How do I know what size circuit breaker I need?

Your breaker size comes from the wire, not the appliance. The breaker exists to protect the conductor, so 14 gauge copper gets 15 amps, 12 gets 20, 10 gets 30, 8 gets 40 or 50 depending on type.

The load then fine-tunes within that frame: continuous loads count at 125 percent, so a 16 amp heater needs a 20 amp circuit even though 16 is under 15 plus change.

Appliance nameplates state their required circuit outright, and motors and AC units follow their own article where the breaker may legitimately exceed the wire's number.

Never size a breaker to stop nuisance trips by going bigger; that removes the protection while keeping the problem.

The breaker size calculator walks wire, load, and the 125 percent factor in one pass.