Ohm's Law Calculator
Enter any two of the known values below:
What Is Ohm's Law?
Ohm's Law states that the current flowing through a conductor between two points is directly proportional to the voltage across those points. In formula form: V = I × R, where V is voltage in volts, I is current in amperes (amps), and R is resistance in ohms (Ω).
Power (P) adds a fourth variable: P = V × I (also expressible as P = I²R and P = V²/R). Together, these four variables and their relationships form the complete Ohm's Law circle — knowing any two, you can solve for all four.
Named after German physicist Georg Simon Ohm (1827), this law is foundational to electrical engineering, electronics design, circuit analysis, and any work involving resistive components.
How to Use This Ohm's Law Calculator
- Select the variable you want to solve for: Voltage, Current, Resistance, or Power.
- Enter any two of the remaining values that you know. The missing field (what you're solving for) is hidden.
- Click Calculate to see all four values — voltage, current, resistance, and power — simultaneously.
- The calculator will indicate if the combination you entered doesn't provide enough information to solve.
All 12 Ohm's Law Formulas
| Solve for | Formula 1 | Formula 2 | Formula 3 |
|---|---|---|---|
| Voltage (V) | V = I × R | V = P / I | V = √(P × R) |
| Current (I) | I = V / R | I = P / V | I = √(P / R) |
| Resistance (R) | R = V / I | R = V² / P | R = P / I² |
| Power (P) | P = V × I | P = V² / R | P = I² × R |
Worked Examples
Example 1: Find current in a 12V, 6Ω circuit
I = V / R = 12 / 6 = 2 amps. Power = V × I = 12 × 2 = 24 watts.
Example 2: Find resistance for a 60W bulb on 120V
I = P / V = 60 / 120 = 0.5A. R = V / I = 120 / 0.5 = 240 ohms.
Example 3: Find voltage for a 1000W heater at 8.33A
V = P / I = 1000 / 8.33 ≈ 120V. Confirms standard US household voltage.
Practical Applications of Ohm's Law
- LED resistor sizing: Calculate the correct series resistor for an LED given supply voltage and LED forward voltage/current requirements.
- Wire gauge selection: Determine if a wire can handle expected current without excessive voltage drop or heat.
- Fuse sizing: Calculate the maximum current a circuit will draw and select an appropriately rated fuse.
- Battery drain estimation: With a known load resistance, calculate how fast a battery will drain (I = V/R).
- Speaker impedance: Audio systems must match amplifier output impedance with speaker impedance to maximize power transfer.
- Solar panel systems: Calculate current at different voltages to size charge controllers and inverters.
Frequently Asked Questions About Ohm's Law
What is Ohm's Law formula?
The basic formula is V = I × R (Voltage equals Current times Resistance). Rearranged: I = V/R and R = V/I. Adding power: P = V × I = I²R = V²/R.
How do I calculate watts from volts and amps?
Power (watts) = Voltage (volts) × Current (amps). A circuit with 120V and 5A draws 600 watts.
How do I find resistance without knowing current?
If you know voltage and power: R = V² / P. For a 100W bulb at 120V: R = 120² / 100 = 14,400 / 100 = 144 ohms.
What is the unit of resistance?
Resistance is measured in ohms, symbolized by the Greek letter omega (Ω). One ohm allows one ampere of current when one volt is applied across it.
Does Ohm's Law apply to AC circuits?
Ohm's Law applies directly to purely resistive AC circuits. For circuits with capacitors or inductors, the concept extends to impedance (Z), which replaces resistance and accounts for reactive components. V = I × Z in the general case.
What is the relationship between voltage and current?
In a resistive circuit, voltage and current are directly proportional — double the voltage, double the current (with constant resistance). This linear relationship is what defines an "ohmic" conductor.
How many amps is 1000 watts at 120V?
I = P / V = 1000 / 120 ≈ 8.33 amps. This is why standard household circuits are rated 15A or 20A — they support multiple 1000W+ appliances simultaneously.
What are the limitations of Ohm's Law?
Ohm's Law only applies to ohmic materials where resistance is constant regardless of voltage or current direction. It doesn't apply to semiconductors (diodes, transistors), electrolytes, or components where resistance changes with temperature or current level.