Ohm's Law Calculator: Solve for Voltage, Current, Resistance & Power

Ohm’s Law Calculator: Solve for Voltage, Current, Resistance & Power

Understanding the relationship between voltage, current, resistance, and power is the cornerstone of all electronics. Whether you’re a student, a DIY hobbyist, or an engineer, our Ohm’s Law calculator helps you instantly solve for any missing value in your circuit. Simply enter any two known values to find the other two.

Calculate Voltage, Current, Resistance, and Power.

Voltage (V)

Current (I)

Resistance (R)

Power (P)

Ohm's Law Wheel

How to Use Our Ohm’s Law Calculator

This calculator is a flexible tool designed to solve for any two unknown variables. To use it, you only need to know two of the four fundamental values of your circuit.

Enter Your Known Values: Input any two of the following four values into their respective fields.
Leave Unknowns Blank: Leave the two fields you want to solve for empty.
Calculate: The calculator will automatically compute the two missing values based on your inputs.

Input/Output Fields Explained:

Voltage (V): Measured in Volts (V). This represents the electric potential difference or “pressure” in a circuit.
Current (I): Measured in Amperes (A), or “amps”. This is the rate of flow of electric charge.
Resistance (R): Measured in Ohms (Ω). This is the opposition to the flow of current.
Power (P): Measured in Watts (W). This is the rate at which electrical energy is transferred or consumed in the circuit.

For example, if you have a 12V source and a 100Ω resistor, you would enter “12” for Voltage and “100” for Resistance to solve for Current and Power.

Understanding Your Results

The numbers our calculator provides are the missing electrical properties of your circuit, all derived from two fundamental laws: Ohm’s Law and the Power Law. Grasping these concepts will allow you to not just get an answer, but truly understand what’s happening in your circuit.

The Core Concept: Ohm’s Law Explained with an Analogy

The easiest way to understand Ohm’s Law is with the classic water pipe analogy:

Voltage (V) is like the water pressure in the pipe. It’s the force pushing the water. A water tower creates high pressure; a small pump creates low pressure.
Current (I) is the flow rate of the water. It’s the actual volume of water moving through the pipe per second.
Resistance (R) is the width of the pipe. A wide pipe has low resistance and allows a high flow rate. A narrow, constricted pipe has high resistance and restricts the flow rate.

Ohm’s Law is the mathematical relationship between these three:

V = I \times R

This formula tells us that the pressure (V) is equal to the flow rate (I) multiplied by the pipe’s narrowness (R). You can rearrange it to solve for any component:

To find Current: $I = f r a c V R$ (Flow equals pressure divided by narrowness)
To find Resistance: $R = f r a c V I$ (Narrowness equals pressure divided by flow)

The Fourth Piece: Electrical Power

Power is the rate at which energy is being used. In our analogy, it’s like the total force the flowing water could exert to turn a water wheel. It’s calculated using the Power Law:

P = V \times I

Power (P) is equal to the pressure (V) multiplied by the flow rate (I). A high-pressure, high-flow stream has a lot of power.

The Ohm’s Law Wheel: All 12 Formulas

By combining Ohm’s Law ( $V = I R$ ) and the Power Law ( $P = V I$ ), we can derive formulas to solve for any variable using any pair of known values. These relationships are often shown in a diagram called an Ohm’s Law Wheel. Our calculator uses these 12 formulas to work its magic.

To Find…	Given V & I	Given V & R	Given I & R	Given P & I	Given P & V	Given P & R
Voltage (V)	–	–	$V = I t im es R$	$V = f r a c P I$	–	$V = s q r t P t im es R$
Current (I)	–	$I = f r a c V R$	–	–	$I = f r a c P V$	$I = s q r t f r a c P R$
Resistance (R)	$R = f r a c V I$	–	–	$R = f r a c P I^{2}$	$R = f r a c V^{2} P$	–
Power (P)	$P = V t im es I$	$P = f r a c V^{2} R$	$P = I^{2} t im es R$	–	–	–

Frequently Asked Questions

What is the easiest way to remember Ohm’s Law?

Use the Ohm’s Law Triangle, a simple mnemonic device. Draw a triangle and divide it in half horizontally. Divide the bottom half vertically. Place V in the top section, and I and R in the bottom sections.

To solve for Voltage (V), cover the ‘V’. You are left with I × R.
To solve for Current (I), cover the ‘I’. You are left with V / R.
To solve for Resistance (R), cover the ‘R’. You are left with V / I.

How can I use Ohm’s Law to choose the right resistor for an LED?

This is a perfect practical application. An LED requires a specific voltage (its “forward voltage,” $V_f$ ) and current to light up safely. If you connect it to a higher voltage source, you need a resistor to limit the current.

Concrete Example: You have a 9V battery and a standard red LED with a forward voltage ( $V_f$ ) of 2V and a desired current ( $I$ ) of 20mA (which is 0.020A).

Find the voltage the resistor must handle: The resistor needs to “drop” the excess voltage. Voltage across Resistor (V_R) = Battery Voltage - LED Voltage V_R = 9V - 2V = 7V
Use Ohm’s Law to find the required resistance: Now you know the voltage across the resistor (7V) and the current that must flow through it (0.020A). R = V_R / I R = 7V / 0.020A = 350Ω

The closest standard resistor value is 390Ω, which would be a safe choice.

Does Ohm’s Law apply to both AC and DC circuits?

Yes, but with a key difference.

In DC (Direct Current) circuits, Ohm’s Law ( $V = I R$ ) works perfectly as written.
In AC (Alternating Current) circuits, components like capacitors and inductors create a type of opposition to current called reactance. The total opposition in an AC circuit, which includes both resistance and reactance, is called Impedance (Z). The AC version of Ohm’s Law is therefore $V = I t im es Z$ . For simple resistive loads like a toaster, impedance and resistance are the same.

What happens if resistance is zero (a short circuit)?

A short circuit occurs when the resistance in a circuit becomes nearly zero (e.g., a frayed wire touches the metal case). According to Ohm’s Law ( $I = V / R$ ), as resistance (R) approaches zero, the current (I) approaches infinity. In reality, the current will surge to a very high level, limited only by the voltage source’s capability. This massive current generates intense heat ( $P = I^{2} R$ ), instantly melting wires, destroying components, and creating a serious fire hazard. This is why we use fuses and circuit breakers—to safely interrupt the circuit during a high-current short.

What happens if resistance is infinite (an open circuit)?

An open circuit is a break in the path, like a flipped switch or a broken wire. Here, the resistance is effectively infinite. According to Ohm’s Law ( $I = V / R$ ), as resistance (R) approaches infinity, the current (I) approaches zero. No current can flow because there is no complete path.

How does Ohm’s Law explain heat in a circuit?

The power dissipated as heat in a resistor is calculated by the formula $P = I^{2} t im es R$ . This means the heat generated is proportional to the resistance and, more importantly, to the square of the current. Doubling the current flowing through a resistor will quadruple the heat it produces. This is the principle behind devices like electric toasters and space heaters, which use highly resistive wires to generate heat purposefully.

Is high voltage or high current more dangerous?

While high voltage can be dangerous, it’s the current flowing through the body that causes injury or death. However, voltage and current are linked by Ohm’s Law. Your body has a certain electrical resistance. A high voltage is dangerous because it has the “pressure” to push a lethal amount of current through your body’s resistance. A low voltage source, like a 1.5V AA battery, simply doesn’t have enough pressure to push a harmful current through you.

Why do my lights dim when a large appliance turns on?

The wiring in your house has a small amount of resistance. When a large appliance with a motor (like a refrigerator or air conditioner) starts up, it draws a huge “inrush” current for a moment. According to Ohm’s Law ( $V = I R$ ), this large current (I) flowing through the resistance of your house wiring (R) causes a larger voltage drop (V) along the wires. This leaves less voltage available for everything else on that circuit, causing your lights to momentarily dim.

How do I measure V, I, and R in a real circuit?

You use a tool called a multimeter.

To measure Voltage: Set the multimeter to V. Place the probes in parallel with (across) the component you want to measure.
To measure Current: Set the multimeter to A (or mA). You must break the circuit and place the multimeter in series with the circuit, so the current flows through the meter.
To measure Resistance: Set the multimeter to Ω. The component must be removed from the circuit or the circuit’s power must be off. Place the probes across the component.

How are voltage, current, and power related?

They are inextricably linked.

Voltage is the cause (pressure).
Current is the effect (flow).
Resistance is the opposition.
Power is the rate of work being done.

You cannot have current without voltage. You cannot calculate power without knowing two of the other three variables. Our calculator uses all the relationships in the Power Wheel to solve the complete picture for you.

Now that you’ve mastered Ohm’s Law, you might need to identify the value of a specific component. Use our Resistor Color Code Calculator to quickly find the resistance of any resistor. To see how these principles apply to dividing voltage in a circuit, check out our Voltage Divider Calculator.

Creator

Ismael Vargas

An experienced software developer specializing in React, JavaScript, Django and Python, with more than six years’ expertise building full‑stack applications, data visualizations and cloud‑hosted solutions. He has a strong background in API integration, testing, and AWS services, delivering polished web products.

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