What does this tool do?
The Ohm's Law Calculator finds the value you need from voltage (V), current (I), resistance (R), and power (P) by entering two known quantities and applying V = I × R and P = V × I. It is a fast reference for electronics students, hobbyists, and technicians designing circuits.
How to use it
- Enter your values into the fields.
- The result is calculated instantly.
- Read or copy the result.
Linking the four quantities
Use the buttons at the top to choose which value you want to find, then enter the two remaining known values. The tool computes your target instantly and displays all four quantities, voltage, current, resistance, and power, below so you can read the whole circuit picture at a glance. You can derive volts from amps and ohms, or wattage from voltage and current. The result updates live as you type values in decimal steps and copies with a single click.
Power formulas and units
The tool uses the classic relation P = VI, which is equivalent to the alternative formulas P = I²R and P = V²/R. Every value works in base SI units: voltage in volts (V), current in amperes (A), resistance in ohms (Ω), and power in watts (W). Convert quantities like milliamps or kilohms to the base unit before entering them, for example type 0.1 for 100 mA or 4700 for 4.7 kΩ. This is ideal for limiting current to an LED or checking the power dissipated by a resistor.
Frequently asked questions
Can I enter milliamps instead of amperes?
No, the tool works in base SI units, so enter current in amperes. For 250 mA, type 0.25. Likewise convert kilohms to ohms and milliwatts to watts before entering them.
What happens if I enter zero resistance?
When solving for current or resistance this can cause a division by zero, and the result shows as infinity or undefined. Since zero resistance is also physically meaningless in a real circuit, enter a positive value.
Why use P = VI for power instead of P = I²R?
All three formulas are mathematically equivalent. When Ohm's law holds, P = VI, P = I²R, and P = V²/R give the same answer. The tool uses P = VI because it is the most direct fit for the voltage and current you enter.
Is Ohm's Law Calculator free?
Yes. Ohm's Law Calculator and all 300+ tools on MagmaNex are completely free with no signup or account required.
Is my data safe?
Yes. Everything runs locally in your browser. The data you enter is never sent to a server, stored, or shared.
Does it work on mobile?
Yes. Ohm's Law Calculator works on phones, tablets and desktops — no app download needed.
🔒 Privacy: everything runs in your browser, your data is never sent to a server.