Electronics CalculatorsOhm's law explained with examples
One equation connects the three basic quantities of a circuit.
The three quantities
- Voltage (V) is the push, measured in volts. It exists between two points.
- Current (I) is the flow, measured in amperes (amps). It flows through a part.
- Resistance (R) is the opposition to the flow, measured in ohms.
A water pipe is a common picture: voltage is the pressure, current is the amount of water flowing per second, and resistance is how narrow the pipe is.
The law
V = I × R. Turned around: I = V / R and R = V / I.
Worked examples
| Situation | Calculation | Result | Power |
|---|---|---|---|
| 5 V across 220 Ω | I = 5 / 220 | 22.73 mA | 113.6 mW |
| 12 V, 20 mA flows | R = 12 / 0.02 | 600 Ω | 240 mW |
| 2.2 kΩ with 3 mA | V = 0.003 × 2200 | 6.6 V | 19.8 mW |
Power
Power is the rate at which energy is turned into heat or light: P = V × I. Using Ohm's law this is also I² × R and V² / R. The heat is why a resistor has a power rating.
Units and prefixes
1 kΩ is 1,000 Ω, 1 mA is 0.001 A and 1 µF is 0.000001 F. Writing a current in milliamps in a formula that needs amps is the most common mistake: 20 mA is 0.02 A. The calculators accept 20m.
Limits
Ohm's law describes parts whose resistance stays constant, such as resistors. An LED or a diode does not follow it: its voltage barely changes with current, which is why it needs a resistor in series.
Is it V = IR or V = I/R?
V = I × R. A higher resistance needs more voltage for the same current.
Does it work with AC?
For resistors with RMS values, yes.
This is an independent guide. It is not connected with any standards body or company named here. The formulas are the usual textbook ones; the colour code, E-series and AWG values are those printed in common references, because the standards IEC 60062, IEC 60063 and ASTM B258 are paid documents that we did not read. Tables and examples were calculated with Python on 14 November 2026. Check data sheets and local safety rules before you build.