Free Ohm's Law Calculator – Voltage, Current, Resistance & Power

Solving Voltage, Current, Resistance, and Power Together

Ohm's Law is the single relationship that underpins almost all everyday electrical and electronics work, and this calculator lets you apply it in any direction you need. At its heart is V = I × R: the voltage across a component equals the current through it multiplied by its resistance. Knowing any two of those quantities immediately gives you the third. Power joins the family through P = V × I, and this tool ties all four together so you can start from whatever pair of readings you actually have and finish with a complete picture of the circuit. Here is what each symbol means and how the equations link. Voltage (V) is measured in volts and is the electrical pressure pushing charge along. Current (I) is measured in amperes and is the rate of charge flow. Resistance (R) is measured in ohms and opposes that flow. Power (P) is measured in watts and is the rate energy is used. From V = I × R and P = V × I you can derive the rest of the classic Ohm's Law wheel by substitution: P = I² × R (handy when you know current and resistance) and P = V² / R (handy when you know voltage and resistance). The calculator picks the right form automatically based on the two values you enter. A quick example makes it click. Suppose a resistor sits across a 12-volt supply and you measure 4 ohms of resistance. Current is V ÷ R = 12 ÷ 4 = 3 amperes. Power follows from P = V × I = 12 × 3 = 36 watts, or equally from P = I² × R = 3² × 4 = 36 watts. Both routes agree, which is a good way to sanity-check your own arithmetic and confirm the component must be rated for at least 36 watts. Try a mains-side case as well: a 230-volt supply feeding a lamp that draws 0.5 amperes gives R = 230 ÷ 0.5 = 460 ohms and P = 230 × 0.5 = 115 watts. You can also work backwards from a nameplate, and the same wheel still holds — a 2,000-watt geyser on 230 volts pulls 2,000 ÷ 230 = 8.7 amperes, which means its element sits near 26.5 ohms. Those few numbers already tell you what cable, switch, and fuse rating the circuit needs. These relationships come up constantly in practice. You use them to size a current-limiting resistor, to check whether a part can handle the power it will dissipate, to work out how much current a device pulls from a supply, or to verify a meter reading against theory. Hobbyists wiring LED circuits, students learning circuit basics, and technicians tracing a fault all reach for Ohm's Law many times a day. Picture four specific jobs. A student wiring a breadboard checks that a 220-ohm resistor on 5 volts passes a safe 22.7 milliamps. A solar hobbyist confirms that a 12-volt panel feeding a 6-ohm load draws 2 amperes, so a 5-amp fuse is enough. A technician who measures 230 volts and 8.7 amperes at a geyser deduces a 26.5-ohm element and 2,000 watts, then compares that against the nameplate to decide whether the element is degrading. A car-audio installer works out the current an amplifier pulls before choosing cable. Two cautions worth keeping in mind. This simple form applies to purely resistive DC circuits or the resistive part of AC circuits; capacitors and inductors add impedance and phase that the basic model does not capture. And always respect a component's power rating — a resistor asked to dissipate more watts than it is built for will overheat and fail, sometimes dangerously. The mistake beginners make most often is mixing units, entering 20 for milliamps where the formula wants 0.02 amps, which throws the answer out by a factor of a thousand. Convert milliamps and kilohms to base units first. Treat every result as an engineering estimate: any mains wiring must follow your local electrical code and be carried out by a qualified electrician, and nothing here replaces a proper measurement on the live circuit. Everything runs locally in your browser, so nothing you type is uploaded or stored.

Ohm's Law Calculator Formula & Method

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