What it means
A headline interest rate on its own is incomplete information. An account paying 6% credited once a year and an account paying 6% credited monthly both advertise 6%, yet the second leaves you better off because each month's interest starts earning interest of its own.
The AER solves this by asking a single question: if all the interest were paid once at the end of the year instead, what rate would produce the same result? That figure can then be compared directly across accounts with different payment schedules.
In the United Kingdom, AER is the standard disclosure for savings accounts and is required to be shown prominently. The equivalent concept on the borrowing side is APR, which additionally includes compulsory fees and charges, and in some markets the term effective annual rate or EAR is used for the same underlying calculation.
The gap between a nominal rate and its AER widens as compounding becomes more frequent and as the rate itself rises. At 2% the difference between monthly and annual compounding is a fraction of a percentage point, while at 12% it becomes material enough to change which product is genuinely better.
One practical caution applies to accounts with introductory bonuses. A quoted AER often assumes the bonus rate persists for the full twelve months, so an account showing an attractive AER may drop sharply once a six-month bonus period ends.
In practice
Real-world examples.
Example
A saver compares two accounts, one quoting 4.80% paid annually and one quoting 4.72% paid monthly. The monthly account has an AER of 4.82%, so despite the lower headline figure it is marginally the better deal.
Example
A finance manager parking $500,000 of surplus cash for twelve months uses AER rather than the nominal rate to rank three bank offers. The ranking changes once compounding frequency is included, and the chosen account earns roughly $1,900 more than the one with the highest headline rate.
Example
A building society advertises an account at 5.25% AER, which includes a 1.00% bonus for the first six months. A customer reading the small print calculates that the underlying rate after the bonus expires is closer to 4.25%, and plans to move the money at that point.
Formula
Calculation
AER = (1 + r / n) ^ n - 1
Here r is the nominal annual interest rate expressed as a decimal, and n is the number of times interest is compounded each year.
A savings account quotes a nominal rate of 6% with interest credited monthly. Then r = 0.06 and n = 12, so the monthly rate is 0.06 / 12 = 0.005, or 0.5%.
AER = (1 + 0.005) ^ 12 - 1 = 1.0616778 - 1 = 0.0616778, or 6.17% when rounded to two decimal places.
Putting $10,000 into that account and leaving it untouched for a year gives a closing balance of $10,000 x 1.0616778 = $10,616.78, so the interest earned is $616.78. An account paying a flat 6% once at the end of the year would pay only $600.00, so the monthly compounding is worth an extra $16.78 on this balance.Case study
Seen in the real world.
This case study is illustrative and the company is fictional. Bramwell Foods, an invented food wholesaler, held $2,000,000 of working capital reserves in a business deposit account paying a nominal 3.60% credited annually.
The finance director asked a junior analyst to compare alternatives. Three banks quoted rates between 3.55% and 3.62%, but the compounding frequencies differed: annual, quarterly and monthly. Converting each to an AER showed that the 3.58% monthly-compounding offer produced an AER of about 3.64%, higher than the 3.62% annual offer.
The difference was small in percentage terms but worth roughly $390 a year on the balance held, and the analysis took under an hour. The illustrative lesson the team took away was to always request compounding frequency alongside any quoted rate, because a headline number on its own cannot be ranked.
Watch out
Common mistakes.
- Comparing quoted nominal rates directly across accounts. Two accounts with identical headline rates can pay different amounts if one compounds monthly and the other annually.
- Assuming AER includes fees. AER covers compounding only, so an account charge or a withdrawal penalty reduces the real return without changing the advertised AER.
- Applying AER to a deposit held for less than a year. AER assumes the money stays invested for a full twelve months, so a three-month deposit will not earn the full advertised figure.
Questions
People also ask.
Is AER the same as APR?
No, AER is used for savings and reflects compounding, while APR is used for borrowing and also includes compulsory fees and charges in the calculation.
Why is AER always at least as high as the nominal rate?
Because compounding can only add to the return, and when interest is credited just once a year the two figures are identical.
Does AER account for tax?
No, AER is a gross figure, so any tax due on the interest must be deducted separately to see the actual net return.
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