Ohm's Law Calculator

Ohm's Law

Georg Ohm published in 1827 what remains the most useful relationship in electronics: the current through a conductor is proportional to the voltage across it and inversely proportional to its resistance.

V = I × R

Combined with the power relationship P = V × I, these two equations generate twelve formulas covering every combination of the four quantities. Knowing any two lets you find the other two.

The Four Quantities

Quantity	Symbol	Unit	Water analogy
Voltage	V	Volt (V)	Pressure in the pipe
Current	I	Ampere (A)	Flow rate through the pipe
Resistance	R	Ohm (Ω)	Narrowness of the pipe
Power	P	Watt (W)	Work the flow can do

The water analogy is imperfect but useful: raising pressure increases flow, and narrowing the pipe reduces it. The symbol I for current comes from the French intensité de courant.

The Twelve Formulas

Find	Formula 1	Formula 2	Formula 3
Voltage (V)	I × R	P ÷ I	√(P × R)
Current (I)	V ÷ R	P ÷ V	√(P ÷ R)
Resistance (R)	V ÷ I	V2 ÷ P	P ÷ I2
Power (P)	V × I	V2 ÷ R	I2 × R

Power Rises With the Square

This is the consequence people most often miss. At fixed resistance, doubling the voltage doubles the current but quadruples the power, because P = V²/R.

Voltage across 10 Ω	Current	Power
5 V	0.5 A	2.5 W
10 V	1.0 A	10 W
20 V	2.0 A	40 W
40 V	4.0 A	160 W

It is why a resistor rated for 1/4 watt survives at 5 V and burns at 20 V, and why a small increase in supply voltage can destroy a component that was comfortable before.

Series and Parallel

	Series	Parallel
Total resistance	R1 + R2 + …	1 ÷ (1/R1 + 1/R2 + …)
Current	Same through every component	Divides between branches
Voltage	Divides across components	Same across every branch
Effect on total R	Always increases	Always decreases

Two equal resistors in parallel give half the resistance of one. Adding any parallel path always lowers total resistance, which is why a short circuit — a near-zero resistance path — draws enormous current.

Common Working Values

Device	Voltage	Current	Power
Standard LED	2 V	20 mA	0.04 W
USB port (2.0)	5 V	0.5 A	2.5 W
USB-C (PD)	20 V	5 A	100 W
Phone charger	5 V	2 A	10 W
Laptop	19 V	3.4 A	65 W
Car headlight	12 V	4.6 A	55 W
Kettle (UK)	230 V	13 A	3,000 W
Hair dryer (US)	120 V	15 A	1,800 W

Where Ohm's Law Does Not Apply

Ohm's law describes ohmic materials, whose resistance is constant. Many components are not ohmic:

• Diodes and LEDs conduct almost nothing below a threshold voltage and then conduct heavily — which is why an LED needs a current-limiting resistor rather than a chosen voltage.
• Incandescent bulbs have far lower resistance cold than hot, which is why they usually fail at switch-on.
• Thermistors change resistance with temperature by design.
• Semiconductors generally have voltage-dependent behaviour.

In AC circuits, resistance is replaced by impedance, which adds the frequency-dependent effects of capacitance and inductance.

Frequently Asked Questions

Why do I need to enter exactly two values?

Two are enough to determine the other two. Entering three over-constrains the problem and would usually be inconsistent.

Does higher voltage mean more current?

Only at constant resistance. A device drawing a fixed amount of power draws less current at higher voltage, which is why power grids transmit at very high voltage — less current means less loss in the wires.

What kills, voltage or current?

Current, but voltage is what drives it through the body's resistance. Around 100 mA through the chest can be fatal; skin resistance means it takes roughly 50 V or more to deliver that under typical conditions.

How do I choose a resistor for an LED?

Subtract the LED's forward voltage from the supply, then divide by the desired current. A 2 V LED at 20 mA on a 5 V supply needs (5 − 2) ÷ 0.02 = 150 Ω.