Ohm's Law Calculator - Free Voltage, Current, Resistance & Power Calculator
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
Results:
Calculation Steps:
⚡ Ohm's Law & Power Formulas
Basic Ohm's Law
V = Voltage (Volts), I = Current (Amps), R = Resistance (Ohms)
Power Formulas
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
- Select your calculation mode: Basic Ohm's Law, Power Calculations, or use the Interactive Wheel
- Enter any two known values - the calculator will find the missing parameters
- For Basic mode: Enter two of V, I, or R to find the third plus power
- For Power mode: Enter any two values to calculate all four parameters
- Results include unit conversions and practical information like wire gauge recommendations
- 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
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•
LED Circuit Design: Calculate current-limiting resistor values to protect LEDs from overcurrent
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•
Power Consumption: Determine electrical costs and heat dissipation for devices and appliances
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•
Wire Sizing: Select appropriate wire gauge based on current requirements
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Circuit Troubleshooting: Diagnose problems by comparing calculated vs measured values
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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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