A like-for-like rate comparison
This comparison converts each quoted APY to an equivalent monthly growth rate, then applies the same starting balance and end-of-month deposits to both accounts. That isolates the rate difference. It does not assume that the higher-yield account is permanently better; it only shows what its stated yield does over the selected five-year path.
Worked account comparison
With $12,000 initially and $300 added monthly for five years, 4.4% APY produces $34,931.31. The same deposits at 0.45% reach $30,472.60. The $4,458.71 difference includes interest on the original balance and on each deposit after it arrives, rather than five years of interest on every dollar.
A yield can change
APY is the right comparison label because it expresses one year of compounding, but accounts can revise it at any time. A promotional rate, balance tier, direct-deposit requirement, or monthly withdrawal limit can make the actual path unlike the fixed-rate illustration. Record the conditions attached to each quoted yield.
Stress-test the rate gap
Try a lower-rate case before using the difference to justify a move. A rate gap that lasts only three months has a much smaller effect than one that lasts five years. Also compare any minimum balance, transfer delay, service charge, and convenience cost; a higher advertised APY can be offset by terms that do not fit the cash reserve.
Different from a CD or investment
This page is not a certificate comparison and not an investment return forecast. It assumes accessible cash, level deposits, and no withdrawals. Tax treatment, account fees, deposit coverage, and a bank's eligibility rules are outside the model, as is the risk and volatility of market investments.
Keep the offer conditions
When reviewing accounts, note the APY date and the balance tier to which it applies. Recalculate after a rate notice rather than preserving a stale five-year projection. The useful outcome is a transparent estimate of the cost of a rate difference, not a recommendation to move every dollar.
A balance-tier check
If an advertised APY applies only up to a stated balance, split the projection at that threshold instead of applying the headline rate to every dollar. A larger reserve can earn a blended yield that differs materially from the rate in an advertisement.
Transfers have a timing cost
The comparison begins with both accounts funded on the same day. A transfer that spends several days outside either interest-bearing account creates a small gap, and a new account may not receive the advertised yield until a condition is met. For a short promotional period, the opening date and qualification date can matter more than the rounded five-year difference.
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It is an educational estimate, not financial, tax, or legal advice.