APY vs APR: what your savings account actually earns

Show me two savings accounts that both advertise 4.50% and I will show you two accounts that pay different interest. One is quoting a nominal rate — the number savers hunt for when they search out a savings account apr calculator — and the other is quoting an APY, the yield you actually keep after a year of compounding. The gap between the two is small in any single year and quietly large over a decade, and it exists for one reason: compounding itself earns money, and only one of the two numbers counts it. Here I will define both rates the way the CFPB does, run one $10,000 balance through each, and give you the single comparison rule I use so a headline rate never fools me again.

Two rates, two definitions

Start with the honest label problem. On a loan, APR is a legal term: the cost of credit, fees included. On a deposit account, banks are not really quoting an APR at all — they quote an interest rate (the nominal rate) and an APY (annual percentage yield). Savers use “APR” for the nominal rate anyway, and most “apr calculator savings account” searches are really asking one question: which of these two numbers do I actually earn? The answer is the APY. The nominal rate is an ingredient; the APY is the finished dish.

The nominal rate is the raw yearly rate before compounding is allowed to help. If your account compounds monthly, the bank divides it by twelve and credits you that slice each month. The APY then asks: after a full year of those credits — each one itself earning interest for the rest of the year — what did the balance really yield? That is the number every savings account yield calculator is solving for, and it is always the bigger of the two whenever compounding happens more than once a year:

APY = (1 + r/n)ⁿ − 1, where r is the nominal rate and n is how often interest compounds per year.

US deposit rules (Truth in Savings, overseen by the Consumer Financial Protection Bureau) require banks to disclose the APY in advertisements and at account opening, precisely so that you can compare a monthly-compounding account against a daily-compounding one on a single standard basis. When I compare accounts, the APY is the only headline I trust.

One balance, two “4.50%”s: the $10,000 test

Take $10,000 and give it a year. In the first account, 4.50% is the nominal rate, compounded monthly: each month the balance grows by 0.375%, and the year ends at $10,459.40 — $459.40 of interest, because the effective yield is really 4.59%. In the second account, 4.50% is the APY itself: the year ends at exactly $10,450.00, $450.00 of interest. Same advertised number, $9.40 apart in year one. Leave both accounts alone for five years and the gap widens to $56.14: $12,517.96 against $12,461.82. Nothing went wrong in either account — the two 4.50%s were never the same rate.

Nominal rate in, true yield out — the conversions behind every honest savings calculator APY comparison. At 6.00% nominal the hidden uplift is worth a full 0.17 points of yield.

The conversion also runs backwards. When an account advertises 4.50% APY with monthly compounding, the nominal rate doing the work underneath is 4.41% — the bank is paying a lower raw rate and letting compounding present it honestly as the yield. And the compounding schedule itself nudges the yield: that same 4.50% nominal rate becomes a 4.58% APY compounded quarterly and 4.60% compounded daily.

The trap: when the “lower” rate pays more

Now the comparison that actually bites. Account A advertises 4.65% — as a nominal rate, compounded monthly. Account B advertises 4.70% — as an APY. B looks better. Convert A first: its APY is 4.75%, which is higher than B’s 4.70%. Put $25,000 in each for three years and A ends at $28,734.72 against B’s $28,693.27 — the “lower” rate pays $41.45 more, because it was never lower. This is why every high yield account calculator worth trusting ranks accounts by APY rather than by whichever number the banner ad chose to print.

Money market accounts play by exactly the same rules

A money market account is a deposit account with a debit card attached, not a different species of math. It discloses an APY under the same rules, and a money market apy calculator performs the identical conversion — nominal rate and compounding frequency in, effective yield out. Two wrinkles are worth knowing: money market APYs are often tiered, so the yield on a $3,000 balance may be a fraction of the headline yield reserved for $50,000 balances; and like savings APYs, they are variable — a yield quoted today can be revised next month, unlike the fixed APY on a CD. Neither wrinkle changes the arithmetic. Both change which APY in the table is actually yours.

Reading your own account’s two numbers

When you open your own bank’s rate page, you will usually find both numbers printed side by side: a modest “interest rate” and a slightly larger “APY.” Now you know the relationship between them — the second is the first, after a year of its own compounding — and you know which of the two your balance will actually experience. My habit is to write down the APY and nothing else. If a rival account wants my money, its APY has to beat that recorded number; its nominal rate, its compounding schedule and its marketing adjectives are all, mathematically, just ingredients of the one figure I wrote down.

Sources: definitions and disclosure framing per the Consumer Financial Protection Bureau (CFPB) and the Truth in Savings Act (Regulation DD); APY = (1 + r/n)^n − 1. All rates are illustrative examples, not live market quotes; every figure was recomputed for this article.