Ohm's Law Calculator
Calculate voltage, current, resistance, or power using Ohm's Law and the power equation. Enter any two known values and solve for the remaining unknowns. See also our Voltage Divider Calculator and Wire Size Calculator.
How to Use Ohm's Law
Ohm's Law defines the fundamental relationship between voltage, current, and resistance in an electrical circuit. Discovered by Georg Simon Ohm in 1827, this law states that the current flowing through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance. Combined with the power equation, you can solve for any electrical quantity given two known values.
To use this calculator, select which value you want to solve for, enter any two of the remaining three values, and click Calculate. The calculator will determine the unknown value and show the formula used. This is essential for circuit design, troubleshooting, and understanding electrical systems.
Ohm's Law Formulas
Basic Ohm's Law:
V = I × R (Voltage = Current × Resistance)
I = V / R (Current = Voltage / Resistance)
R = V / I (Resistance = Voltage / Current)
Power Equations:
P = V × I (Power = Voltage × Current)
P = I² × R (Power = Current² × Resistance)
P = V² / R (Power = Voltage² / Resistance)
Derived Formulas:
V = √(P × R)
I = √(P / R)
R = P / I²
R = V² / P
Example Calculation
Suppose you have a 12V battery connected to a circuit drawing 2 amps of current. Using Ohm's Law:
Given: V = 12V, I = 2A
Resistance: R = V / I = 12 / 2 = 6 Ω
Power: P = V × I = 12 × 2 = 24 W
Verify: P = I² × R = 4 × 6 = 24 W ✓
Verify: P = V² / R = 144 / 6 = 24 W ✓
This means the circuit has a total resistance of 6 ohms and dissipates 24 watts of power. If this were a resistor, it would need to be rated for at least 24W to avoid overheating — in practice, you would choose a resistor rated for at least twice the calculated power dissipation for safety margin.
Ohm's Law Reference Table
| Voltage (V) | Current (A) | Resistance (Ω) | Power (W) |
|---|---|---|---|
| 1 V | 1 A | 1 Ω | 1 W |
| 5 V | 0.5 A | 10 Ω | 2.5 W |
| 9 V | 0.5 A | 18 Ω | 4.5 W |
| 12 V | 1 A | 12 Ω | 12 W |
| 12 V | 2 A | 6 Ω | 24 W |
| 24 V | 2 A | 12 Ω | 48 W |
| 120 V | 10 A | 12 Ω | 1200 W |
| 120 V | 15 A | 8 Ω | 1800 W |
| 220 V | 5 A | 44 Ω | 1100 W |
| 240 V | 20 A | 12 Ω | 4800 W |
| 240 V | 30 A | 8 Ω | 7200 W |
| 480 V | 10 A | 48 Ω | 4800 W |
Frequently Asked Questions
What is Ohm's Law?
Ohm's Law states that the current through a conductor between two points is directly proportional to the voltage across the two points and inversely proportional to the resistance. Mathematically: V = I × R. This fundamental law applies to all resistive circuits and is the foundation of electrical engineering.
How do I calculate resistance from voltage and current?
Divide voltage by current: R = V / I. For example, if you measure 12V across a component with 0.5A flowing through it, the resistance is 12 / 0.5 = 24 ohms. This is useful for identifying unknown resistors or checking circuit continuity.
What is the relationship between power and resistance?
Power and resistance are related through two formulas: P = I²R (power increases with resistance for constant current) and P = V²/R (power decreases with resistance for constant voltage). Which formula to use depends on whether your source maintains constant current or constant voltage.
Does Ohm's Law apply to AC circuits?
Ohm's Law applies to AC circuits when you replace resistance with impedance (Z). The generalized form is V = I × Z, where Z includes resistance, inductive reactance, and capacitive reactance. For purely resistive AC loads, the basic V = IR still applies using RMS values.
What are common voltage and current values in household circuits?
In the US, standard outlets provide 120V with 15A or 20A circuits (1,800W or 2,400W max). Large appliances use 240V with 30A or 50A circuits (7,200W or 12,000W). In Europe and most other countries, standard outlets provide 220-240V at 10-16A.
Why is my calculated resistance different from the resistor color code?
Resistors have tolerance ratings (typically ±1%, ±5%, or ±10%). A 100Ω resistor with 5% tolerance can actually be anywhere from 95Ω to 105Ω. Temperature also affects resistance — most resistors increase in resistance as they heat up. Use precision resistors (±0.1%) for critical applications.
Understanding the Ohm's Law Wheel
The Ohm's Law wheel (also called the power wheel or electrical formula circle) is a visual tool that shows all 12 formulas relating voltage, current, resistance, and power. The wheel is divided into four quadrants — one for each quantity you might want to solve for. Each quadrant contains three formulas using different combinations of the other two known values. This calculator implements all 12 formulas automatically.
Practical Applications of Ohm's Law
- LED current limiting: Calculate the resistor needed to limit current through an LED (e.g., R = (Vsupply - Vled) / Iled)
- Fuse selection: Determine maximum current draw to select appropriate fuse rating
- Wire sizing: Calculate voltage drop across wire resistance to ensure adequate power delivery
- Battery life estimation: Divide battery capacity (Ah) by circuit current to estimate runtime
- Heater design: Calculate resistance needed for a heating element to produce desired wattage
- Sensor circuits: Design voltage dividers and current sensing resistors for measurement applications
Limitations of Ohm's Law
Ohm's Law assumes a linear relationship between voltage and current, which holds true for ohmic materials (most metals and standard resistors). However, it does not apply to non-ohmic devices such as diodes, transistors, thermistors, and varistors where resistance changes with voltage or current. For these components, you need their characteristic V-I curves or specialized equations. Additionally, at very high frequencies, parasitic inductance and capacitance become significant, and simple DC resistance calculations are insufficient.