Calculate voltage, current, resistance, and electrical power accurately using Ohm's Law and Watt's Law. Perfect for electrical engineering students, hobbyists, and professionals designing circuits and troubleshooting electronics
Click on any parameter to calculate it
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Volts (V)
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Amperes (A)
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Ohms (Ω)
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Watts (W)
Ohm's Law is one of the most fundamental principles in electrical engineering and physics. Discovered by German physicist Georg Simon Ohm in 1827, this law describes the relationship between voltage, current, and resistance in electrical circuits. It states that the current through a conductor between two points is directly proportional to the voltage across the two points and inversely proportional to the resistance between them.
Key Principle: Ohm's Law forms the foundation for understanding electrical circuits and is essential for anyone working with electronics, from hobbyists to professional engineers. Use our scientific calculator for complex electrical calculations.
The law applies to ohmic materials, which are materials that have a constant resistance regardless of the voltage or current applied. Most metals at room temperature behave as ohmic materials, making Ohm's Law widely applicable in practical electrical work.
Georg Ohm's discovery was initially met with skepticism by the scientific community. However, his work laid the groundwork for the development of electrical theory and the eventual creation of our modern electrical infrastructure.
Ohm's Law can be expressed in three different ways, depending on which parameter you want to calculate. These three formulas are the cornerstone of electrical calculations.
Voltage = Current × Resistance
Example:
I = 2A, R = 10Ω
V = 2 × 10 = 20V
Current = Voltage / Resistance
Example:
V = 12V, R = 6Ω
I = 12 / 6 = 2A
Resistance = Voltage / Current
Example:
V = 9V, I = 3A
R = 9 / 3 = 3Ω
Memory Aid: Remember the Ohm's Law triangle! Put your finger over the unknown value, and the remaining two show you the formula. Use our basic calculator for quick calculations.
Watt's Law complements Ohm's Law by describing the relationship between power, voltage, and current. Named after James Watt, this law is crucial for understanding energy consumption and heat generation in electrical circuits.
Power = Voltage × Current
Power = Current² × Resistance
Power = Voltage² / Resistance
Cover the unknown to find the formula
Power dissipated in a resistor is converted to heat. This is crucial for:
Example: LED Current Limiting
5V supply, 2V LED, 20mA current
Resistor: R = (5-2)/0.02 = 150Ω
Power: P = (5-2) × 0.02 = 0.06W
Energy Calculation: Energy (kWh) = Power (kW) × Time (hours). Use our percentage calculator to determine efficiency percentages.
Ohm's Law is used daily by electricians, engineers, and technicians in various applications. Understanding these practical uses helps bridge the gap between theory and real-world problem-solving.
Circuit Breaker Sizing
I = P / V determines breaker rating
Example: 2400W heater ÷ 240V = 10A breaker
Wire Sizing
Current determines minimum wire gauge
Higher current = thicker wire needed
Energy Bills
kWh = (V × I × hours) / 1000
Calculate appliance operating costs
Motor Control
Calculate starting current and power
Size contactors and protection devices
Heating Elements
P = V² / R for heating calculations
Design furnaces and industrial heaters
Power Distribution
Voltage drop calculations: V = I × R
Ensure adequate voltage at load
LED Circuits
Current limiting resistor calculation
R = (Vsupply - VLED) / ILED
Voltage Dividers
Create reference voltages
Vout = Vin × (R2 / (R1 + R2))
Sensor Interfaces
Convert resistance changes to voltage
Temperature, pressure, light sensors
Battery Testing
Load testing using known resistance
Measure voltage drop under load
Fuse Selection
I = P / V determines fuse rating
Protect circuits from overcurrent
Lighting Systems
Calculate bulb power and current
Size alternator and wiring
Design Tip: Always include a safety margin in your calculations. For power ratings, use components rated at least 25% higher than calculated values. Use our scientific calculator for complex design calculations.
While Ohm's Law applies to both AC and DC circuits, AC calculations involve additional considerations such as impedance, power factor, and reactive components. Understanding these differences is crucial for working with modern electrical systems.
Simple Ohm's Law
V = I × R (exactly)
Resistance is constant
Power Calculation
P = V × I (real power only)
No reactive power
Applications
Batteries, electronics, solar panels
Constant voltage and current
Impedance (Z)
V = I × Z (complex)
Z = R + jX (reactance)
Power Factor
P = V × I × cos(φ)
φ = phase angle
RMS Values
Use RMS for calculations
VRMS = Vpeak / √2
S² = P² + Q² (Power Triangle)
Important: In AC circuits, always use RMS values for voltage and current in Ohm's Law calculations. Peak values will give incorrect results for power calculations.
Working with electricity requires strict adherence to safety protocols. Understanding the relationship between voltage, current, and resistance helps identify potential hazards and implement appropriate safety measures.
Remember: It's current that kills, not voltage. However, higher voltages can drive more current through the body's resistance. Always respect electricity and follow proper safety procedures.
Ohm's Law is the foundation for more complex circuit analysis techniques. Understanding how to apply it in series, parallel, and combination circuits is essential for electrical troubleshooting and design.
R1 — R2 — R3
Parallel branches
Series + Parallel
Analysis Tip: Start with what you know and work systematically. Draw clear circuit diagrams and label all known values. Use our scientific calculator for complex calculations.
Ohm's Law is an invaluable tool for diagnosing electrical problems. By measuring voltage, current, and resistance, technicians can quickly identify faulty components and circuit issues.
Open Circuit
Infinite resistance, zero current
Full voltage across open
Short Circuit
Zero resistance, maximum current
Zero voltage across short
High Resistance
Reduced current, voltage drop
Poor connections, corrosion
Symptoms: LED circuit not lighting
Tests:
Symptoms: Motor overheating
Tests:
Safety First: Always follow lockout/tagout procedures and use properly rated test equipment. Never work on live circuits unless absolutely necessary and properly trained.
Beyond basic Ohm's Law applications, there are advanced concepts that extend its principles to more complex electrical phenomena and modern applications.
Resistance changes with temperature affect Ohm's Law calculations:
Temperature Coefficient
R(T) = R₀[1 + α(T - T₀)]
α = temperature coefficient
At high frequencies, additional factors affect circuit behavior:
Some components don't follow Ohm's Law linearly:
Capacitors and inductors store energy, affecting circuit behavior:
Accurate measurements are essential for applying Ohm's Law effectively. Understanding the proper use of measurement instruments ensures reliable results and safe operation.
Measurement Accuracy: The accuracy of your calculations depends on the accuracy of your measurements. Always use properly calibrated instruments and consider measurement uncertainty in critical applications.
Even experienced professionals can make errors when applying Ohm's Law. Understanding common pitfalls helps prevent mistakes and ensures accurate calculations.
Unit Confusion
Mixing mA with A, kΩ with Ω
Always convert to base units first
AC vs DC
Using peak values instead of RMS
RMS values for power calculations
Circuit Loading
Meter affects circuit behavior
Consider meter impedance
Double-Check Units
Write units with every number
Use dimensional analysis
Verify Results
Check if answers make sense
Use multiple calculation methods
Document Work
Show all calculation steps
Include assumptions made
Pro Tip: When in doubt, go back to basics. Draw the circuit, label known values, and work step by step. Use our calculator above to verify your manual calculations.
In DC circuits, Ohm's Law applies directly: V = I × R. In AC circuits, you must use RMS values for voltage and current, and consider impedance (Z) instead of just resistance (R). For AC: V = I × Z, where Z includes both resistance and reactance components.
Ohm's Law only applies to ohmic materials that have a linear relationship between voltage and current. Non-ohmic components like diodes, LEDs, and transistors have non-linear I-V characteristics and don't follow Ohm's Law directly.
Power can be calculated using three formulas: P = V × I (most direct), P = I² × R (when you know current and resistance), or P = V² / R (when you know voltage and resistance). Choose the formula based on the values you have available.
All electrical power dissipated in a resistor is converted to heat. This is why components have power ratings - exceeding these ratings causes overheating and potential failure. Always ensure your components can handle the calculated power with a safety margin.
Always convert to base units: volts (V), amperes (A), ohms (Ω), and watts (W). For example, convert 5mA to 0.005A and 2.2kΩ to 2200Ω before calculating. This prevents unit-related errors in your calculations.
Yes, but you must first calculate the equivalent resistance. For series: RT = R1 + R2 + R3. For parallel: 1/RT = 1/R1 + 1/R2 + 1/R3. Then apply Ohm's Law to the equivalent circuit, and work backwards to find individual component values.
Always remember that current kills, not voltage. However, higher voltages can drive more current through your body. Use proper PPE, follow lockout/tagout procedures, and never work on live circuits unless absolutely necessary and properly trained. Respect all electrical safety protocols.