About resistance calculator
Resistance, in ohms, measures how much a component opposes current. It can be found from voltage and current or from power and current.
Formulas
- R = V ÷ I.
- R = P ÷ I².
Example
An LED circuit with a 5 V supply, 2 V LED drop and 20 mA needs a resistor of (5 − 2) ÷ 0.02 = 150 Ω.
Another worked example
A circuit with a 9 V supply and a 0.3 A current has a resistance of 9 ÷ 0.3 = 30 Ω. A component that dissipates 2 W at 0.5 A has a resistance of 2 ÷ 0.5² = 8 Ω.
Series and parallel
Resistors in series add up: 10 Ω and 20 Ω make 30 Ω. In parallel the total is lower than the smallest, so 10 Ω and 10 Ω in parallel make 5 Ω.
