Use Ohm’s law: R = V / I
Identify the voltage across the resistor
Identify the current through the resistor
Divide voltage by current to get resistance
For series circuits: R_total = R1 + R2 + R3 + …
For parallel circuits: 1 / R_total = 1 / R1 + 1 / R2 + 1 / R3 + …
For a single resistor in a circuit, use the required voltage and current to calculate R
Check power rating with P = V × I, P = I²R, or P = V² / R
Choose the nearest standard resistor value if an exact value is unavailable