Voltage equals current times resistance, written V = I × R. Push harder (more volts) and more current flows; add resistance and less does. It holds for ordinary resistive components at a steady temperature, which covers most of what you meet in practice.

How do you calculate watts from volts and amps?

Multiply them: P = V × I. A device drawing 2 amps at 12 volts uses 24 watts. If you know resistance instead of one of those, use P = I²R or P = V²/R — all three give the same answer.

How do I calculate amps from watts?

Divide watts by volts: I = P / V. A 1,200 W heater on a 120 V circuit draws 1200 ÷ 120 = 10 amps. On 240 V the same heater draws only 5 amps, which is why higher-voltage circuits use thinner wire for the same power.

What is the difference between volts and amps?

Voltage is the electrical pressure pushing charge along; current is the rate charge actually flows. The usual analogy is water in a pipe — voltage is the pressure, current is the litres per second, and resistance is how narrow the pipe is.

How do you find resistance?

Divide voltage by current: R = V / I. Twelve volts producing 2 amps means 12 ÷ 2 = 6 ohms. If you know the power instead, R = V²/P or R = P/I² work just as well.

Does Ohm's law work for AC circuits?

Only partly. For a purely resistive AC load it works with RMS values. Once inductance or capacitance is involved you need impedance in place of resistance, and real power becomes P = V × I × power factor rather than simply volts times amps.

What is a component that does not obey Ohm's law?

Diodes, LEDs, transistors and filament lamps are all non-ohmic. A diode barely conducts below its forward voltage and then conducts heavily above it, so its resistance is not a constant and a single V/I figure only describes one operating point.

Ohm's Law Calculator: Volts, Amps, Ohms and Watts

Almost every electrical calculation on this site is Ohm’s law wearing a different hat. Volts to amps, watts to amps, wire sizing, voltage drop — they all come back to one relationship between four quantities, plus the power equation that ties them together. Learn the two and you can derive the other twelve on demand.

Ohm’s law

is voltage in volts, is current in amperes, and is resistance in ohms. Rearranged the two other ways:

The physical reading is straightforward. Voltage is the pressure driving charge through a circuit, current is how much charge actually moves per second, and resistance is what opposes it. Raise the pressure and more flows; raise the resistance and less does.

The power equation

Ohm’s law alone cannot tell you how much energy a circuit is using. That takes a second relationship:

Substituting Ohm’s law into it gives two more forms that avoid needing a quantity you may not have:

is the one that explains why wire gets hot. Power lost as heat in a conductor rises with the square of the current, so doubling the current through a cable quadruples the heating. That single fact drives wire gauge tables, fuse ratings and the whole case for high-voltage transmission.

All twelve formulas

The Ohm's law wheel: twelve formulas for voltage, current, resistance and power — Every rearrangement of V = IR and P = VI, grouped by what you are solving for

Solving for	Form 1	Form 2	Form 3
Voltage
Current
Resistance
Power

You never need to memorise all twelve. Memorise and , then substitute one into the other whenever you are missing a term.

Worked example

A 6-ohm resistor carrying 2 amps.

Step 1 — voltage across it, straight from Ohm’s law.

Step 2 — power dissipated, using the form that needs only what we started with.

Step 3 — cross-check with a different form.

Two routes, same answer. That agreement is the standard sanity check — if two forms of the power equation disagree, one of the inputs is wrong.

Twenty-four watts is worth pausing on. A resistor dissipating that much needs to be physically large or heatsinked; a quarter-watt component in that position would fail within seconds. The power figure is usually the one that decides whether a design is practical.

A worked example that runs the other way

A 1,200-watt space heater on a 120-volt outlet. What does it draw, and what does it look like electrically?

Step 1 — current.

Step 2 — effective resistance.

Ten amps on a 15-amp circuit is 67 percent of its rating, which is why running two of these on one circuit trips the breaker. Move the same heater to 240 volts and it draws 5 amps instead — same power, half the current, a quarter of the heating loss in the supply cable.

Reference values

Situation	Voltage	Typical current
AA battery, small load	1.5 V	0.01–0.5 A
Car electrical system	12 V	5–100 A
USB-C phone charging	5–20 V	1.5–3 A
US household outlet	120 V	up to 15 A
US dryer / range circuit	240 V	30–50 A
Standard LED indicator	2 V drop	0.02 A

An LED is a useful case to run the numbers on. Feeding a 2-volt LED from a 5-volt supply means dropping 3 volts across a series resistor at 20 milliamps:

That is the single most common practical application of Ohm’s law there is.

Where Ohm’s law stops working

Non-ohmic components. Diodes, LEDs, transistors and filament bulbs do not have a constant resistance. A tungsten filament’s resistance rises roughly tenfold between cold and operating temperature, which is why bulbs draw a large inrush current and usually fail at switch-on.

AC with reactive loads. With motors, transformers and capacitors, current lags or leads voltage and you need impedance rather than plain resistance. Real power then becomes , and the three-phase power calculator handles that case.

Temperature. Even ordinary resistors drift with temperature. For copper, resistance rises by about 0.4 percent per degree Celsius, which matters over a long cable run on a hot day.

Common mistakes

Mixing milliamps with amps. 20 mA is 0.02 A. Leaving it as 20 makes every downstream figure a thousand times too big.
Using on a reactive AC load. That gives apparent power in VA, not real power in watts. Multiply by the power factor.
Forgetting to square the current in . It is the squaring that makes heating grow so fast with current.
Using supply voltage instead of the voltage across the component. For a series resistor it is the drop across that resistor that matters, not the whole supply.
Treating a battery as an ideal source. Real cells have internal resistance, so terminal voltage sags under load — noticeably so on a nearly flat battery.

For the everyday one-step conversions this page generalises, the site has dedicated tools: volts to amps, amps to watts and watts to volts. For splitting a supply voltage across two resistors, use the voltage divider calculator.

About the formula: Ohm's law is V = I × R. Combined with P = V × I it gives twelve rearrangements, any of which lets you find two unknowns from two knowns.

Ohm's Law Calculator