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⚡ Ohm's Law Calculator

Calculate voltage, current, resistance and power using Ohm's Law. Enter any two known values of V, I and R to instantly find the third, plus the power dissipated in the circuit.

Voltage (V)

Current (I)

Resistance (R)

What is this tool?

Ohm's Law is one of the most fundamental principles in electrical engineering and physics. It describes the relationship between voltage (V), current (I) and resistance (R) in an electrical circuit. Named after the German physicist Georg Simon Ohm, who published the law in 1827, it states that the current flowing through a conductor between two points is directly proportional to the voltage across those points and inversely proportional to the resistance between them. The three core formulas are V = I × R, I = V ÷ R, and R = V ÷ I. Beyond the three basics, Ohm's Law also connects to electrical power through P = V × I, which lets you compute the energy dissipated as heat in a resistor or component. This Ohm's Law calculator is designed for students, hobbyists, electricians, engineers and anyone who works with electrical circuits. Whether you are sizing a resistor for an LED, checking a household circuit, troubleshooting a broken appliance or analysing a circuit design, you simply enter any two known values of voltage, current and resistance, and the calculator instantly returns the third, along with the power figure. You can use volts (V), amperes (A) and ohms (Ω) directly — no unit conversion needed.

How it works

The calculator takes any two of the three Ohm's Law quantities and solves for the unknown. If you know current (I) and resistance (R), it multiplies them to find voltage: V = I × R. If you know voltage (V) and resistance (R), it divides them to find current: I = V ÷ R. If you know voltage (V) and current (I), it divides them to find resistance: R = V ÷ I. In all three cases, power (P) is computed as P = V × I. You can also derive power from the other two known values using P = I² × R or P = V² ÷ R. The table below shows common reference values for standard resistors, along with the current that flows through each resistor when 5 V is applied across it: | Resistance (Ω) | Voltage (V) | Current (mA) | Power (mW) | |---|---|---|---| | 100 | 5 | 50.0 | 250.0 | | 220 | 5 | 22.7 | 113.6 | | 330 | 5 | 15.2 | 75.8 | | 1,000 (1 kΩ) | 5 | 5.0 | 25.0 | | 4,700 (4.7 kΩ) | 5 | 1.06 | 5.32 | | 10,000 (10 kΩ) | 5 | 0.50 | 2.50 | | 100,000 (100 kΩ) | 5 | 0.05 | 0.25 | These values follow directly from Ohm's Law. A lower resistance allows more current to flow and dissipates more power as heat. A higher resistance chokes the current and keeps power dissipation low.

How to use

Enter any two of Voltage (V), Current (I) and Resistance (R).
Leave the third field blank — that is what will be calculated.
Click the Calculate button.
Review the result for the unknown value plus Power (P).
Double-check that the units and power rating are safe for your circuit.

Frequently Asked Questions

What units does the Ohm's Law calculator use?

The calculator works in base SI units: volts (V) for voltage, amperes (A) for current, ohms (Ω) for resistance and watts (W) for power. You can enter fractional values directly, for example 0.02 A to represent 20 mA or 4700 Ω to represent 4.7 kΩ.

Can I calculate power (P) as well?

Yes. Power is always derived from P = V × I. If you enter voltage and current, power is immediate. If you enter voltage and resistance, the calculator finds the current first, then computes power. If you enter current and resistance, it finds the voltage first, then computes power.

What happens if I enter all three values?

When all three fields are filled, the calculator verifies the relationship V = I × R and displays all four quantities (V, I, R, P). This is handy for checking whether a measured circuit satisfies Ohm's Law.

Does Ohm's Law apply to AC circuits?

For purely resistive AC loads (heaters, incandescent lamps) Ohm's Law works exactly as with DC. For circuits containing capacitors or inductors, you must use impedance Z instead of resistance R, and the relationship becomes V = I × Z. This calculator does not account for phase angle and is intended for DC or purely resistive AC loads.

Why is my calculated resistance zero or infinite?

If the current you entered is zero, resistance cannot be computed (division by zero). If the voltage you entered is zero, resistance calculates as zero. Both situations usually indicate a short circuit (R = 0) or an open circuit (R → ∞), which is physically meaningful but worth double-checking.

How accurate is this calculator?

The calculator uses standard IEEE-754 double-precision floating point arithmetic and rounds display output to six decimal places. The underlying physics is exact; real-world deviations come from component tolerance, temperature drift and measurement error.

Tips & Advice

When using this calculator, leave exactly one field blank — the one you want to find. You can mix units freely (for example 0.5 A instead of 500 mA) because the calculator works with any decimal input. Keep in mind that real-world resistors have a tolerance (typically ±5 % or ±1 %), so your measured result may differ slightly from the calculated value. In AC circuits with reactive components (capacitors and inductors), Ohm's Law becomes V = I × Z, where Z is the impedance rather than the simple DC resistance. Also remember that power dissipation matters: a 1/4 W resistor carrying too much current will overheat and burn out, so always verify that P < the resistor's rated wattage. For LED circuits, aim for a forward current of about 20 mA and use the calculator to find the correct current-limiting resistor.

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