This calculator answers a narrow, common question: if I just leave a lump sum sitting in a savings account earning interest — no further deposits — how much will it grow?
APY vs. interest rate
Annual Percentage Yield (APY) already accounts for compounding within the year, so it's the number to use here directly — it's usually slightly higher than a bank's stated 'interest rate' for the same account.
When this differs from other savings calculators
Most savings tools assume you're adding money regularly. This one is specifically for a single deposit left to grow on its own — see the Monthly Savings Calculator or Savings Goal Calculator if you're also contributing over time.