Voltage Calculator
Find the voltage across a component from the current through it and its resistance — V = I × R, with the power it dissipates alongside.
Any two of volts, amps and ohms give the third — and the watts.
How the voltage calculator works
Ohm's law rearranged: voltage equals current times resistance. This is the form you need when you know what a circuit is drawing and what is in the way — working out the voltage dropped across a series resistor, or checking how much of a supply a component is actually seeing. The power row matters as much as the answer: it is what turns into heat.
Formula: V = I R · P = V I
Worked examples
| Inputs | Result | Note |
|---|---|---|
| 2 A through 6 Ω | 12 | 12 V, dissipating 24 W |
| 0.02 A through 220 Ω | 4.4 | 4.4 V — a typical LED resistor drop |
| 10 A through 0.1 Ω | 1 | 1 V lost in the cable |
FAQFrequently asked questions
How do I calculate voltage?
Multiply the current in amps by the resistance in ohms: 2 A through 6 Ω gives 12 V.
What is voltage drop?
The voltage used up across a component or a length of cable. Long thin cable has resistance, so some of the supply never reaches the load.
Why does my LED need a resistor?
To drop the difference between the supply and the LED's forward voltage. Without it the current is limited only by the LED, which then fails.
Is voltage the same as potential difference?
Yes — voltage is the difference in electrical potential between two points, which is why it is always measured across something.
What is a safe voltage?
Below about 50 V AC or 120 V DC is generally considered low-risk, but current through the body is what harms, and wet skin changes everything.
Where these figures come from
- NIST — CODATA 2018 fundamental physical constants — G, g₀, R, c
- NIST Special Publication 811 — Guide for the use of the International System of Units — unit conversions
- The Engineering ToolBox — material properties — specific heats, expansion coefficients, densities
- National Measurement Institute — Australia's measurement authority
Last checked: September 2026. Constants are the CODATA 2018 values; formulas are the standard textbook forms.