Ohm's Law Calculator
Enter any two values and leave the third blank. Then click Calculate.
What is Ohm's Law?
Ohm's Law is one of the most useful relationships in electronics and basic electrical work. It connects three key values in a circuit: voltage (V), current (I), and resistance (R). If you know any two, you can calculate the third instantly.
In practical terms, this helps you size resistors, estimate current draw, and check whether a power source is appropriate for your project. Whether you're building an LED circuit, troubleshooting a battery-powered device, or designing a lab experiment, Ohm's Law is the starting point.
The 3 core equations
- Voltage: V = I × R
- Current: I = V ÷ R
- Resistance: R = V ÷ I
How to use this calculator
- Enter values for any two fields.
- Leave the field you want to find empty.
- Click Calculate.
- The result appears below and is also filled into the empty input.
Example 1: Find current
You have a 9V supply and a 3Ω resistor. Leave current blank, enter 9 for voltage and 3 for resistance:
I = V ÷ R = 9 ÷ 3 = 3A
Example 2: Find resistance
If a device runs at 2A on a 24V line, then:
R = V ÷ I = 24 ÷ 2 = 12Ω
Common mistakes to avoid
- Entering all three values: this calculator expects one unknown.
- Dividing by zero: resistance and current cannot be zero in division steps.
- Mixing units: milliamp values should be converted to amps when needed (e.g., 500 mA = 0.5 A).
- Ignoring tolerance: real components have tolerances and measurement error.
Why this matters in real projects
Ohm's Law is foundational for safe and reliable circuit design. It tells you how much current will flow through a component and helps prevent overheating, blown fuses, and damaged parts.
For example, when powering an LED, you can calculate a safe resistor value that limits current. In battery systems, you can estimate load current to predict runtime. During troubleshooting, you can quickly check whether measured readings are physically reasonable.
Quick reference checklist
- Use volts (V), amps (A), and ohms (Ω).
- Keep one variable unknown for each calculation.
- Double-check decimal places.
- Round only at the end for best accuracy.
- Validate with a multimeter whenever possible.
Final thoughts
A simple Ohm's Law calculator can save time and reduce mistakes. Use it whenever you're planning or debugging circuits, and pair it with good measurement habits for the best results. If you work with electronics regularly, this is one tool you'll return to again and again.