Foundations
Ohm’s law: resistance
- where
- resistance (Ω)
- voltage (V)
- current (A)
Explain this equation
Works out resistance from measured voltage and current at the same moment. Useful for checking a load or element from live readings.
R is the resistance at that operating point. V is the voltage across the component and I is the current through it, measured together.
It is Ohm’s law rearranged: resistance is the volts needed per ampere of current. A high ratio means the part strongly resists flow.
Sense check: 12 V and 2 A gives 6 Ω. A 230 V, 3 kW heater draws about 13 A, so its hot resistance is about 17.6 Ω.
Watch out: many parts, such as lamp filaments and heater elements, change resistance as they warm up. A cold meter reading will not match the hot value.
Use the voltage across a component and current through it at the same operating point. Their ratio gives its resistance at that point.
For a filament lamp, resistance changes greatly as it heats. A hot operating resistance need not match a cold meter reading.
Enter your values to see the result.
Common mistake
Mixing a cold resistance with a hot operating current.
Limits
Current must be nonzero; this does not imply all components have constant resistance.
Sources
The tools calculate the displayed equation. They do not automatically choose cable sizes, approve installations, supply current regulatory tables, certify equipment or replace inspection and testing.