Ohm's law
V = I × R
Ohm's law says voltage equals current times resistance: V = I × R.
Rearranged: I = V ÷ R (how much current) and R = V ÷ I (which resistor). Cover the one you want in the V-I-R triangle.
It holds for resistors and wires, not for LEDs, diodes or transistors on their own.
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Everything on this subject: Ohm's Law and Circuit Basics, Simplified