Ohm's Law Calculator
Calculate voltage, current, resistance, and power from any two known values.
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Formulas
V = I × RV = Voltage (V), I = Current (A), R = Resistance (Ω)
P = V × I = I² × R = V² / RP = Power (Watts)
Understanding Ohm's Law
Ohm's Law describes the relationship between voltage, current, and resistance. Formulated by Georg Simon Ohm in 1827, it states that current is directly proportional to voltage and inversely proportional to resistance.
Practical example
A 12V battery with a 100Ω resistor: Current = 12V / 100Ω = 0.12A. Power = 12V × 0.12A = 1.44W.
Limitations
Applies to linear (ohmic) materials only. Does not apply to diodes, transistors, or temperature-dependent devices.
Understanding Ohm's Law
Ohm's law is the most fundamental relationship in electronics. It states that the current through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance. From a single equation you can find any one of voltage, current, resistance, or power given the other values.
V = I × RV is voltage in volts, I is current in amps, and R is resistance in ohms. Rearranged: I = V/R and R = V/I.
Power ties in through P = V × I, which combines with Ohm's law to give P = I²R and P = V²/R. These four quantities form the basis of nearly every DC circuit calculation.
Worked Examples
A 12 V supply across a 100 Ω resistor:
I = V / R = 12 / 100 = 0.12 A (120 mA)
Same circuit, the power dissipated by the resistor:
P = V × I = 12 × 0.12 = 1.44 W
A standard 1/4 W resistor would overheat here — you would need at least a 2 W part.
The Ohm's Law Wheel
Every combination of two known quantities lets you solve for the other two. The table summarises the twelve standard rearrangements.
| To find | From V, I | From V, R | From I, R |
|---|---|---|---|
| Voltage (V) | — | — | I × R |
| Current (I) | — | V / R | — |
| Resistance (R) | V / I | — | — |
| Power (P) | V × I | V² / R | I² × R |
Frequently Asked Questions
Does Ohm's law apply to AC circuits?
For purely resistive AC loads, yes, using RMS values. When capacitance or inductance is present, resistance is replaced by impedance (Z), and you must account for phase.
Which components don't obey Ohm's law?
Non-ohmic devices such as diodes, transistors, and incandescent bulbs have resistance that changes with voltage, current, or temperature, so the simple linear relationship does not hold.
How do I remember the rearrangements?
The triangle trick: cover the quantity you want in the V-I-R triangle and the remaining two show the formula. V sits on top, I and R below.
Why does my resistor get hot?
It is dissipating power as heat. Check P = V·I against the resistor's wattage rating; if power exceeds the rating, choose a higher-wattage part.