Why Your Savings Interest Falls Short of the Advertised APY
An installment savings account, where you deposit a fixed amount every month, doesn't earn interest the same way a lump-sum deposit does. Your first deposit earns interest for the entire term, but each later deposit earns interest for a shorter and shorter period — your final deposit earns just one month's worth. That means even at a 4% advertised rate, your blended return on the full balance ends up noticeably lower. On top of that, the interest itself is taxed at your ordinary federal marginal rate rather than a flat withholding rate, so your actual take-home amount depends heavily on your tax bracket.
How the Calculation Works
| Step | Item | Formula |
|---|---|---|
| 1 | Gross Interest | Monthly Deposit × n(n+1)/2 × (Rate/12) |
| 2 | Estimated Tax | Gross Interest × Your Marginal Rate |
| 3 | Net Interest | Gross Interest − Tax |
| 4 | Amount at Maturity | Total Principal + Net Interest |
This calculator uses a standard simple-interest installment savings formula and federal tax only — it does not include state income tax, promotional bonus rates, or early-withdrawal penalties. Check your bank's specific terms before opening an account.
Frequently Asked Questions
Each deposit earns interest only for its remaining months until maturity, not the full term, so the blended yield is lower.
At your ordinary federal marginal rate (10%-37%) plus any state tax — there's no flat withholding rate.
Yes, interest inside an IRA grows tax-deferred or tax-free, and municipal bond interest is often federally tax-exempt.
※ Actual returns depend on your bank's terms, promotional rates, and tax situation. This is a simplified estimate for reference only.