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Ohm’s Law Calculator: Voltage, Current and Power
Calculate voltage, current, resistance and power from two known values. Explore circuit diagrams, accurate graphs, worked formulas and LED resistor sizing.
Two known quantities
Choose two known quantities and enter their values and units. All results update as you type; changing a unit preserves the physical value. Decimal commas and scientific notation are accepted; do not use thousands separators.
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Circuit results
Voltage—
Current—
Resistance—
Power—
Live circuit diagram
Formula and calculation
Current versus voltage · Power versus voltage
Resistance stays fixed. Current increases linearly with voltage; power increases with its square. The orange dot is the calculated operating point (when within range); the teal dot explores another voltage without changing the inputs.
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Current versus voltage
Power versus voltage
Calculated operating pointExploration point
Resistor power loading
Rating utilization
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The percentage compares calculated dissipation with the rating you enter; it is not a safety approval. Check the component datasheet, ambient temperature, mounting conditions and derating. Operating below the label rating alone does not guarantee suitability.
LED series resistor calculator
This simplified nominal LED model uses R = (Vs − Vf) / I. Supply voltage must exceed the LED forward voltage, and current must be positive. Choose an available resistor and verify the resulting current, tolerances and power margin against both datasheets. The calculated resistance is not a universal component recommendation.
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Required nominal resistance—
Resistor dissipation—
LED power—
Supply power—
Formula and calculation
Model and limitations
This is a steady-state DC model of a positive, constant, ohmic resistance. Values are nonnegative magnitudes. It does not model AC impedance, component temperature or installation safety.
Use a positive resistance. Zero voltage, current or power is supported when resistance is known; other zero-value pairs may not determine a finite positive resistance. Enter finite values; extremely small or large results are outside this tool’s range.
Worked examples and explanations
Understanding Ohm’s Law
Ohm’s Law is a fundamental principle in electrical engineering that describes the relationship between voltage (\(V\)), current (\(I\)), and resistance (\(R\)) in an electrical circuit. The law 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.
The Formula
The mathematical expression of Ohm’s Law is:
\[ V = I \times R \]Where:
- \(V\) is the voltage in volts (V).
- \(I\) is the current in amperes (A).
- \(R\) is the resistance in ohms (Ω).
Using the Ohm’s Law Calculator
Choose two known quantities and enter their values and units. All results update as you type; changing a unit preserves the physical value. Decimal commas and scientific notation are accepted; do not use thousands separators.
Deriving the Formulas
From the basic formula \(V = I \times R\), we can derive the other two formulas:
- To find current (\(I\)) when voltage (\(V\)) and resistance (\(R\)) are known: \[ I = \frac{V}{R} \]
- To find resistance (\(R\)) when voltage (\(V\)) and current (\(I\)) are known: \[ R = \frac{V}{I} \]
Example 1
Suppose you have a circuit with a current of 2 amperes and a resistance of 5 ohms. To find the voltage:
\[ V = I \times R = 2 \, \text{A} \times 5 \, \Omega = 10 \, \text{V} \]Example 2
If you know the voltage is 12 volts and the resistance is 3 ohms, you can calculate the current:
\[ I = \frac{V}{R} = \frac{12 \, \text{V}}{3 \, \Omega} = 4 \, \text{A} \]Example 3
Given a voltage of 9 volts and a current of 3 milliamperes, you can find the resistance:
\[ I = 3 \, \text{mA} = 0.003 \, \text{A} \] \[ R = \frac{V}{I} = \frac{9 \, \text{V}}{0.003 \, \text{A}} = 3000 \, \Omega = 3 \, \text{kΩ} \]Complex Example with Unit Conversion
Consider a circuit where the voltage is 15 kilovolts (kV) and the current is 5 milliamperes (mA). Find the resistance.
\[ V = 15 \, \text{kV} = 15 \times 1000 \, \text{V} = 15000 \, \text{V} \] \[ I = 5 \, \text{mA} = 5 \times 0.001 \, \text{A} = 0.005 \, \text{A} \] \[ R = \frac{V}{I} = \frac{15000 \, \text{V}}{0.005 \, \text{A}} = 3000000 \, \Omega = 3 \, \text{MΩ} \]Additional Example: Power Calculation
Power (\(P\)) in an electrical circuit can also be calculated using Ohm’s Law. The power dissipated in a resistor can be found using the following formulas:
\[ P = V \times I \] \[ P = I^2 \times R \] \[ P = \frac{V^2}{R} \]For example, if the voltage is 12 volts and the current is 2 amperes, the power dissipated in the resistor is:
\[ P = V \times I = 12 \, \text{V} \times 2 \, \text{A} = 24 \, \text{W} \]Alternatively, using the resistance \(R = \frac{V}{I} = \frac{12 \, \text{V}}{2 \, \text{A}} = 6 \, \Omega\):
\[ P = I^2 \times R = (2 \, \text{A})^2 \times 6 \, \Omega = 4 \, \text{A}^2 \times 6 \, \Omega = 24 \, \text{W} \] \[ P = \frac{V^2}{R} = \frac{(12 \, \text{V})^2}{6 \, \Omega} = \frac{144 \, \text{V}^2}{6 \, \Omega} = 24 \, \text{W} \]Practical Application: LED Circuit
Consider a simple LED circuit where an LED requires a forward voltage (\(V_f\)) of 2 volts and a current (\(I_f\)) of 20 milliamperes. To determine the required series resistor (\(R\)) to limit the current through the LED when connected to a 9-volt battery, follow these steps:
- Calculate the voltage drop across the resistor (\(V_R\)): \[ V_R = V_{\text{battery}} – V_f = 9 \, \text{V} – 2 \, \text{V} = 7 \, \text{V} \]
- Convert the current to amperes: \[ I_f = 20 \, \text{mA} = 0.02 \, \text{A} \]
- Calculate the resistance using Ohm’s Law: \[ R = \frac{V_R}{I_f} = \frac{7 \, \text{V}}{0.02 \, \text{A}} = 350 \, \Omega \]
This simplified nominal LED model uses R = (Vs − Vf) / I. Supply voltage must exceed the LED forward voltage, and current must be positive. Choose an available resistor and verify the resulting current, tolerances and power margin against both datasheets. The calculated resistance is not a universal component recommendation.
Applications of Ohm’s Law
Ohm’s Law is widely used in various applications, including:
- Designing and analyzing electrical circuits.
- Calculating power consumption in devices.
- Troubleshooting electrical issues.
- Determining the appropriate components for a circuit.
- Ensuring safety in electrical systems by preventing overcurrent conditions.
- Optimizing energy efficiency in electronic devices.
- Understanding and designing analog and digital circuits.
Final notes
Mastering Ohm’s Law is crucial for anyone working with electricity and electronics. The Ohm’s Law Calculator simplifies the process of calculating voltage, current, and resistance, making it easier to understand and apply this fundamental principle. Whether you’re a student, hobbyist, or professional, having a solid grasp of Ohm’s Law is essential for designing and troubleshooting electrical circuits. By understanding the relationships between voltage, current, and resistance, you can build more efficient and reliable electrical systems.
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