Ohm’s Law Calculator (V, I, R, P)

Understanding the Ohm's law calculator — concept and principle

Ohm's law links the voltage across a component, the current through it and its resistance. From a known voltage and resistance this calculator gives the current, the power dissipated as heat and the conductance. It is the first check in almost every electronics job: sizing a resistor, choosing a fuse, checking whether a part will overheat or finding the current drawn by a heater.

Cover the unknown in the triangle: 12 V ÷ 6 Ω = 2 A, and 12 V × 2 A = 24 W.

How it is calculated

Current: I = V / R (amperes, A)
Power: P = V · I = V² / R = I² · R (watts, W)
Conductance: G = 1 / R (siemens, S)

V is in volts and R in ohms. All three outputs follow from these two inputs alone.

The principle

In many conductors at a steady temperature, the current is proportional to the applied voltage; the constant of proportionality is the resistance, V = IR. Georg Simon Ohm published this relationship in 1827. Physically, the voltage is energy per unit charge, and the current is charge per second, so their product V · I is energy per second, which is power. Substituting I = V/R or V = IR gives the other two power forms. Conductance is simply the reciprocal of resistance and is convenient when adding parallel branches, because conductances in parallel add directly.

Worked example

Inputs: V = 12 V, R = 6 Ω.
Current: I = 12 / 6 = 2 A.
Power: P = 12² / 6 = 144 / 6 = 24 W (check: 12 V × 2 A = 24 W).
Conductance: G = 1 / 6 ≈ 0.1667 S.

Each point is the calculator re-run with only “Resistance (Ω)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Resistance (Ω)	Current (A)
1.5	8
3	4
4.5	2.666666667
6	2
7.5	1.6
9	1.333333333
12	1

How to read the result

The power figure tells you how much heat the resistor must handle. Choose a resistor rated well above it; a common rule of thumb is at least twice the calculated dissipation. In this example a 24 W load would destroy an ordinary 0.25 W resistor instantly. Doubling the voltage across a fixed resistance doubles the current but quadruples the power, so power grows much faster than voltage.

Common mistakes

Mixing units, such as entering kΩ values as ohms or mA as A.
Applying Ohm's law to non-linear parts such as diodes, LEDs or lamps whose resistance changes with temperature.
Using peak AC voltage instead of RMS when calculating average power.
Forgetting the source's internal resistance and wiring resistance in high-current circuits.

Keep in mind: V = IR applies to ohmic (linear) resistances at a stable temperature. Mains voltage is dangerous; real installations must follow local electrical codes and be done by a qualified electrician.

Frequently asked questions

Is Ohm's law valid for AC circuits?

For pure resistors, yes, using RMS values. With inductors and capacitors you replace R with the impedance Z and phase matters.

Why does a light bulb draw more current when it is first switched on?

A cold tungsten filament has much lower resistance than a hot one, so the inrush current is several times the running current.

How do I find resistance from power and voltage?

Rearrange P = V²/R to R = V²/P. A 230 V, 1,000 W heater is about 52.9 Ω when hot.

Sources: G. S. Ohm, Die galvanische Kette, mathematisch bearbeitet (1827) · P. Horowitz and W. Hill, The Art of Electronics, Cambridge University Press

Checked 8 Nov 2026. Standard textbook formulas in SI units, computed in your browser. For learning and estimates — real designs must follow your local codes and be checked by a qualified engineer.

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