Ohm's Law Calculator

How to use

1. 1Pick which two quantities you know with the segmented buttons at the top: Volts + Amps, Volts + Ohms, Volts + Watts, Amps + Ohms, Amps + Watts, or Ohms + Watts.
2. 2Type the two values into the input boxes. Use the unit chips next to each box to switch between mV/V/kV, μA/mA/A, mΩ/Ω/kΩ/MΩ, or mW/W/kW so you don't have to convert by hand.
3. 3Read the four result tiles on the right. The two values you provided are tagged given; the two derived values are tagged solved and highlighted in emerald.
4. 4Open the Working panel to see the exact formula used and the substituted numbers (for example, P = V × I = 12 × 0.5).
5. 5Click any preset (red LED at 20 mA, USB-C charger 5 V × 3 A, 230 V kettle 2200 W, etc.) to load a typical scenario, then edit the inputs to match your circuit.
6. 6Use Copy on any tile to grab a single result, or Copy full report to copy the four values plus the working steps in a plain-text block.

About this tool

Ohm's Law Calculator is a complete browser-based solver for the four core electrical quantities: voltage (V), current (I), resistance (R), and power (P). Pick any two values you know and the tool derives the remaining two by walking through the standard rearrangements of V = I × R and P = V × I. Inputs accept any common metric prefix (mV, V, kV for voltage, μA, mA, A for current, mΩ, Ω, kΩ, MΩ for resistance, mW, W, kW for power), and the result panel auto-scales each output to the most readable unit so a 0.0027 A current is shown as 2.7 mA without you doing the conversion. Open-circuit, short-circuit, and indeterminate combinations (zero current with non-zero voltage, zero resistance with non-zero voltage, zero power with non-zero voltage, and similar) are detected and shown as a clear physical-meaning error rather than NaN, so you always know whether the inputs make sense. A working panel beneath the results prints out the exact formula used and the substituted numbers (for example, P = V × I = 12 × 0.5), which is useful for homework, lab reports, and sanity-checking a circuit design. Quick presets cover the calculations engineers and hobbyists hit every day: a red LED at 20 mA forward current, a USB-C charger at 5 V × 3 A, a 12 V automotive bulb at 21 W, a 120 V or 230 V mains heater, a 9 V battery driving a 470 Ω resistor, and a 1 A current through a 1 kΩ load to demonstrate power scaling. A full formula reference at the bottom lists every Ohm's Law and electrical-power rearrangement (twelve formulas in total) so you can solve in your head and verify with the tool. Useful for sizing series resistors for LEDs, picking the right resistor wattage to avoid burnout, checking whether a power supply can deliver the current a load needs, computing the heater rating for a kettle or space heater, working out whether a wire gauge is safe at a given current, and any back-of-the-envelope check during electronics design, hobby projects, or coursework. The tool is fully client-side: every calculation runs in your browser, the values you enter never leave your device, and there is no signup or upload at any point.

Free to use. Works in your browser. No signup, no login.