Ohm's Law Calculator — Solve V=IR for Any Variable
Ohm's Law is the foundation of electronics: V = I × R. But in practice, you often know two of the four electrical quantities (voltage, current, resistance, power) and need to find the others. The free Ohm's Law calculator on PublicSoftTools solves any combination with one click.
The Four Electrical Quantities
Every DC circuit can be described by four related quantities:
- Voltage V — electrical potential difference, measured in Volts (V)
- Current I — flow of charge, measured in Amps (A)
- Resistance R — opposition to current flow, measured in Ohms (Ω)
- Power P — rate of energy consumption, measured in Watts (W)
These four quantities are related by six equations, all derivable from V = IR and P = VI:
| Equation | Solves for | Requires |
|---|---|---|
| V = I × R | Voltage | Current + Resistance |
| I = V / R | Current | Voltage + Resistance |
| R = V / I | Resistance | Voltage + Current |
| P = V × I | Power | Voltage + Current |
| P = I² × R | Power | Current + Resistance |
| P = V² / R | Power | Voltage + Resistance |
How to Use the Calculator
- Open the Ohm's Law calculator.
- Select which variable you want to solve for (V, I, R, or P) using the tabs at the top.
- Enter the values you know in the remaining three fields. You only need two known values — the calculator tries all applicable formulas.
- The result appears automatically with the unit label.
Advanced Workflows
Series circuits
In a series circuit, resistances add: R_total = R₁ + R₂ + R₃. Calculate the total resistance first, then use the calculator to find total current and voltage drop. For example, three 100Ω resistors in series give R_total = 300Ω. At 9V: I = 9/300 = 30 mA.
Parallel circuits
For parallel resistors: 1/R_total = 1/R₁ + 1/R₂. Two 100Ω resistors in parallel give R_total = 50Ω. At 9V: I = 9/50 = 180 mA total. Each branch draws 90 mA. The total power drawn is P = 9 × 0.18 = 1.62W.
Resistor power rating
When designing circuits, use P = I²R to check whether a resistor will overheat. If a 1kΩ resistor carries 20 mA: P = (0.02)² × 1000 = 0.4W. Choose a resistor rated for at least 0.5W (double for safety margin).
LED current limiting resistor
A standard red LED needs 2.0V forward voltage at 20 mA. Running from 5V: V_R = 5 − 2 = 3V. R = V_R / I = 3 / 0.02 = 150Ω. Use the calculator: solve for R, enter V = 3V and I = 0.02A to get 150Ω.
Common Questions
Does Ohm's Law apply to all components?
No. Ohm's Law applies only to ohmic (linear) components at constant temperature. Diodes, transistors, capacitors, and inductors have non-linear V-I relationships. For these, you need more specialized models.
What about AC circuits?
In AC circuits, resistance is replaced by impedance (Z), which includes inductive and capacitive reactance. V = I × Z still holds, but Z is a complex number. This calculator covers DC circuits and resistive AC loads only.
Solve Ohm's Law Now
Enter any two electrical values to instantly calculate the third using V=IR and power formulas.
Open Ohm's Law Calculator