Ohm's Law Calculator — Voltage, Current, and Resistance
Calculate voltage, current, or resistance using Ohm's Law V = IR. Includes power formulas, parallel/series circuits, and practical electronics examples.
Ohm's Law is the starting point for almost everything in electronics. It describes how voltage, current, and resistance relate to each other in a linear circuit. Learn these three quantities and their relationship and you can understand most of what's happening in any circuit you encounter.
The CalcHub Ohm's Law calculator solves for any one of the three variables when you know the other two.
The Formula
V = I × R- V = voltage (Volts, V)
- I = current (Amperes, A)
- R = resistance (Ohms, Ω)
A useful memory aid: think of electricity like water. Voltage is water pressure, current is flow rate, and resistance is the pipe's narrowness. More pressure or wider pipe = more flow.
The Power Trio
Combine Ohm's Law with the power formula and you get very flexible tools:
| Formula | Solves for |
|---|---|
| V = IR | Voltage |
| I = V/R | Current |
| R = V/I | Resistance |
| P = VI | Power from V and I |
| P = I²R | Power from I and R |
| P = V²/R | Power from V and R |
Practical Reference
| Device | Voltage | Current | Resistance |
|---|---|---|---|
| AA battery | 1.5 V | (varies) | — |
| Phone charger | 5 V | 2 A | 2.5 Ω |
| 60W incandescent | 120 V | 0.5 A | 240 Ω |
| LED (typical) | 3.3 V | 20 mA | 165 Ω |
Worked Example
You want to connect a 9V battery to an LED that needs 20 mA (0.02 A) and has a 2V forward voltage drop. What resistor do you need?
Voltage across resistor = 9 − 2 = 7 V
R = V/I = 7 / 0.02 = 350 Ω
Pick the nearest standard value: 360 Ω. This is the most common beginner electronics calculation, and Ohm's Law is all you need.
Does Ohm's Law work for all materials?
Only for ohmic conductors — materials where resistance stays constant regardless of voltage or current. Most metals behave this way at constant temperature. Non-ohmic devices include diodes, transistors, and LEDs, where V-I curves are nonlinear.
What's the difference between AC and DC for Ohm's Law?
In DC circuits, it's straightforward. In AC circuits, resistance becomes impedance (Z), which includes capacitive and inductive reactance: V = IZ. The basic relationship holds, but Z is complex-valued and frequency-dependent.
Why does resistance increase with temperature in metals?
Higher temperature means more atomic vibration, which increases collisions with charge carriers (electrons). This increases resistivity. Tungsten (used in light bulb filaments) has much higher resistance when hot than cold — which is why startup current for bulbs is high.