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Ohm’s Law Calculator | Solve Voltage, Current & Power

Quick reference

Ohm's law relationships:

Know	Find voltage	Find power
I and R	V = I x R	P = I^2 x R
V and R	—	P = V^2 / R
V and I	—	P = V x I
P and I	V = P / I	—

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Responsive design tested down to a 320px-wide phone screen. The input fields use native number/date keyboards on iOS and Android, the calculate button is thumb-sized, and the result still reads big and bold on the smallest screen. No app to download, ever.

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Interpretation guide

Quantities and units:

Symbol	Quantity	Unit
V	Voltage	Volts
I	Current	Amperes
R	Resistance	Ohms
P	Power	Watts

lightbulb Worked example — current through a 240 Ω resistor

A 240 Ω resistor across a 12 V supply. Current I = V ÷ R = 12 ÷ 240 = 0.05 A. Power P = V × I = 12 × 0.05.

Result: I = 0.05 A (50 mA); P = 0.6 W.

What this means: Use at least a 1 W resistor — 0.6 W dissipation is uncomfortably close to a standard ¼ W part's rating and it would overheat. The same resistor across 120 V mains would draw 0.5 A and dissipate 60 W: instant smoke, which is why the power check matters.

Formula & methodology

Formula: V = I x R; P = V x I; combine to get P = I^2 R = V^2 / R

How the four quantities relate

Ohm's law: V = I × R. Power law: P = V × I. Combined: P = I²R = V²/R. Any two known values determine the other two.

Voltage + resistance → current = V ÷ R.
Power + voltage → current = P ÷ V.
Always check the power rating — a resistor dissipating near its wattage limit will overheat and fail.

Authoritative source: timeanddate.com

Frequently asked questions

What is Ohm's law?

The Ohms Law Calculator is free, private, and accurate: it runs entirely in your browser (no uploads, no accounts), applies the standard calculation, and explains each step so you can verify the result. Solves Ohm's law problems for voltage, current, resistance, and power. Enter any two values — for example voltage and resistance — and the tool computes the missing ones using V = IR and P = VI, with the working displayed. A fast check for electronics There is no limit on usage, and it works on any device.

How do I calculate power from voltage and current?

Enter the required values into the input fields and press the calculate button. The tool applies the formula V = I x R; P = V x I; combine to get P = I^2 R = V^2 / R and returns the result with a short explanation of each step, so you can check the working yourself.

How do I find current from power and voltage?

Use the power-law rearrangement I = P ÷ V: a 60 W bulb on 120 V draws 0.5 A. Combined with Ohm's law V = IR, the calculator solves for any two known values among voltage, current, resistance, and power. Current is what heats wires and determines breaker sizing, so this is the value electricians check first.

What is resistance measured in?

Resistance is measured in ohms (symbol Ω), named after Georg Ohm. One ohm is the resistance that allows one ampere to flow when one volt is applied — 1 Ω = 1 V/A. Larger values use kilo-ohms (kΩ) and mega-ohms (MΩ), and small values use milli-ohms. The calculator accepts these unit prefixes and converts them automatically.

Can I find resistance from power and current?

Yes — rearrange the power and Ohm's law equations: R = P ÷ I². A 60 W device drawing 0.5 A has a resistance of 60 ÷ 0.25 = 240 Ω. The calculator solves for resistance from any valid pair — voltage and current, voltage and power, or power and current — using the four rearrangements of V = IR and P = VI.

Which two values do I need to enter?

Any two of voltage, current, resistance, or power. Voltage + resistance is the classic pair (I = V ÷ R); power + voltage suits appliance questions (I = P ÷ V). The calculator derives the other two and shows which rearrangement it used.

When do I use P = I²R vs P = V²/R?

Use whichever matches your knowns: I²R when you know current (handy for wire heating — a 15 A circuit's wire loss), V²/R when you know the voltage across the part. They are algebraically identical via V = IR, so both give the same watts.

Is my data sent anywhere when I calculate?

No — the math runs on the page itself, so your circuit values never leave your device.

How precise are the results?

Exact — Ohm's law is a definition-level relationship for resistive circuits. The caveat is reality: real resistors heat up and drift, and the law doesn't apply to non-ohmic parts like diodes or LEDs, where voltage and current aren't proportional.

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