Ohm's Law Calculator
Enter any 2 values — Voltage, Current, Resistance, or Power — and the other 2 calculate instantly.
Resistor Color Code Reference
4-band resistors: Band 1 & 2 = digits, Band 3 = multiplier, Band 4 = tolerance.
| Color | Digit | Multiplier | Tolerance |
|---|---|---|---|
| Black | 0 | ×1 | — |
| Brown | 1 | ×10 | ±1% |
| Red | 2 | ×100 | ±2% |
| Orange | 3 | ×1 k | — |
| Yellow | 4 | ×10 k | — |
| Green | 5 | ×100 k | ±0.5% |
| Blue | 6 | ×1 M | ±0.25% |
| Violet | 7 | ×10 M | ±0.1% |
| Grey | 8 | — | ±0.05% |
| White | 9 | — | — |
| Gold | — | ×0.1 | ±5% |
| Silver | — | ×0.01 | ±10% |
Example: Red (2) → Violet (7) → Orange (×1 k) → Gold (±5%) = 27,000 Ω ±5%
Understanding Ohm's Law
Ohm's Law is the fundamental relationship governing voltage, current, and resistance in an electrical circuit. Discovered by German physicist Georg Simon Ohm in 1827, the law states that the voltage across a resistive conductor is directly proportional to the current passing through it, provided temperature remains constant. In its most common form: V = I × R.
This simple equation unlocks a complete system of circuit analysis. Because power is also defined as the rate of energy transfer — P = V × I — combining the two equations gives you four interchangeable formulas connecting all four fundamental electrical quantities: voltage (V), current (I), resistance (R), and power (P).
The Four Core Formulas
To solve for any variable, you only need two known values. The complete set of derived formulas is:
- Voltage: V = I × R | V = P / I | V = √(P × R)
- Current: I = V / R | I = P / V | I = √(P / R)
- Resistance: R = V / I | R = V² / P | R = P / I²
- Power: P = V × I | P = I² × R | P = V² / R
This calculator detects which two fields you have filled in and automatically selects the appropriate formula, displaying which equation was used so you can verify your work.
Voltage (V) — Electrical Pressure
Voltage, measured in volts (V), represents the electrical potential difference between two points — essentially the "pressure" that drives current through a circuit. Common voltage levels include: 1.5 V (AA battery), 3.3 V (microcontroller logic), 5 V (USB power), 9 V (battery), 12 V (car electronics), 120 V (US mains), and 240 V (European mains). Higher voltage with a fixed resistance means proportionally higher current.
Current (I) — Flow of Charge
Current, measured in amperes (A) or milliamperes (mA), is the rate at which electric charge flows through a circuit. 1 A = 1,000 mA. Small electronic components like LEDs typically draw 10–30 mA, while a laptop power supply might draw 3–5 A. Exceeding a component's rated current causes overheating and permanent damage. The letter "I" comes from the French word intensité du courant (current intensity).
Resistance (R) — Opposition to Current
Resistance, measured in ohms (Ω) or kilohms (kΩ), quantifies how strongly a material opposes the flow of current. Resistors are the most common passive electronic component and come in standard values following the E12 and E24 series. Common values range from 1 Ω to 10 MΩ. Resistors are physically color-coded — the standard 4-band system encodes the value and tolerance directly onto the component body.
Power (P) — Energy Dissipation
Power, measured in watts (W) or milliwatts (mW), is the rate at which a circuit component converts electrical energy into heat (or other forms of energy). This is a critical design constraint: every resistor has a maximum power rating, and exceeding it causes the resistor to overheat, change value, or fail. Standard through-hole resistors are rated at 1/8 W, 1/4 W (most common), 1/2 W, 1 W, 2 W, and 5 W. Our calculator warns you when calculated power exceeds the common 1/4 W (0.25 W) threshold.
Practical Applications
LED current limiting: LEDs require a series resistor to limit current. If your supply is 5 V, LED forward voltage is 2 V, and target current is 20 mA: R = (5 - 2) / 0.02 = 150 Ω. Power = 0.02² × 150 = 0.06 W — well within the 1/4 W limit.
Battery drain estimation: If a circuit draws 50 mA from a 9 V battery, power consumption is 9 × 0.05 = 0.45 W. This helps estimate run time from battery capacity (mAh).
Speaker matching: Audio amplifiers specify an output impedance; connecting an 8 Ω speaker to an amplifier designed for 4 Ω halves the available power. Ohm's Law shows why impedance matching matters.
Voltage dividers: Two resistors in series split a voltage proportionally. With a 12 V supply and two equal 10 kΩ resistors, the midpoint is 6 V — confirmed by Ohm's Law: I = 12 / 20000 = 0.6 mA, V across lower resistor = 0.0006 × 10000 = 6 V.
Limitations of Ohm's Law
Ohm's Law applies to linear, resistive components at constant temperature. It does not directly apply to non-linear components like diodes, transistors, or capacitors, whose voltage-current relationships are governed by different equations. It also assumes DC or low-frequency AC circuits; at high frequencies, inductance and capacitance become significant through the broader concept of impedance (Z). Despite these limitations, Ohm's Law remains the essential starting point for any circuit analysis.