Ohm's Law Calculator
Interactive Ohm's Law and power calculator: choose what to solve for (V, I, R, or P), then enter any two of the remaining three values. The tool automatically picks the correct formula, fills in the missing value, and shows normalized circuit stats.
Calculation Steps
Circuit Design · Ohm’s Law
Guide: How to Master Ohm’s Law Calculations
Learn how to use the Ohm’s Law calculator above, see the key \( V = I \times R \) formulas, explore the Ohm’s Law wheel, and work through real-world examples for voltage, current, resistance, and power.
What is Ohm’s Law? (V = I × R)
Ohm’s Law describes the relationship between voltage (V), current (I), and resistance (R) in an electrical circuit. The basic Ohm’s Law formula is:
If you know any two of these values, you can calculate the third. The Ohms Law calculator above automates this process and also includes the related power equations so you can solve for volts, amps, ohms, or watts without doing algebra by hand.
In simple terms:
- Voltage (V) is like pressure, pushing electrons through the circuit.
- Current (I) is the flow of electrons, measured in amperes (A).
- Resistance (R) is how much the circuit resists that flow, measured in ohms (Ω).
How to use the Ohm’s Law Calculator
The tool above is designed to be the fastest way to check your circuit math. Here is the workflow:
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Select your goal: In the Solve For dropdown, pick the value you are missing (e.g., if you want to find amps, select Current).
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Enter two knowns: Fill in the two remaining fields. For example, to find current, you must enter voltage and resistance.
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Check units: You can toggle between volts (V) and millivolts (mV), or ohms (Ω) and kilo-ohms (kΩ). The calculator handles the conversion automatically and normalizes the result to readable units like mA or kΩ.
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Review the steps: Use the Show Steps button to see which Ohm’s Law or power equation was used and how the calculator rearranged it.
Which mode should I use?
The calculator supports all four key circuit quantities. Choose the mode that matches what you need to solve:
Solve for Voltage (V)
Use when you know the current flowing through a load and its resistance.
\( V = I \cdot R \)
\( V = \dfrac{P}{I} \)
Solve for Current (I)
Use when checking if a wire or fuse can handle the load connected to a battery or power supply.
\( I = \dfrac{V}{R} \)
\( I = \dfrac{P}{V} \)
Solve for Resistance (R)
Essential for LED circuits, creating voltage dividers, or picking a resistor to limit current.
\( R = \dfrac{V}{I} \)
\( R = \dfrac{V^2}{P} \)
Solve for Power (P)
Use this to check if your resistor or device will overheat and to size components safely.
\( P = V \cdot I \)
\( P = I^2 \cdot R \)
Worked Examples: Real-world Calculations
The best way to learn Ohm’s Law is by doing. Use these example problems and click “Load into Calculator” to send the values into the Ohm’s Law calculator automatically.
Example 1: Finding LED Resistor Current
Scenario: You have a 5V power supply and a 220Ω resistor in series with an LED. You need to know if the current is safe (usually under 20 mA).
Result: 22.7 mA. This is slightly high for standard indicator LEDs; you might want to increase resistance to 330Ω to bring the current closer to 15 mA.
Example 2: Resistor Wattage (Safety Check)
Scenario: You are dropping 12V across a 100Ω resistor. Will a standard 1/4 Watt resistor survive?
Result: 1.44 Watts. A 1/4 W resistor will burn out instantly. You need a large 2 W or 5 W power resistor and some airflow.
Example 3: Speaker Load on an Amplifier
Scenario: A small audio amplifier is rated for 20 W into an 8Ω speaker at 12 V. You want to know the current and check that your power supply is sized correctly.
Step 1: Use the power formula to find current.
Step 2: Double-check resistance using Ohm’s Law:
Result: The actual load is close to 8Ω, and the amplifier will draw around 1.7 A at full power. Make sure your power supply can safely deliver at least 2–3 A.
Ohm’s Law in Series vs. Parallel Circuits
The formulas in the calculator above apply to individual components, but real circuits often combine components in series and parallel. That changes how you calculate total voltage, current, and resistance.
Series Circuits
Components are lined up in a single path (like a string of old Christmas lights).
- Current (I): The same through all components.
- Voltage (V): Splits between components.
- Resistance (R): Adds up (\(R_{\text{total}} = R_1 + R_2 + R_3 + \dots\)).
If one fails, the whole circuit stops.
Parallel Circuits
Components share the same connection points.
- Voltage (V): The same across all branches.
- Current (I): Splits between the branches.
- Resistance (R): Drops as you add paths: \( \dfrac{1}{R_{\text{total}}} = \dfrac{1}{R_1} + \dfrac{1}{R_2} + \dots \)
This is how house wiring and power distribution are typically implemented.
Real World App: Estimating Battery Life
One of the most common uses for an Ohm’s Law calculator is determining how long a battery will last. You can use the Power and Current modes to estimate runtime.
Scenario: How long will my battery last?
Given: A 12 V car battery with 50 Ah (amp-hours) capacity, connected to a 40 W camping cooler.
Step 1: Find the current draw.
Use the calculator in Power mode (Solve for I).
Inputs: \( P = 40\ \text{W} \), \( V = 12\ \text{V} \).
Result: \( I \approx 3.33\ \text{A} \).
Step 2: Divide capacity by current.
The Ohm’s Law Wheel and All 12 Formulas
You don’t need to memorize every algebraic rearrangement of Ohm’s Law. The Ohm’s Law wheel (sometimes called the Ohm’s Law triangle) shows how to solve for any variable using the two you already know. The calculator above implements all twelve of these formulas.
\( V = I \cdot R \)
\( V = \dfrac{P}{I} \)
\( V = \sqrt{P \cdot R} \)
\( I = \dfrac{V}{R} \)
\( I = \dfrac{P}{V} \)
\( I = \sqrt{\dfrac{P}{R}} \)
\( R = \dfrac{V}{I} \)
\( R = \dfrac{V^2}{P} \)
\( R = \dfrac{P}{I^2} \)
\( P = V \cdot I \)
\( P = I^2 \cdot R \)
\( P = \dfrac{V^2}{R} \)
When does Ohm’s Law fail? (Limitations)
While \( V = I R \) is fundamental, it does not apply cleanly to every electrical component. Understanding these exceptions is what separates beginners from experts.
1. Non-Ohmic Conductors
Some components change their effective resistance based on voltage, current, or temperature.
- Diodes & LEDs: They only allow current to flow one way and have a relatively fixed forward voltage drop regardless of current.
- Transistors: Act as active switches or amplifiers. Their behavior is controlled by currents and voltages, not a simple fixed resistance.
- Filament lightbulbs: As the filament heats up, its resistance increases drastically. A cold bulb pulls much more current than a hot one.
2. AC Circuits & Impedance
This calculator focuses on DC (direct current). In AC (alternating current) circuits—like your wall outlet—we must consider frequency. Resistance becomes impedance (Z), which combines:
- Resistance (R): Constant, frequency-independent opposition to current.
- Inductive reactance (XL): Opposition due to inductors and coils, which increases with frequency.
- Capacitive reactance (XC): Opposition due to capacitors, which decreases with frequency.
The same basic idea of Ohm’s Law still appears in AC as \( V = I Z \), but you must use complex numbers and phasors to capture phase shifts. For pure DC resistor networks, the Ohm’s Law calculator above is exact. For AC power factor and impedance calculations, you’ll need additional tools and formulas.
Safety & Component Selection
If the calculator says you are dissipating 0.4 W, do not use a 0.5 W resistor. It will run extremely hot and may discolor the board. Use a 1 W resistor instead, and make sure it has some room to breathe.
The same logic applies to voltage and current ratings: leave safety margins on every spec, not just resistance and power.
Frequently Asked Questions
What is Ohm’s Law in simple terms?
Ohm’s Law says that voltage equals current times resistance. If you increase voltage or decrease resistance, current goes up. The basic formula is \( V = I \times R \).
How do I use an Ohm’s Law calculator?
Pick which value you want to solve for (volts, amps, ohms, or watts), then enter any two of the remaining values. The calculator automatically rearranges the Ohm’s Law and power equations to find the missing quantity.
How do I calculate volts from ohms and amps?
Use the basic Ohm’s Law equation \( V = I \times R \). Multiply the current in amps (A) by the resistance in ohms (Ω) to get voltage in volts (V). In the calculator, choose Solve for Voltage, then enter current and resistance.
How do I calculate resistance using Ohm’s Law?
Rearrange the formula to \( R = \dfrac{V}{I} \). Divide the voltage in volts (V) by the current in amps (A). In the calculator, choose Solve for Resistance and enter your known voltage and current values.
What is the difference between Ohm’s Law and Watt’s Law?
Ohm’s Law relates voltage, current, and resistance: \( V = I \times R \). Watt’s Law relates power, voltage, and current: \( P = V \times I \). The Ohm’s Law calculator above combines both sets of equations so you can solve any of the four quantities.
Can this calculator determine wire gauge?
Indirectly. You can use it to find the current (amps) your load requires. Once you know the current, consult a wire gauge (AWG) chart to pick a wire size that handles that current safely for the length of run you have.
Why is my result in mW or mA?
In electronics, values are often very small. 0.02 A is harder to read than 20 mA (milliamps). This calculator automatically formats the result into milli- or kilo-units so it’s easier to interpret, without changing the actual underlying value.