Tiered savings-account return: on the whole balance or on each tier?

There are two methods, and the bank decides which one applies. In the first, once your balance passes a tier's threshold, the whole balance earns that tier's rate. In the second, each part of the balance earns its own tier's rate: the part up to the threshold at one rate, and only the part above it at the higher rate. So the same tiers and the same advertised rates can give different returns depending on the method.

Other things also affect a savings account's return: the balance basis (daily balance or the lowest balance in the month), so one withdrawal can lower the whole month's base if the bank uses the lowest balance; the minimum balance that earns a return, since some accounts pay nothing below it; and recurring fees, because they are fixed amounts that weigh more on a small balance.

The key-facts statement for savings accounts must state the tiers, the balance basis, the minimum balance that earns a return and every fee. Check those, and if the tier method is not clear, ask the bank directly: does the tier rate apply to the whole balance or only to the part above the threshold? And ask for a worked example at your balance.