High-Yield Savings Calculator

How high-yield savings interest compounds each month

There's no closed-form shortcut hiding here — the tool simply walks forward one month at a time for the whole term, 120 steps on the defaults. Each month the balance earns one-twelfth of the nominal rate behind your APY, then your deposit lands on top. Do that 120 times and you have the ending balance.

That word nominal is doing quiet work. An APY is what a year is worth after compounding, so it can't also be the rate that gets compounded — dividing 4.50% by 12 and running it twelve times would produce 4.594%, not 4.50%. The tool therefore converts first: it asks what monthly rate compounds to a 4.50% year, gets 4.4098%, and uses that. Enter 4.50% and a year returns 4.50%, which is the only behaviour that makes the APY you typed mean what the bank meant by it.

The order matters more than it looks. Deposits are added after the month's interest is credited, so the $300 you put in during month one earns nothing in month one. This is the conservative convention, and it's why the tool's ending balance sits slightly below what you'd get from a real account that credits interest daily on a rising balance.

nominal rate = 12 × ((1 + APY) ^ (1 ÷ 12) − 1) ← back out the rate that compounds to the APY monthly rate = nominal rate ÷ 12 repeat for each of the (years × 12) months: balance = balance × (1 + monthly rate) balance = balance + monthly deposit interest earned = ending balance − total deposited total deposited = initial deposit + monthly deposit × months

APY

The annual percentage yield you enter — converted to its equivalent nominal rate before compounding, so twelve months of growth returns exactly the APY you typed

initial deposit

What you open the account with — compounds for the full term, so it does the most work per dollar

monthly deposit

What you add at the end of each month — the last one earns no interest at all — it arrives as the projection ends

years

How long the money stays put — multiplied by 12 to get the number of compounding steps

balance

The running total, carried from month to month — starts at the initial deposit; the final value is your ending balance

The comparison panel runs the identical loop a second time at 0.01%, a rate that stands in for a large brick-and-mortar bank's default savings product. That figure is hardcoded in the calculator rather than tracked, so read it as an illustration of the low end and not as a claim about any specific institution.

For a sense of the real spread: the FDIC's national savings rate — a deposit-weighted average across every insured bank and credit union it collects data from — was 0.38% as of June 15, 2026, while money market accounts averaged 0.61%. Because the average weights each institution by its share of deposits, and most deposits sit at the largest banks, the national rate is pulled far below what accounts competing for new money actually pay. The average isn't the market. It's a measure of where the money already is.

Worked examples

Frequently asked questions

Where this high-yield savings projection parts company with a real account

The arithmetic is a monthly compounding loop, and the distance between that loop and a real account is mostly about things the loop has no way to represent.

• Interest is credited monthly, not daily — The loop takes twelve steps a year, and your deposit lands after the month's interest is credited — so the $300 you add in month one earns nothing that month, and the final deposit earns nothing at all. A real account crediting interest daily on a rising balance finishes a few dollars ahead. The convention is deliberate and it errs against the projection rather than for it. What it doesn't distort is the yield: the APY you enter is converted to its equivalent nominal rate first, so a year still returns exactly the rate you typed regardless of how the year is chopped up.
• Tax is absent — Every figure is pre-tax. Interest is ordinary income in the year it's credited, so a saver in the 22% bracket keeps roughly 78 cents of each dollar the chart shows — and unlike an unrealised capital gain, there's no deferral. The tax is due the year the interest is credited whether or not you withdraw anything.
• The rate never moves — One APY, held flat for up to 40 years, against a product whose defining feature is a rate the bank can change at will. Deposit rates follow the federal funds rate, and any projection past a year or two is arithmetic about an assumption rather than a forecast about a bank.
• The 0.01% comparison is an illustration, not a tracked rate — That figure is fixed in the calculator's code and represents the low end of large-bank savings pricing rather than any particular bank's current offer. The FDIC's deposit-weighted national savings average was 0.38% as of June 15, 2026 — higher than the comparison used here, and still far below what accounts actively competing for deposits pay. Both numbers move.
• Inflation isn't subtracted — The ending balance is in future dollars, not today's. At 3% inflation, $52,907.55 ten years out buys roughly what $39,368 buys today — less than the $41,000 you actually deposited. The account grew; the purchasing power didn't quite keep up. That gap is the real cost of holding long-term money in a deposit account, and no APY slider position on this page fixes it.