⚡ Ohm's Law Calculator
Enter any two of voltage, current, resistance and power — we instantly compute the other two with Ohm's law. Everything runs in your browser.
What is the Ohm's Law Calculator?
The Ohm's law calculator uses the relationship between voltage (V), current (I), resistance (R) and power (P) to instantly find the two values you don't know from the two values you do. It is a practical tool for Arduino and electronics hobbyists, electrical and electronic engineering students, technicians and engineers. It makes everyday tasks easier — picking a series resistor for an LED, estimating the current a power supply will draw, or working out how much power (watts) a component dissipates. Every calculation happens in your browser; none of the values you enter are ever sent to a server.
The Ohm's law formula
Ohm's law states that the current through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance. The core relationship is V = I × R. From this equation:
- Voltage: V = I × R
- Current: I = V / R
- Resistance: R = V / I
The power triangle and power formulas
Electrical power is the energy used per unit of time, measured in watts (W). Combined with Ohm's law, it produces several formulas, often shown as a power triangle:
- P = V × I (when voltage and current are known)
- P = I² × R (when current and resistance are known)
- P = V² / R (when voltage and resistance are known)
Likewise, if power is known, the other quantities can be derived: V = √(P × R), I = √(P / R), R = V² / P and R = P / I².
Worked example
Suppose you want to drive an LED from a 5 V supply and you want 0.02 A (20 mA) flowing through the circuit. If the LED drops 2 V, that leaves 3 V across the series resistor. Enter 20 mA in the current field and 3 V in the voltage field, then press Calculate: the tool returns the resistance R = V / I = 3 / 0.02 = 150 Ω and the power the resistor dissipates P = V × I = 3 × 0.02 = 0.06 W (60 mW). So a 1/8 W (0.125 W) resistor is more than enough.
Usage tips
- Fill in exactly two fields. If you enter one or more than two, the tool will warn you.
- Use the unit-prefix selector beside each field (mA, kΩ, mW…) to enter very small or very large values easily.
- Entering zero resistance causes a "divide by zero" condition, which the tool flags as a warning.
- Results are shown in base units (V, A, Ω, W) and formatted with a convenient prefix (e.g. mA, kΩ) for readability.
Frequently Asked Questions
How does the Ohm's law calculator work?
Enter any two of voltage, current, resistance and power. The tool uses Ohm's law (V = I × R) and the power formula (P = V × I) to compute the remaining two instantly. All processing happens in your browser.
What is the Ohm's law formula?
The core relationship is V = I × R. From it, I = V / R and R = V / I are derived. Power is P = V × I = I² × R = V² / R.
Which units should I enter?
Each field has a unit-prefix selector: mV, V, kV for voltage; µA, mA, A for current; mΩ, Ω, kΩ, MΩ for resistance; mW, W, kW for power. The tool converts everything to base units before calculating.
Is my data stored?
No. The tool runs entirely in your browser; the values you enter are never uploaded or saved.