Resistors in series add: Rtotal = R1 + R2 + …. Resistors in parallel combine as conductances: 1/Rtotal = 1/R1 + 1/R2 + ….
Why combine parts
You may not have an exact E24 value on hand. Two resistors in series or parallel can land closer to a divider ratio. The parallel resistor calculator handles two or more branches.
Power
Each resistor dissipates I²R. In parallel, current splits; verify each leg stays within its power rating.