Ohm's Law Calculator
Calculate voltage, current, resistance, and power using Ohm's Law equations. Enter any 2 values and the calculator will automatically compute the remaining values in real-time.
Input Values
Voltage
Current
Resistance
Power
Applied Formulas
Ohm's Law
V = I × R
I = V / R
R = V / I
Power Formulas
P = V × I
P = V² / R
P = I² × R
Values Distribution
Circuit Diagram
┌─────────────┐
│ │
│ Voltage │
┌───┤ Source ├───┐
│ │ │ │
│ └─────────────┘ │
│ │
│ +V │
│ │
─┴─ ┌───┐ │
─┬─ ←I │ R │ │
│ └───┘ │
│ │
│ GND │
│ │
└─────────────────────┘
V = Voltage (110 V)
I = Current (0.2 A)
R = Resistance (? Ω)
P = Power (? W)
What is Ohm's Law?
Ohm's Law is one of the most fundamental principles in electronics and electrical engineering, named after German physicist Georg Simon Ohm who published it in 1827. The law states that the current flowing through a conductor between two points isdirectly proportional to the voltage across the two points, provided the temperature and other physical conditions remain constant.
In simpler terms, Ohm's Law describes the relationship between three fundamental electrical quantities:voltage (V), current (I), and resistance (R). This relationship is expressed by the equation V = I × R, which can be algebraically rearranged to solve for any of the three variables when the other two are known.
The Three Forms of Ohm's Law
V = I × R
Voltage equals Current times Resistance
I = V / R
Current equals Voltage divided by Resistance
R = V / I
Resistance equals Voltage divided by Current
Understanding Voltage, Current, and Resistance
To truly understand Ohm's Law, you need to understand what each of the three fundamental electrical quantities represents and how they interact in an electrical circuit.
Voltage (V) - Electrical Pressure
Voltage, also called electric potential difference, is the "pressure" that pushes electrical charge through a conductor. It's measured in volts (V)and represents the amount of energy available to move electrons from one point to another.
Water Analogy:
Think of voltage as water pressure in a pipe. Higher pressure (voltage) pushes more water (current) through the pipe. A 12V battery has more "pressure" than a 9V battery.
Common Voltages:
- • AA Battery: 1.5V
- • USB Power: 5V
- • Car Battery: 12V
- • Household Outlet (US): 120V
- • Household Outlet (Europe): 230V
Current (I) - Flow of Charge
Current is the rate of flow of electrical charge through a conductor. It's measured in amperes or amps (A), where one ampere equals one coulomb of charge passing through a point per second.
Water Analogy:
Current is like the flow rate of water through a pipe—gallons per minute. More current means more electrons flowing past a point per second.
Typical Current Levels:
- • LED: 20mA (0.02A)
- • Smartphone Charging: 1-2A
- • Laptop: 3-5A
- • Microwave Oven: 10-15A
- • Electric Car Charger: 30-80A
Resistance (R) - Opposition to Flow
Resistance is the opposition to current flow in an electrical circuit. It's measured in ohms (Ω) and determines how much current will flow for a given voltage. Materials with low resistance (conductors) allow current to flow easily, while materials with high resistance (insulators) restrict current flow.
Water Analogy:
Resistance is like the diameter of a pipe or friction inside it. A narrow pipe (high resistance) restricts water flow, while a wide pipe (low resistance) allows more flow.
Resistance Examples:
- • Copper Wire (1m, 1mm²): 0.017Ω
- • LED Current Limiting: 220Ω
- • Typical Resistor: 1kΩ - 10kΩ
- • Multimeter Input: 10MΩ
- • Insulator: >1GΩ
Understanding Electrical Power
Electrical power is the rate at which electrical energy is transferredby an electric circuit. It's measured in watts (W), named after Scottish inventor James Watt. Power represents how quickly energy is being used or generated—one watt equals one joule of energy transferred per second.
In resistive circuits, electrical power can be calculated using several formulas derived from combining Ohm's Law with the basic power equation. These formulas allow you to calculate power when you know different combinations of voltage, current, and resistance.
Power Formulas (Joule's Law)
P = V × I
Basic power formula
Power = Voltage × Current
P = V² / R
When you know V and R
Substituting I = V/R
P = I² × R
When you know I and R
Substituting V = I×R
Power in Everyday Life
💡 Household Appliances
• LED Bulb: 10W
• Laptop: 45-65W
• Desktop PC: 200-500W
• Microwave: 1,000-1,500W
• Electric Heater: 1,500-2,000W
⚡ Power Consumption
Energy vs Power: Power is the rate of energy use. A 100W bulb uses 100W of power continuously. If left on for 10 hours, it consumes 1,000 watt-hours (1 kWh) of energy.
Cost Example: At $0.12 per kWh, that 100W bulb costs $0.12 to run for 10 hours.
Using the Ohm's Law Formula Wheel
The Ohm's Law Formula Wheel (also called the Ohm's Law Circle or Wheel Chart) is a visual tool that shows all the relationships between voltage (V), current (I), resistance (R), and power (P). It's an excellent memory aid for quickly determining which formula to use based on which values you know and which you need to find.
How to Use the Formula Wheel
Finding Voltage (V)
• V = I × R (if you know I and R)
• V = P / I (if you know P and I)
• V = √(P × R) (if you know P and R)
Finding Current (I)
• I = V / R (if you know V and R)
• I = P / V (if you know P and V)
• I = √(P / R) (if you know P and R)
Finding Resistance (R)
• R = V / I (if you know V and I)
• R = V² / P (if you know V and P)
• R = P / I² (if you know P and I)
Finding Power (P)
• P = V × I (if you know V and I)
• P = V² / R (if you know V and R)
• P = I² × R (if you know I and R)
Step-by-Step Example
Problem: A circuit has a 12V battery and a 6Ω resistor. Find the current and power.
Step 1: Find current using I = V / R
I = 12V / 6Ω = 2A
Step 2: Find power using P = V × I
P = 12V × 2A = 24W
Or use: P = V² / R = 144 / 6 = 24W ✓
Practical Applications of Ohm's Law
Ohm's Law isn't just theoretical—it's used constantly in real-world electrical and electronics work. Understanding these applications helps you design circuits, troubleshoot problems, and ensure safe operation of electrical systems.
Circuit Design
When designing circuits, Ohm's Law helps determine the correct resistor values for specific purposes:
LED Current Limiting
LEDs need constant current. For a 20mA LED with 2V forward voltage on a 5V supply: R = (5V - 2V) / 0.02A = 150Ω
Voltage Dividers
Create specific voltages from a higher voltage source using resistor ratios. Essential for sensor interfaces and reference voltages.
Troubleshooting
Ohm's Law is invaluable for diagnosing circuit problems:
🔍 Short Circuits
If measured current is much higher than expected, calculate the actual resistance. Very low resistance indicates a short.
🔍 Open Circuits
If current is zero but voltage is present, the circuit is open (broken connection or failed component).
Electrical Safety
Understanding current and power relationships is critical for safety:
⚠️ Wire Sizing
Calculate current draw to select appropriate wire gauge. Too thin wire for high current causes overheating and fire risk. Use I = P / V to find current for known power loads.
⚠️ Fuse/Breaker Selection
Calculate maximum current to size protective devices correctly. A 1,500W heater on 120V draws 12.5A, requiring at least a 15A circuit breaker.
Battery Life Calculations
Estimate how long a battery will power a device:
Example: A device draws 100mA from a 2,000mAh battery:
Runtime = Battery Capacity / Current Draw
Runtime = 2,000mAh / 100mA = 20 hours
(Actual runtime is usually 70-80% of calculated due to voltage drop and efficiency losses)
Common Mistakes & Pro Tips
Common Mistakes to Avoid
❌ Unit Confusion
Always use consistent units: V in volts, I in amps, R in ohms, P in watts. Convert milliamps to amps, kilohms to ohms, etc.
❌ Applying to Non-Resistive Circuits
Ohm's Law doesn't directly apply to capacitors, inductors, or semiconductors (diodes, transistors). These require more complex analysis.
❌ Ignoring Power Ratings
Always check if calculated power exceeds component ratings. A ¼W resistor dissipating 1W will overheat and fail.
❌ Parallel vs Series Confusion
In series circuits, current is the same everywhere. In parallel circuits, voltage is the same across branches. Mix these up and your calculations will be wrong.
Pro Tips for Success
Draw the Circuit
Always sketch the circuit before calculating. Label all voltages, currents, and resistances you know and what you need to find.
Use Standard Values
Resistors come in standard values (E12, E24, E96 series). When designing, round calculated values to the nearest standard value.
Add Safety Margins
For power calculations, use at least 2× safety factor. If calculated power is 0.5W, use a 1W or higher rated resistor.
Measure and Verify
After building, measure actual voltages and currents. Real components have tolerances—a "10kΩ" resistor might be 9.5kΩ or 10.5kΩ.
Think About Heat
Power dissipated as heat needs to go somewhere. High-power resistors need ventilation or heat sinks. Dense circuits need airflow.
Key Takeaways & Quick Reference
Essential Formulas
Ohm's Law:
V = I × R
I = V / R
R = V / I
Power:
P = V × I
P = I² × R
P = V² / R
Units Reference
Common prefixes: m (milli, ×10⁻³), k (kilo, ×10³), M (mega, ×10⁶), µ (micro, ×10⁻⁶)
When to Use This Calculator
Circuit Design
Calculate component values before building
Troubleshooting
Verify circuit behavior matches theory
Learning
Explore relationships between V, I, R, P