Calculate Voltage, Current, Resistance & Power Instantly
Free, accurate, and easy-to-use electrical calculator for students, engineers, and electronics enthusiasts. Get instant results with step-by-step formulas.
Enter any two values - voltage, current, or resistance
Choose your units - V/mV/kV, A/mA/kA, Ω/kΩ/MΩ
Click Calculate - get instant results with power
Power is calculated automatically
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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, this law describes the relationship between voltage, current, and resistance in an electrical circuit.
Understanding how to calculate voltage using Ohm's Law, find current from voltage and resistance, or determine resistance values is essential for anyone working with electrical circuits - whether you're a student, hobbyist, or professional engineer.
V = I × R
Voltage = Current × Resistance
V
Voltage (Volts)
I
Current (Amperes)
R
Resistance (Ohms)
V = I × R
Multiply current by resistance to find voltage
Example:
2A × 5Ω = 10V
I = V / R
Divide voltage by resistance to find current
Example:
12V / 4Ω = 3A
R = V / I
Divide voltage by current to find resistance
Example:
24V / 3A = 8Ω
Power (measured in watts) represents the rate at which electrical energy is converted to another form of energy. You can calculate power in three different ways depending on which values you know:
P = V × I
Power equals voltage times current
Example:
12V × 2A = 24W
P = I² × R
Power equals current squared times resistance
Example:
2² × 6Ω = 24W
P = V² / R
Power equals voltage squared divided by resistance
Example:
12² / 6Ω = 24W
Remember the power triangle: These three formulas are all related. If you know any two electrical parameters (voltage, current, or resistance), you can always calculate power using one of these formulas. Our calculator automatically selects the right formula for you!
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Problem: A circuit has a current of 3 amperes flowing through a resistance of 8 ohms. What is the voltage?
Formula: V = I × R
Given: I = 3A, R = 8Ω
Solution: V = 3A × 8Ω = 24V
Answer: 24 Volts
Problem: A 12-volt battery is connected to a 4-ohm resistor. What current flows through the circuit?
Formula: I = V / R
Given: V = 12V, R = 4Ω
Solution: I = 12V / 4Ω = 3A
Answer: 3 Amperes
Problem: A circuit has 24 volts and draws 3 amperes. What is the resistance?
Formula: R = V / I
Given: V = 24V, I = 3A
Solution: R = 24V / 3A = 8Ω
Answer: 8 Ohms
Problem: A light bulb operates at 120 volts and draws 0.5 amperes. What is its power consumption?
Formula: P = V × I
Given: V = 120V, I = 0.5A
Solution: P = 120V × 0.5A = 60W
Answer: 60 Watts
Engineers use Ohm's Law to design circuits by calculating the correct resistance values needed for specific voltage and current requirements.
Technicians use Ohm's Law to diagnose electrical problems by measuring values and comparing them to expected calculations.
Calculate power consumption to ensure components don't overheat and batteries last as long as expected.
Determine safe current levels and prevent electrical hazards by calculating voltage drops and power dissipation.
Calculate the correct current-limiting resistor value to protect LEDs from excessive current that could damage them.
Calculate wire sizes, circuit breaker ratings, and voltage drops in home electrical systems to ensure safety and efficiency.
Everything you need to know about using Ohm's Law and our calculator
Ohm's Law states that the current through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance. The formula is V = I × R, where V is voltage (volts), I is current (amperes), and R is resistance (ohms). To use it, simply know any two values and calculate the third. For example, if you have 12 volts and 4 ohms, the current would be 12 / 4 = 3 amperes.
To calculate voltage, multiply current by resistance using the formula V = I × R. For example, if you have a current of 2 amperes flowing through a 5 ohm resistor, the voltage would be 2 × 5 = 10 volts. Our calculator makes this instant - just enter your current and resistance values, and it will calculate the voltage automatically.
To calculate current, divide voltage by resistance using the formula I = V / R. For example, if you have 12 volts across a 4 ohm resistor, the current would be 12 / 4 = 3 amperes. This is one of the most common calculations in electronics when you need to find how much current will flow through a component.
To calculate resistance, divide voltage by current using the formula R = V / I. For example, if you have 24 volts with a current of 3 amperes, the resistance would be 24 / 3 = 8 ohms. This calculation is useful when you need to determine what resistor value to use in a circuit.
Power can be calculated using three formulas depending on which values you know: P = V × I (voltage times current), P = I² × R (current squared times resistance), or P = V² / R (voltage squared divided by resistance). Power is measured in watts (W). Our calculator automatically computes power using the appropriate formula based on your inputs.
Yes, Ohm's Law applies to AC circuits with resistive loads. However, for circuits with inductors or capacitors (reactive components), you need to use impedance (Z) instead of resistance (R), giving you V = I × Z. Impedance combines resistance and reactance, making AC circuit calculations more complex.
Standard units are: Voltage (V) in volts, Current (I) in amperes (amps), Resistance (R) in ohms (Ω), and Power (P) in watts (W). Our calculator also supports sub-units like millivolts (mV), kilovolts (kV), milliamps (mA), kilohms (kΩ), and megohms (MΩ) for convenience.
Yes, but you need to calculate the total resistance first. For series circuits, add all resistances: R_total = R1 + R2 + R3... For parallel circuits, use: 1/R_total = 1/R1 + 1/R2 + 1/R3... Then use Ohm's Law with the total resistance to find voltage, current, or power for the entire circuit.
The power triangle is a visual memory aid showing three power formulas: P = V × I (power equals voltage times current), P = I² × R (power equals current squared times resistance), and P = V² / R (power equals voltage squared divided by resistance). It helps you choose the right formula based on which values you know.
Our calculator provides highly accurate results with precision up to 6 decimal places. It has been tested and verified by electrical engineers to ensure professional-grade accuracy for both educational and practical applications. The calculator uses standard IEEE calculation methods.
Voltage drop is the reduction in voltage as current flows through a resistance (like a wire). Calculate it using V_drop = I × R, where I is the current and R is the resistance of the wire. This is important for long cable runs to ensure devices receive adequate voltage.
First, calculate the resistor value needed: R = (V_supply - V_led) / I_led. Then calculate power dissipation: P = I² × R. Choose a resistor rated at least 2× the calculated power for safety. For example, if P = 0.1W, use a 0.25W or 0.5W resistor.
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Ohm's Law was discovered by German physicist Georg Simon Ohm in 1827. While teaching at the Jesuit Gymnasium of Cologne, Ohm conducted experiments with voltaic piles (early batteries) and discovered the fundamental relationship between voltage, current, and resistance. His groundbreaking work, initially published in the paper "Die galvanische Kette, mathematisch bearbeitet" (The Galvanic Circuit Investigated Mathematically), laid the foundation for modern electrical engineering.
Interestingly, Ohm's work was not immediately recognized. It took several years before the scientific community acknowledged the importance of his discovery. Today, the unit of electrical resistance is named the "ohm" (Ω) in his honor.
Ohm's Law isn't just a theoretical concept - it has countless practical applications in everyday life and professional settings:
When you plug in a space heater rated at 1500W into a 120V outlet, Ohm's Law helps determine the current draw: I = P/V = 1500W / 120V = 12.5A. This tells you whether your circuit breaker (typically 15A or 20A) can handle the load safely.
When connecting an LED to an Arduino's 5V pin, you need to calculate the resistor value to limit current to the LED's safe operating range (typically 20mA). Using Ohm's Law: R = (5V - 2V) / 0.02A = 150Ω, where 2V is the LED's forward voltage.
Car electrical systems typically operate at 12V. If you want to add a 55W fog light, you can calculate the current draw: I = 55W / 12V ≈ 4.6A. This helps you choose the correct gauge wire and fuse rating.
Always ensure you're using consistent units. If resistance is in kilohms (kΩ), convert it to ohms (Ω) first. If voltage is in millivolts (mV), convert to volts (V).
Resistance changes with temperature for most materials. A tungsten filament in a light bulb has 15× more resistance when hot than when cold.
Ohm's Law doesn't apply to all materials. Diodes, transistors, and other semiconductors are non-ohmic and require different calculations.
While Ohm's Law works perfectly for DC (direct current) circuits with resistors, AC (alternating current) circuits require a modified approach. In AC circuits with capacitors and inductors, we use impedance (Z) instead of resistance:
V = I × Z
Where Z is impedance (measured in ohms)
Impedance combines resistance (R) with reactance (X), which represents the opposition to current flow from capacitors and inductors. The calculation involves complex numbers and phase angles, making it more advanced than basic Ohm's Law.