Ohm's Law Calculator - Free Voltage, Current, Resistance & Power Calculator

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Ohm's Law Calculator

Calculate voltage, current, resistance, and power using Ohm's Law and power formulas. Essential tool for electrical engineers and technicians.

Basic Ohm's Law (V = I × R)

Enter any two values to calculate the third

V
A
Ω

⚡ Ohm's Law & Power Formulas

Basic Ohm's Law

V = I × R
I = V ÷ R
R = V ÷ I

V = Voltage (Volts), I = Current (Amps), R = Resistance (Ohms)

Power Formulas

P = V × I
P = I² × R
P = V² ÷ R

P = Power (Watts)

📊 Common Electrical Values

Common Voltages

AA Battery 1.5V
Car Battery 12V
USB 5V
US Household 120V
EU Household 230V

Device Currents

LED 20mA
Phone Charger 1-3A
Light Bulb (60W) 0.5A
Microwave 10A
Electric Heater 12.5A

Standard Resistors

Pull-up/down 10kΩ
LED Current Limit 220-470Ω
Base Resistor 1-10kΩ
Voltage Divider 1-100kΩ
Current Sense 0.1-1Ω

🔧 Real-World Examples

Example 1: LED Circuit

Calculating resistor for an LED with 9V battery:

  • • Battery voltage: 9V
  • • LED forward voltage: 2V
  • • LED current: 20mA (0.02A)
  • • Voltage across resistor: 9V - 2V = 7V
  • Required resistance: 7V ÷ 0.02A = 350Ω

Example 2: Power Consumption

Calculate power used by a 120V, 15A space heater:

  • • Voltage: 120V
  • • Current: 15A
  • Power: 120V × 15A = 1,800W
  • • Monthly cost (8hr/day @ $0.12/kWh):
  • • 1.8kW × 8hr × 30days × $0.12 = $51.84

⚠️ Electrical Safety Tips

Always Check Ratings

Ensure components can handle calculated current and power

Use Safety Margins

Choose components rated 20-50% above calculated values

High Voltage Warning

Voltages above 50V can be dangerous - use proper precautions

Power Dissipation

High power means heat - ensure adequate cooling

❌ Common Mistakes to Avoid

Unit Confusion

Always use base units: Volts, Amps, Ohms (not milliamps or kilohms unless converting)

Ignoring Power Ratings

A 1/4W resistor can't handle 1W of power - check P = I²R

AC vs DC Confusion

Ohm's Law applies directly to DC; AC requires impedance considerations

Temperature Effects

Resistance changes with temperature - especially in semiconductors

How to use this calculator

⚡ How to Use This Calculator

  1. Select your calculation mode: Basic Ohm's Law, Power Calculations, or use the Interactive Wheel
  2. Enter any two known values - the calculator will find the missing parameters
  3. For Basic mode: Enter two of V, I, or R to find the third plus power
  4. For Power mode: Enter any two values to calculate all four parameters
  5. Results include unit conversions and practical information like wire gauge recommendations
  6. Use the Ohm's Law Wheel tab for a visual reference of all formulas

📐 Essential Formulas

Ohm's Law

V = I × R

  • • V = Voltage (Volts)
  • • I = Current (Amperes)
  • • R = Resistance (Ohms)

Power Formulas

P = V × I

P = I² × R

P = V² ÷ R

P = Power (Watts)

🔧 Common Applications

  • LED Circuit Design: Calculate current-limiting resistor values to protect LEDs from overcurrent
  • Power Consumption: Determine electrical costs and heat dissipation for devices and appliances
  • Wire Sizing: Select appropriate wire gauge based on current requirements
  • Circuit Troubleshooting: Diagnose problems by comparing calculated vs measured values
  • Component Selection: Choose resistors with appropriate power ratings for your circuit

📊 Quick Reference Guide

Unit Conversions

  • 1000 mA = 1 A
  • 1000 Ω = 1 kΩ
  • 1000 kΩ = 1 MΩ
  • 1000 mW = 1 W
  • 1000 W = 1 kW

Safety Voltages

  • < 50V DC: Generally safe
  • < 30V AC: Generally safe
  • > 50V: Dangerous
  • > 1000V: High voltage

Power Margins

  • Resistors: Use 2× power
  • Capacitors: Use 2× voltage
  • Wire: 80% of rating
  • Fuses: 125% of load

💡 Pro Tips

Always verify units

Convert to base units (V, A, Ω) before calculating

Include safety margins

Components should be rated above calculated values

Consider temperature

Resistance changes with temperature

Account for tolerances

Real components vary from nominal values

About this calculator

Calculate voltage, current, resistance, and power using Ohm's Law. Essential electrical engineering tool with interactive formulas and circuit examples.

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