Ohms Law Calculator
Calculate voltage, current, resistance, and power using Ohm's Law equations
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The V=IR Formula and Its Three Variations
Ohms Law calculator uses the fundamental relationship between voltage, current, and resistance discovered by Georg Ohm in 1827. The formula V = I × R states that voltage equals current multiplied by resistance. This principle forms the foundation of all electrical circuit analysis and design.
From this single equation, you can derive three variations: V = I × R (find voltage), I = V / R (find current), and R = V / I (find resistance). Our calculator automatically applies the correct formula based on which value you want to find.
Voltage Calculation: V = I × R
When you know current and resistance, multiply them to find voltage. For example, if 3 amps flow through a 4-ohm resistor, the voltage drop is 3 × 4 = 12 volts. This calculation helps determine power supply requirements for circuits.
Current Calculation: I = V / R
Divide voltage by resistance to find current flow. A 12-volt battery connected to a 6-ohm load produces 12 / 6 = 2 amps of current. This helps size wires and fuses correctly for safety.
Resistance Calculation: R = V / I
When measuring voltage and current, divide to find resistance. If you measure 9 volts across a component with 0.5 amps flowing, the resistance is 9 / 0.5 = 18 ohms. Useful for identifying unknown components.
What is the Power Formula P = V × I?
Power in watts equals voltage multiplied by current. This extends Ohm's Law to calculate electrical power consumption. A device running at 120 volts drawing 2 amps consumes 120 × 2 = 240 watts. You can also calculate power using P = I²R or P = V²/R by substituting Ohm's Law.
Power Dissipation in Resistors
Resistors convert electrical energy to heat. A 100-ohm resistor with 0.1 amps flowing dissipates P = 0.1² × 100 = 1 watt as heat. Always choose resistors rated above the calculated power to prevent overheating and failure.
Calculating Energy Cost from Power
Multiply power by time to get energy in watt-hours. A 60-watt bulb running 5 hours uses 300 watt-hours or 0.3 kWh. At $0.12 per kWh, that costs about 3.6 cents. Use our watt calculator for detailed power analysis.
How Does Ohms Law Apply to Series and Parallel Circuits?
In series circuits, resistances add directly: R_total = R1 + R2 + R3. Current stays the same through all components while voltage divides proportionally. In parallel circuits, use 1/R_total = 1/R1 + 1/R2 + 1/R3. Voltage stays the same while current divides.
Series Circuit Example
Three resistors of 2Ω, 3Ω, and 5Ω in series have total resistance of 10Ω. With 20V applied, current is 20/10 = 2A through all resistors. Voltage drops are 4V, 6V, and 10V respectively, adding to 20V total.
Parallel Circuit Example
Two 6Ω resistors in parallel have combined resistance of 3Ω (half of either one). With 12V applied, total current is 4A, split equally as 2A through each resistor. Each resistor sees the full 12V.
Ohms Law Formula Reference Table
| Find | Formula | Known Values | Example |
|---|---|---|---|
| Voltage (V) | V = I × R | Current, Resistance | 2A × 5Ω = 10V |
| Current (I) | I = V / R | Voltage, Resistance | 12V / 4Ω = 3A |
| Resistance (R) | R = V / I | Voltage, Current | 9V / 3A = 3Ω |
| Power (P) | P = V × I | Voltage, Current | 12V × 2A = 24W |
| Power (P) | P = I² × R | Current, Resistance | 2A² × 6Ω = 24W |
| Power (P) | P = V² / R | Voltage, Resistance | 12V² / 6Ω = 24W |
LED Resistor Selection Using Ohms Law
LEDs require current-limiting resistors to prevent burnout. To calculate the resistor value, use R = (V_supply - V_LED) / I_LED. For a red LED (2V forward voltage, 20mA current) on a 5V supply: R = (5 - 2) / 0.02 = 150Ω. Always round up to the next standard resistor value.
Wire Gauge Selection for Current Capacity
Wires have resistance that causes voltage drop over distance. For a 50-foot run of 14 AWG wire (2.5Ω per 1000 feet) carrying 10A, voltage drop is 10 × 0.125 = 1.25V. Keep voltage drop under 3% for proper equipment operation. Use our power calculator for related calculations.
Battery and Load Matching
Batteries have internal resistance that affects output voltage under load. A 12V battery with 0.1Ω internal resistance supplying 5A delivers only 12 - (5 × 0.1) = 11.5V at the terminals. Higher current draws cause greater voltage sag.
Common Mistakes When Applying Ohms Law
The most frequent error is mixing units - always use volts, amps, and ohms (not milliamps or kilohms) unless you adjust accordingly. 500mA is 0.5A, and 2.2kΩ is 2200Ω. Another mistake is applying Ohm's Law to non-linear components like diodes, which don't follow V=IR.
Temperature affects resistance in most materials. A light bulb's resistance when cold differs significantly from when hot. Measure resistance with power off, but understand operating conditions may differ. For complex circuits, try our kinetic energy calculator for physics problems.
Last Updated: January 2026 | Reviewed for accuracy