Ohm's Law Calculator (Voltage, Current, Resistance)
Use Ohm's law V=IR to calculate any one of voltage, current, or resistance from the other two values. Power (W) is calculated at the same time.
What Ohm’s law is
Ohm’s law is the basic relation of an electric circuit, tying voltage V, current I and resistance R together as **V = I × R**. Georg Ohm, the German physicist, published it in 1827. Knowing two of the three settles the remaining one, so this tool asks which value you want and then takes the two you already have. It also works out the power as **P = V × I**.
**Every input — voltage, current and resistance alike — must be greater than zero.** A resistance of 0 Ω is a short circuit: on paper the current diverges without bound, and in a real circuit it means dangerous overcurrent, so it is left outside the calculation. **The tool also presumes a direct-current circuit.** Alternating current calls for impedance, which carries a phase shift and a frequency response, and a plain resistance will not give a correct answer there. The units are fixed as V, A and Ω.
How to use the calculator
- Choose the value to solve for Pick voltage, current or resistance. Choosing one brings up the input fields for the other two.
- Enter the two you know Give the values read from the circuit diagram or the meter. Each must be greater than zero.
- Read the result and the power You get the value you asked for, along with the power P worked out beside it. That is the field to look at for consumption.
- Check it against the other power formulas Substituting V = IR into P = VI gives P = I²R, and substituting I = V/R gives P = V²/R. Use whichever form matches the quantities you have.
- Do not use it on AC circuits Alternating current needs impedance. Entering a bare resistance will not produce a correct figure.
Tips for getting more out of it
- Select the variable you want to find under "Value to solve for", and the other two input fields will appear. Enter the two known values from your circuit diagram or multimeter readings.
- Power (P) is automatically calculated alongside using the formula P=VI, derived from Ohm's law. Check this field if you want to know the power consumption.
- When resistance is close to 0Ω (a short circuit), current becomes extremely large and dangerous (overcurrent) in a real circuit, so this tool excludes input values of 0 or below from calculation.
- AC circuits require impedance (which can include complex numbers) instead of simple resistance; this tool assumes a DC circuit.
Where the calculator helps
Choosing a resistor in electronics work
With the current you want through an LED and the supply voltage settled, the resistance you need follows.
Checking what a part will dissipate
How many watts a resistor must handle follows from the power. Pick one rated below that and it will fail from heat.
Verifying a meter reading
Compute the third value from the two you measured and see whether it agrees with what you read. A mismatch points to a bad contact or another path.
Studying physics or electrical basics
You can watch the relation I = V/R at work — raise the resistance and the current falls — by changing the numbers.
When you also need to read or combine resistors
For colour bands, see the resistor colour code reader; for series and parallel combinations, the combined resistance calculator; for time constants, the RC time constant calculator.
Terms about Ohm’s law
- Ohm’s law
- The law expressing the relation of voltage, current and resistance as V = I × R. Georg Ohm published it in 1827.
- Voltage (V, volts)
- The quantity answering to the pressure that drives a current. It expresses the difference in potential between two points of a circuit.
- Current (I, amperes)
- The quantity of charge passing per unit of time. At a given voltage, the smaller the resistance the larger it grows.
- Resistance (R, ohms)
- The quantity expressing how much a path opposes a current. Zero ohms amounts to a short circuit and is not handled here.
- Power (P, watts)
- The electrical energy consumed per unit of time. It comes from P = VI, and may equally be written P = I²R or P = V²/R.
- Short circuit
- The state in which a path of nearly no resistance forms and an excessive current flows. It is a cause of overheating and fire.
- Impedance
- The opposition to current in an alternating-current circuit, expressed as a complex number that carries a phase shift alongside resistance. It is the counterpart of DC resistance.
Frequently Asked Questions
Side Note — The Struggle Behind the Discovery of Ohm's Law
Georg Simon Ohm, who published Ohm's law, faced strong criticism from the German physics community when he published it in 1827. At the time, there was a prevailing view in academia that simply expressing electrical phenomena with a mathematical equation was "philosophically shallow," and Ohm is said to have faced such backlash that he was even pushed out of his university position.
It took more than a decade after publication for Ohm's law to become widely accepted. Eventually, in recognition of his achievement, his name was immortalized in the international unit of electrical resistance, the "ohm (Ω)." The Greek letter Ω (omega) is said to have been adopted as the unit symbol to avoid confusion with the letter O, the initial of "Ohm."
In modern electronics hobby projects and DIY work, such as determining the resistor value to connect in series with an LED, Ohm's law remains one of the most fundamental calculations used daily. The simplicity of instantly finding the third value once two of voltage, current, and resistance are known is one reason it hasn't lost its relevance even after nearly 200 years.