearest standard E12 resistor and shows the power the resistor must handle.
What is a voltage divider used for?
A voltage divider produces a smaller voltage from a larger one using two series resistors, with Vout = Vin·R2/(R1+R2). It is common for setting reference voltages, scaling a signal into a microcontroller's input range, or biasing a transistor. Note that drawing current from the output changes the result.
Related tools and uses
Pair this with the resistor color code calculator to read or pick physical resistors, the scientific notation converter for very large or small values, and the formula solver for more physics relationships.
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