Ohm's Law Calculator and Resistor Color Code Decoder

You're at the workbench with a 9V battery and you want to drive an LED that needs about 20 mA. What resistor do you need? That's V = IR rearranged to R = V/I = 9/0.02 = 450 Ω. You'd probably grab a 470 Ω resistor from the bin. Simple enough — but when you're iterating on a circuit design and swapping values constantly, having a calculator open beats doing the algebra every time.

Two tools in one

The Ohm's law calculator has four fields: voltage, current, resistance, and power. Enter any two and it calculates the other two. Need to know how much power a 100 Ω resistor dissipates with 12V across it? Enter V = 12 and R = 100, get I = 0.12 A and P = 1.44 W. That tells you a quarter-watt resistor won't cut it — you need at least a 2W rating.

Below the calculator is a resistor color code decoder. Pick the colors of each band from dropdowns and the tool tells you the resistance value and tolerance. No more squinting at brown-black-red-gold and trying to remember if brown is 1 or 2. (It's 1.)

When you're learning vs. when you're building

Students working through circuit problems can use the Ohm's law calculator to verify their manual work. If you calculated that a series circuit with a 6V source and 30 Ω total resistance draws 0.2 A, plug in the numbers and confirm. It's faster than asking the TA.

For hobbyists and makers, the color code decoder is the real time-saver. You've got a handful of resistors with faded markings and no multimeter nearby. Select the band colors and get the value. Works for both 4-band and 5-band resistors.

If you're also working with AC circuits and need to visualize frequency behavior, the wave visualizer is another useful companion tool.