CircuitLab

Ohm's Law Calculator

V = I · R, plus P = V · I.

Current (A)

Resistance (Ω)

Voltage (V)

200

V = I · R

Power (W): 400

What you entered

Current (A): 2Resistance (Ω): 100

1
Ohm's law: V = I × R
2 × 100= 200 V
2
Power dissipated: P = V × I
200 × 2= 400 W

Result

Voltage (V): 200

A circuit carrying 2A through 100Ω needs 200V and dissipates 400W.

Voltage and power from current and resistance

Ohm's law gives voltage as current times resistance. Power is voltage times current, which is current squared times resistance.

Current (A)	Resistance (ohm)	Voltage (V)	Power (W)
0.02	330	6.600	0.132
1	1	1.000	1.000
2	10	20.000	40.000
0.5	100	50.000	25.000
1	220	220.000	220.000
10	12	120.000	1200.000

The first row is a typical LED with its series resistor - a fifth of a watt, which is why such resistors are physically tiny. The last is a 12 V circuit drawing 10 A, dissipating 1.2 kW, which needs substantial cable. Power rises with the square of current, so doubling the current quadruples the heating - that is why thick wires matter for high-current circuits and why transmission lines use very high voltage to keep current, and therefore losses, low. Resistance in metals also rises with temperature, so a hot component draws less current than a cold one at the same voltage.

The most fundamental circuit equation

Ohm's law relates voltage, current, and resistance in a simple resistive circuit — know any two, and the third is fixed. It's the starting point for essentially all basic circuit analysis.

Why resistance matters for safety

Higher resistance means less current flows for a given voltage — this is why fuses, resistors, and proper wire gauges are chosen deliberately to keep current (and the heat it generates) within safe limits.

Frequently asked questions

A 12 V car battery pushes current through a 4 Ω resistance. How much current flows?

I = V/R = 12/4 = 3 A. The power dissipated is P = VI = 12 × 3 = 36 W — that's the heat the resistor produces.

I replaced a 60 W bulb with a 100 W bulb in the same 120 V socket. What changed?

Current increased from I = 60/120 = 0.5 A to 100/120 = 0.83 A. The 100 W bulb has lower resistance (R = 120²/100 = 144 Ω vs. 240 Ω), which is why more current flows and more power is consumed.

What happens if I wire two resistors in series vs. parallel?

In series, resistances add (total goes up, current goes down). In parallel, the total resistance decreases (more current flows). Two 100 Ω resistors in series = 200 Ω; in parallel = 50 Ω. This calculator handles the single-resistor case.

Why does higher voltage mean more danger?

Current kills, but voltage drives current through your body's resistance (~1,000-100,000 Ω). At 12 V, even through wet skin (1,000 Ω), only 12 mA flows. At 240 V, that's 240 mA — well above the 100 mA lethal threshold.

How is Ohm's law related to the power calculator?

This calculator already shows power (P = VI), which is the rate of energy use. The physics power calculator (P = W/t) is the same concept but from the mechanical side — both measure energy per unit time, just in different contexts.