The effective interest rate (EIR) is the true annual cost of borrowing, or true annual return on an investment, once compounding is taken into account. EIR stands for effective interest rate, and it is the figure that tells you what a rate really costs or earns over a year, not the headline number on the contract. Because it folds in how often interest is added, it is the only fair way to compare two rates side by side.
In accounts receivable, the effective rate is what turns a small discount or a monthly late fee into a number you can actually judge. A 2% early-payment discount or a 1.5% monthly interest charge looks minor until you annualise it, and the effective rate is how you do that. It is the difference between guessing whether a deal is good and knowing.
The real rate, not the headline.EIR is the true annual cost or return once compounding is counted in.
Compounding is the difference.The more often interest is added, the further the effective rate sits above the nominal rate.
It makes rates comparable.Two offers with different compounding are only comparable once both are stated as effective rates.
The effective interest rate formula is EIR = (1 + i/n)^n minus 1, where i is the nominal annual rate and n is the number of compounding periods per year. The logic is simple: interest charged partway through the year starts earning interest itself, so the more often it compounds, the higher the true rate climbs above the nominal one. Enter a rate below to see it.
Defaults show 12% nominal compounded monthly. General information, not financial advice.
Notice what happens as you raise the frequency: 12% compounded annually stays 12%, but compounded monthly it becomes 12.68%, and daily it edges higher still. That gap is the whole point of the effective rate. For overdue invoices, the late payment interest calculator applies the same idea to a real balance and number of days.
The nominal rate is the stated rate before compounding, the effective rate (EIR) is the true rate after compounding, and APR is a regulated rate that adds fees but often ignores compounding. They answer slightly different questions, which is why the same loan can carry three different-looking numbers. This table lays them out.
| Rate | What it includes | Best for |
|---|---|---|
| Nominal rate | The stated annual rate, before compounding and fees. | The headline on a contract |
| Effective rate (EIR) | The nominal rate adjusted for how often it compounds. | Comparing the true cost or return |
| APR | A regulated rate that adds fees, but often without compounding. | Comparing consumer loan offers |
The practical rule is to compare like with like. If one lender quotes a nominal rate and another an effective rate, convert both to the effective rate before deciding. The same discipline lets you compare a financing cost against the return you earn elsewhere, such as the yield on receivables tied up in unpaid invoices.
Say a supplier charges 1.5% interest a month on overdue invoices. The nominal annual rate looks like 1.5% times 12, which is 18%. But because the interest compounds each month, the effective rate is higher: (1 + 0.015)^12 minus 1, which works out to about 19.6%. So a customer who lets an invoice run a full year is paying closer to 19.6% than the 18% the headline suggests.
The same maths runs the other way for early-payment discounts. Classic 2/10 net 30 terms, take 2% off if you pay within 10 days instead of 30, work out to an effective annual rate of roughly 44%. That is why a settlement discount is almost always worth taking if you have the cash: turning it down is like borrowing at 44% a year. Whether you are charging interest or offering a discount, the effective rate is what shows you the figure that actually matters.
For a finance team, the effective rate turns three everyday decisions from guesswork into arithmetic. None of these are visible in the nominal rate alone, which is why reaching for the effective rate is a small habit that consistently leads to better calls on terms, discounts and collections.
It shows the annualised rate you are effectively paying customers to pay early, so you can tell whether an early-payment discount is worth offering.
It tells a customer whether accepting your discount beats their own cost of funds, turning a vague offer into a clear yes or no.
It puts a real number on both the interest you can fairly charge and the hidden cost of cash sitting in overdue invoices.
A worked comparison shows the payoff. Suppose you are deciding whether to offer a 1% discount for payment within 7 days on net 30 terms. That looks cheap, but the effective annual rate you are giving up is well over 15%, money you hand to every customer who would have paid on time anyway. Set against a borrowing cost of, say, 8%, the discount is too generous and you would be better tightening the terms or chasing harder.
Run the same numbers with a customer whose own cost of cash is 20% and the discount suddenly looks like a bargain for them. The effective rate is what lets both sides see the deal clearly, instead of arguing over percentages that are not comparable.
The effective rate is higher because of compounding: interest charged partway through the year is added to the balance, and then itself earns interest for the rest of the year. The nominal rate ignores that and simply states the yearly rate as if interest were added once at the end. So whenever interest compounds more than once a year, the effective rate pulls ahead, and the more often it compounds, the larger the gap.
The size of the gap depends almost entirely on frequency. At 12% nominal, annual compounding leaves the effective rate at 12%, monthly lifts it to 12.68%, and daily nudges it to about 12.75%. The jumps shrink as you go, because there is a mathematical ceiling, continuous compounding, that the rate approaches but never passes. The practical lesson is that a higher nominal rate compounded rarely can cost less than a lower one compounded often, so the frequency on the contract matters as much as the headline percentage. It is exactly the kind of detail the effective rate is designed to expose, and exactly the kind that a quick conversion catches before you sign.
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