Ohm's Law Calculator
V = I × R connects voltage, current and resistance so tightly that knowing any two always gives you the third. This solves for whichever one you're missing — the single most-used relationship in basic electronics, from choosing a resistor to diagnosing why an LED just burned out.
Inputs
Result
12 V
How it works
V = I × R
How the ohm's law calculator works
Voltage: V = I × R.
Current: I = V ÷ R.
Resistance: R = V ÷ I.
Pick which one you're solving for — the tool uses the other two inputs and ignores the field you're solving for.
Worked example: sizing a resistor for an LED
- Supply is 12 V, the LED needs 2 A across the resistor, solve for Resistance.
- R = V ÷ I = 12 ÷ 2 = 6 Ω.
- Swap to solving for Current with V = 12 and R = 6 instead: I = 12 ÷ 6 = 2 A — the same relationship, confirmed from a different starting pair.
Common mistakes to avoid
Leaving 'Solve for' on the wrong field
The tool uses whatever the other two fields say and computes the one you've selected — if 'Solve for' is left on Voltage while you actually changed the resistance, you'll get an answer to a question you didn't ask.
Applying this to AC circuits with reactance
Plain Ohm's Law assumes purely resistive load. Circuits with capacitors or inductors need impedance (which includes a reactive component), not simple resistance, or the numbers will be wrong for AC calculations.
Frequently asked questions
Does Ohm's Law apply to every electrical component?
It applies exactly to ideal resistors (and closely to real ones within their rated range). Diodes, transistors, and other non-linear components don't follow a simple V=IR relationship.
What's the relationship to power (watts)?
Once you know any two of V, I, and R, power follows directly: P = V × I, or equivalently P = I²R or P = V²/R.
Why is resistance sometimes shown as a range rather than a single value?
Real resistors have manufacturing tolerance (often ±5% or ±1%) around their marked value — the Resistor Color Code Calculator shows exactly what tolerance a given colour band represents.
How do I use this for a series circuit with multiple resistors?
Sum the resistances first (R_total = R1 + R2 + ...) then treat that total as the R in this calculator.
Learn more
Ohm's Law: The One Formula Electronics Keeps Coming Back To
V = I × R looks trivial until you realise it's the answer to nearly every 'why isn't this circuit working' question.
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