Your business credit card quotes 24.99%, and your business savings account advertises 4.60%. Both are percentages, both say "annual," and neither means what you think it means. The card will cost you more than 24.99% if you carry a balance, while the savings account will pay you a touch more than its headline rate suggests. The entire gap between those two outcomes is compounding: interest earning interest before the year is even over. Once you see how that one mechanism pushes borrowing costs up and savings returns up, you will never compare the two numbers directly again.
The One-Sentence Version
APR (annual percentage rate) is the price tag on money you borrow. APY (annual percentage yield) is the paycheck on money you save. APR is built on simple interest and deliberately leaves intra-year compounding out of the math, while APY folds compounding in. For the same nominal rate, APY is always equal to or higher than APR — equal only when interest compounds once a year, higher everywhere else. That asymmetry is not an accident: each number is regulated to answer one question honestly, and mixing them up is where the money leaks out.
APR: What Your Borrowing Really Costs
APR exists so you can compare two loans on one scale. Lenders must disclose it under the Truth in Lending Act, implemented through Regulation Z, which is why every credit card offer, auto loan, and mortgage disclosure shows the same standardized figure. But "standardized for comparison" is not the same as "equal to what you will pay." How far apart those two are depends on the kind of debt.
Credit cards and lines of credit: the daily grind
Credit card APRs look like simple interest and behave like compound interest. The issuer divides your APR by 365 to get a daily periodic rate, applies it to your balance every day, and adds the resulting interest to the balance — so tomorrow's interest is charged on today's interest. Most issuers compound daily, which means a carried balance grows faster than the APR alone implies.
Run the numbers on a $10,000 balance carried for a full year at 24.99% APR with daily compounding, and the effective cost lands near 28.4%, not 24.99%. Business lines of credit work the same way: the stated rate understates the cost of money you leave drawn while interest accrues on interest. The APR is still the right number for comparing two cards or two lines against each other. It is just not the number that determines what a year of revolving debt actually costs you.
The practical takeaway for owners is blunt: every week a balance sits on a high-APR card, compounding does its quiet work against you. Paying the statement balance in full each month sidesteps the whole mechanism, because interest never gets a chance to start earning interest.
Term loans and equipment financing: monthly and milder
Auto loans, equipment loans, and most personal loans sit in the middle. Interest typically accrues monthly on the declining balance, so the gap between APR and effective cost is smaller than on a credit card but still nonzero. Each payment covers that month's interest first and the remainder reduces principal, which means early payments are interest-heavy and later payments are principal-heavy.
This amortization shape matters when you are deciding whether to pay a loan off early. Extra principal payments in year one of an equipment loan save far more interest than the same dollars applied in the final year, because every dollar of principal you retire stops compounding against you for the rest of the term.
Mortgages and commercial property: when APR runs higher than the rate
Mortgages flip the pattern: here the APR is usually higher than the interest rate on the note. That is because mortgage APR folds in points, origination fees, broker compensation, and certain closing costs, spreading them across the loan term. A 7.00% note rate with two discount points might carry a 7.25% APR, while a 7.15% no-fee loan shows 7.15%. The APR lets you compare those two offers on one scale.
That fee-spreading comes with a catch owners refinancing commercial property should know: APR amortizes upfront costs over the full loan term. If you sell or refinance after five years, the true cost of the fee-heavy loan is higher than its APR suggests, because you paid a thirty-year spread of fees for five years of borrowing. When you know you will exit early, compare total cash outlay over your actual holding period instead of trusting APR alone.
APY: What Your Savings Really Earn
APY answers the mirror-image question: if you leave money here for a year, how much more will you have? Because deposit accounts credit interest that itself starts earning interest, the only honest answer bakes compounding in. Banks must disclose APY under the Truth in Savings Act, implemented through Regulation DD, precisely so that two banks advertising the same nominal rate cannot hide different compounding frequencies behind it.
The compounding ladder, with real numbers
Start with a 5.00% nominal rate on a $50,000 operating reserve and watch what compounding frequency does to it:
| Compounding | APY | Year-end balance |
|---|---|---|
| Annually | 5.000% | $52,500.00 |
| Quarterly | 5.095% | $52,547.27 |
| Monthly | 5.116% | $52,558.09 |
| Daily | 5.127% | $52,563.35 |
The spread from annual to daily compounding is about $63 on $50,000 — small, but it is free money determined entirely by a word in the account disclosure. At higher rates the ladder steepens: a 12% nominal rate is 12.00% APY compounded annually, 12.68% compounded monthly, and 12.75% compounded daily.
Where business owners actually meet APY
Operating reserves in a business savings account are the obvious case, but APY follows idle cash everywhere it sits: money market accounts holding payroll buffers, certificates of deposit laddering tax reserves, and sweep accounts that move overnight balances into interest-bearing vehicles automatically. Each of these quotes APY, which means each headline number already includes its compounding — you can compare two money market accounts by APY directly without asking about frequency at all.
That comparability breaks the moment you step outside deposit products. A brokerage cash sweep, a Treasury bill, and a high-yield savings account can all show plausible-looking percentages computed under different conventions, so confirm you are looking at APY on both sides before you move reserves chasing a higher number.
The Conversion That Settles Every Argument
One formula connects the two worlds. Given a nominal rate r and n compounding periods per year:
APY = (1 + r/n)^n - 1
With r as a decimal: a 5% nominal rate compounded monthly is (1 + 0.05/12)^12 - 1 = 0.05116, or 5.116% APY. Compounded daily, n is 365 and the same 5% becomes 5.127%. The formula assumes every interest credit stays in the account and joins the balance for the next period — exactly what deposit accounts do, and exactly what carried credit card balances do in reverse.
Two comparison rules fall out of this, and they handle nearly every real decision:
- Borrowing: compare APR to APR. Card against card, loan against loan. Never stack a loan's APR against a savings account's APY.
- Saving: compare APY to APY. Account against account. The frequency is already inside the number, so the higher APY wins on yield alone.
The classic trap is the "should I pay down debt or keep cash in savings" question answered by eyeballing 24.99% against 4.60%. The comparison is directionally right — the debt rate dwarfs the savings yield — but the true spread is wider than it looks, because daily compounding pushes the card's effective cost toward 28% while the savings account delivers almost exactly its quoted APY. When the rates are close, say a 6% equipment loan against a 5% reserve account, run the conversion both ways before deciding; compounding and taxes can flip a close call.
Five Mistakes That Cost Real Money
Comparing APR to APY directly. A 7% loan APR and a 7% savings APY are not equal and opposite. The loan compounds against you while the savings compounds for you, and fees and taxes widen the gap further. Keep each number on its own side of the ledger.
Ignoring compounding frequency on the borrowing side. Two cards can share a 24.99% APR while one compounds daily and the other monthly. The daily compounder costs more on any carried balance. The disclosure tells you the frequency — read it before you revolve.
Chasing headline APY without reading the conditions. Teaser rates that reset after ninety days, tiered yields that pay the top rate only above a high minimum, and monthly fees that erase the interest on small balances all hide behind an accurate APY. A 5.00% APY with a $25 monthly fee is a worse deal than 4.50% with no fee on balances under $60,000.
Forgetting that variable rates move. Most card APRs and many savings APYs float with benchmark rates. The APR on your line of credit and the APY on your reserve account can both shift within a quarter, so revisit the pay-down-versus-save math at least when rates move, not just when you signed up.
Treating the mortgage APR as your payoff math. APR helps you choose between loan offers; it does not tell you what extra payments save. Amortization runs on the note rate, so model prepayments against that — or better, against an amortization schedule — rather than against APR.
How This Lands in Your Books
Interest you pay and interest you earn are different accounts with different tax lives, and clean books keep them apart. Interest paid on business debt is generally deductible as a business expense, recorded under something like Expenses:Interest, while interest earned on reserves is taxable income under Income:Interest. Netting them against each other hides both the true cost of your debt and the true return on your cash.
In a plain-text ledger the two flows look like this:
2026-10-19 * "Business card interest charge"
Expenses:Interest:Credit-Card 212.44 USD
Liabilities:Business-Card -212.44 USD
2026-10-19 * "Reserve account interest credited"
Assets:Business-Savings 189.58 USD
Income:Interest -189.58 USDReconcile both monthly against statements: card interest should tie to the finance charge line, and savings interest to the credit line. If your accounting software buries finance charges inside a lump payment entry, split them out — deductible interest mixed into principal repayments is a deduction you will forget at tax time. The Beancount documentation covers account structure for interest flows, and charting both series over time in Fava makes the compounding curves visible in a way statements never do.
Your Next Move
Pull your two highest-APR debts and your lowest-APY cash balance this week. Convert the debts to effective rates with the formula above, confirm the savings figure is APY and not a teaser nominal rate, and check that your books record the interest flows in separate accounts. That thirty-minute review tells you whether your next spare dollar should attack a balance or pad a reserve — and it is a decision compounding will quietly grade every day until you make it.
