Work out availability zone redundancy cost instantly with clear inputs, formula shown and shareable results.
Surviving the loss of one availability zone means the remaining zones must carry the whole load, so instance count multiplies by z/(z-1): 50 percent overhead with two zones, 33 percent with three, 25 percent with four. That is the quantitative case for three zones. Cross-zone traffic is charged in both directions and is easy to overlook.
Zone redundancy
instances = ceil(capacity x zones / (zones - 1)) rounded up per zone; overhead = (instances - capacity) / capacity
Two zones means each must carry 100 percent of the load alone, so you pay 50 percent overhead rather than 33 percent for three — and lose more capacity per failure.
Zone-aware routing so requests are served within the zone they arrive in, and topology-aware replica selection for reads. Both cut the charge substantially.