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Calculate voltage, current, resistance, and power using Ohm's Law. Essential for electronics, electrical engineering, automotive repair, and DIY projects.
Everything you need to know
Ohm's Law is the fundamental principle of electrical circuits. Discovered by Georg Simon Ohm in 1827, it describes the relationship between voltage (V), current (I), and resistance (R) in an electrical circuit. Whether you're designing electronics, troubleshooting car electrical issues, or wiring a home theater, Ohm's Law is your starting point.
Our calculator solves for any variable when you know the other two:
The core equation: V = I × R
From this, you can derive:
V
/ \
/ \
I × R
Cover the variable you want to solve for:
Scenario: A 12V car battery connected to a 4Ω speaker I = V ÷ R = 12 ÷ 4 = 3 amps
Scenario: A 120V outlet providing 5 amps to a space heater R = V ÷ I = 120 ÷ 5 = 24 ohms
Scenario: A circuit with 2 amps flowing through 60 ohms resistance V = I × R = 2 × 60 = 120 volts
Scenario: A laptop charger outputs 19.5V at 4.62 amps P = V × I = 19.5 × 4.62 = 90.09 watts
| Device | Voltage | Current | Resistance | Power |
|---|---|---|---|---|
| LED bulb | 120V | 0.083A | 1,440Ω | 10W |
| Phone charger | 5V | 2A | 2.5Ω | 10W |
| Laptop charger | 19.5V | 4.6A | 4.2Ω | 90W |
| Space heater | 120V | 12.5A | 9.6Ω | 1,500W |
| Hair dryer | 120V | 15A | 8Ω | 1,800W |
| Electric stove | 240V | 25A | 9.6Ω | 6,000W |
| Car battery | 12V | 100A | 0.12Ω | 1,200W |
| Doorbell transformer | 16V | 1A | 16Ω | 16W |
In series, components are connected end-to-end. The same current flows through all components.
Total Resistance: R_total = R₁ + R₂ + R₃ + ...
Example: Three resistors in series: 10Ω, 20Ω, 30Ω R_total = 10 + 20 + 30 = 60Ω
If connected to 12V: I = 12 ÷ 60 = 0.2 amps
In parallel, components share the same two connection points. The same voltage is across all components.
Total Resistance: 1/R_total = 1/R₁ + 1/R₂ + 1/R₃ + ...
Example: Three resistors in parallel: 10Ω, 20Ω, 30Ω 1/R_total = 1/10 + 1/20 + 1/30 = 0.1 + 0.05 + 0.033 = 0.183 R_total = 1 ÷ 0.183 = 5.45Ω
If connected to 12V: I_total = 12 ÷ 5.45 = 2.2 amps
| Wire Gauge (AWG) | Max Current | Typical Use |
|---|---|---|
| 18 AWG | 5A | Low-power electronics |
| 16 AWG | 10A | Extension cords, lamps |
| 14 AWG | 15A | Household circuits |
| 12 AWG | 20A | Kitchen, bathroom circuits |
| 10 AWG | 30A | Water heaters, dryers |
| 8 AWG | 40A | Large appliances |
| 6 AWG | 55A | Subpanels, EV chargers |
| 4 AWG | 70A | Main service panels |
Circuit breakers protect wires from overheating by limiting current:
| Breaker Size | Wire Required | Common Uses |
|---|---|---|
| 15A | 14 AWG | Lighting, outlets |
| 20A | 12 AWG | Kitchen, garage outlets |
| 30A | 10 AWG | Water heater |
| 40A | 8 AWG | Electric range |
| 50A | 6 AWG | EV charger, hot tub |
Never use a larger breaker than the wire can safely handle.
Long wire runs cause voltage drop. For critical circuits, keep drop under 3%:
Example: 100-foot run of 12 AWG wire carrying 15 amps Voltage drop ≈ 3.84 volts On a 120V circuit: 120 - 3.84 = 116.16V (3.2% drop — acceptable)
Ohm's Law is used to design circuits, select components, troubleshoot electrical problems, and ensure safety in any electrical system.
Yes, but with modifications. In AC circuits, you use impedance (Z) instead of resistance (R), which includes reactance from capacitors and inductors.
A short circuit has near-zero resistance. By Ohm's Law (I = V ÷ R), as R approaches zero, current approaches infinity—causing wires to overheat.
The resistor overheats and may burn out, change value, or catch fire. Always choose resistors with power ratings at least 2× the expected power dissipation.
You don't. Always disconnect power before measuring resistance with a multimeter. Measuring resistance in a live circuit can damage your meter and is dangerous.