How much will my savings grow? Worked examples at 4–5% APY

Every week someone asks me a version of the same question — the one behind every how much will my savings grow calculator search: if I park this money in a high-yield savings account, what is it honestly worth in five years? In ten? I am tired of answers that hide behind “it depends,” so here is the full working: growth tables for $5,000, $10,000 and $25,000 at 4.00%, 4.50% and 5.00% APY over one, three, five and ten years — first left alone, then with $200 a month added — and one real goal worked backwards to the month. Every cell was recomputed for this article.

The method, in one paragraph

Because an APY is already the effective annual yield, a lump sum grows by one clean line: balance = P × (1 + APY)^years. Monthly deposits need the APY’s monthly twin, (1 + APY)^1/12 − 1 — at 4.50% APY that is about 0.3675% a month — with each deposit landing at the end of the month and then compounding like the rest. That is the entire engine. A future savings calculator is just this method with your numbers typed in, and under the hood it is the same arithmetic every savings account growth calculator runs; I am simply showing it with the lid off.

Table 1: lump sums, left alone

No deposits, no withdrawals, APY held constant. Read the middle column as the typical high-yield case.

Two patterns are worth seeing. First, time beats rate at these levels: $10,000 at 4.50% reaches $15,529.69 in ten years, and more than half of that endpoint — $5,529.69 — is interest the money earned by simply staying put. Second, the rate gap compounds quietly: the half-point between 4.50% and 5.00% is worth $1,898.13 on a $25,000 balance over ten years, which is why I bother shopping for APY at all.

Table 2: the same starts, plus $200 a month

This second table is where the tool earns its full name. A compound interest savings account calculator exists for exactly this pattern, because here each deposit starts earning interest the month after it lands, and the deposits’ own growth becomes a third contributor alongside your starting balance and its interest.

Starting balance plus $200 deposited at each month-end ($2,400 a year). The $10,000 / 4.50% / 5-year cell — $25,860.17 — is the same worked example our calculator page uses, so you can check the table against the tool.

Look at the ten-year row for a $10,000 start at 4.50%: $45,624.83. Of that, $34,000 is money you actually deposited ($10,000 at the start, $24,000 along the way) and $11,624.83 is interest — nearly a third of the final balance arrived while you slept. Compare it with the same cell in Table 1 and the deposit habit is worth $30,095.13 all by itself.

A goal, worked backwards: the $30,000 house deposit

Tables answer “how much”; savers usually ask “how long.” So: I have $10,000 today, my account pays 4.50% APY, I can add $400 a month, and I want $30,000. Running the balance forward month by month, it passes three years at $26,777.48 and crosses $30,000 in month 43 — three years and seven months — finishing that month at $30,304.99. By then I have deposited $27,200 in total, which means interest quietly contributed the remaining $3,104.99 of the goal. For contrast, the lump sum alone — no deposits at all — would need ln 3 ÷ ln 1.045, about 24.96 years, to turn $10,000 into $30,000 at the same APY. The deposits did not just speed the goal up; they shrank it from a quarter-century project into a single car-loan term. That is the arithmetic I wish someone had shown me before I spent a year rate-shopping instead of deposit-shopping. Notice what the goal math rewards: at $400 a month the finish line arrives years before the lump-sum-only version of me gets there, and none of it required finding a better rate.

How long until it doubles?

The saver’s party trick is the Rule of 72 — divide 72 by the rate for a rough doubling time. The exact answer is ln 2 ÷ ln(1 + APY), and at savings rates the two are close enough to respect each other: at 4.00% APY the rule says 18 years and the exact figure is 17.67; at 4.50%, 16 against 15.75; at 5.00%, 14.4 against 14.21. I use the rule in conversation and the exponent in planning — and both agree that at high-yield savings rates, doubling is a fifteen-to-eighteen-year project unless deposits do the heavy lifting, which, per Table 2, they happily do.

Sources: growth method per the APY conventions in CFPB consumer guidance on deposit accounts — APY treated as the effective annual yield, monthly rate = (1 + APY)^(1/12) − 1, deposits at month-end. All rates are illustrative; every table cell was recomputed for this article.