Ohm's Law & Electronics Calculator
Solve V, I, R, P · Decode resistor color codes · LED resistor sizing · Series & parallel combinations
Enter any two known values. The other two are calculated automatically.
Formula Reference
What is Ohm's Law Calculator?
An Ohm's Law calculator is an essential tool for electronics students, hobbyists, and engineers. Ohm's Law states that the voltage (V) across a resistor equals the current (I) flowing through it multiplied by its resistance (R): V = I x R. From this single equation, plus the power formula P = V x I, you can derive all four electrical quantities — voltage, current, resistance, and power — from any two known values.
This tool goes beyond the basic formula. It includes a resistor colour code decoder: enter the colour bands printed on a physical resistor and the tool calculates the resistance value and tolerance. This saves time when reading small 4-band or 5-band resistors in the workshop, where the colour bands are easy to misread under poor lighting.
The calculator also includes an LED resistor calculator. When you connect an LED to a power supply, a current-limiting resistor is required to prevent the LED from drawing too much current and burning out. Enter the supply voltage, LED forward voltage, and desired current, and the calculator tells you the correct resistor value — plus the nearest standard E12 or E24 series resistor.
Whether you are doing a quick back-of-envelope check during circuit design, verifying component values before soldering, or working through an electronics homework problem, this tool provides clear, instant answers with relevant context.
How to Use Ohm's Law Calculator
- Select the Ohm's Law mode
Choose which two values you know (e.g. voltage and resistance) and the tool solves for the remaining two.
- Enter the known values
Type in your known voltage, current, resistance, or power values with the appropriate unit (V, A, Ω, W).
- Get instant results
All four electrical quantities are calculated and displayed in real time as you type your inputs.
- Decode resistor colours or calculate LED resistor
Switch to colour code or LED mode for additional workshop-ready tools in the same interface.
Key Benefits
Enter any two known values and instantly get voltage, current, resistance, and power — no rearranging formulas.
Read 4-band and 5-band resistors instantly by selecting colour bands — no more squinting at tiny components.
Find the correct current-limiting resistor for any LED and power supply combination, with nearest standard values.
The entire calculator runs in your browser with no server dependency — perfect for the workshop bench.
Frequently Asked Questions
Ohm's Law states that the voltage across a conductor is proportional to the current through it: V = I x R, where V is volts, I is amperes, and R is ohms. It applies to resistors and many other linear components.
Each colour corresponds to a digit: black=0, brown=1, red=2, orange=3, yellow=4, green=5, blue=6, violet=7, grey=8, white=9. For a 4-band resistor, the first two bands are digits, the third is the multiplier, and the fourth is tolerance.
An LED has a fixed forward voltage drop (typically 1.8-3.3V depending on colour). Without a resistor, the LED draws uncontrolled current from the supply, which can instantly destroy it. The resistor limits current to a safe operating level, typically 10-20 mA.
E12 is a standard set of 12 preferred resistor values per decade (e.g. 10, 12, 15, 18, 22, 27... ohms). Resistors are manufactured in these standard values so you can always find a component close to the calculated value.
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About Ohm's Law & Electronics Calculations
Ohm's Law
Ohm's Law (V = IR) relates voltage, current, and resistance in a conductor. Combined with the power formula P = VI, you can solve for any unknown given two knowns.
Resistor Color Codes
Resistors use colored bands to indicate their value. 4-band resistors give two significant digits; 5-band resistors give three, offering higher precision. The last band indicates tolerance.
LED Current Limiting
LEDs must have a series resistor to limit current. Too much current destroys the LED; too little makes it dim. Use R = (Vcc − Vf) / If for a single LED in series.
Series vs Parallel
Series resistors add directly. Parallel resistors always give a result smaller than the smallest resistor — great for increasing current capacity or fine-tuning values.