Every month a small line appears on your savings statement that says something like “Interest paid: $16.44.” Most people glance at it and move on. But that number comes from a specific set of rules, and a few of them are written into federal law. Once you know how the math works, you can tell whether your bank is paying you what it promised, and you’ll understand why two accounts with the same advertised rate can pay slightly different amounts.
The rule banks have to follow
Interest on consumer deposit accounts is governed by the Truth in Savings Act and its implementing rule, Regulation DD. The section on paying interest, 12 CFR 1030.7, says banks must calculate interest on the full amount of principal in your account for each day. They’re allowed two ways to do it: the daily balance method or the average daily balance method.
The same rule sets a floor on the daily rate. Banks must use a daily rate of at least 1/365 of the annual interest rate (1/366 is allowed in a leap year). A bank may use a bigger slice, such as 1/360, as long as it applies that rate all 365 days of the year. That version pays you slightly more, which is why it’s permitted.
The daily balance method
This is the simpler one. Each day, the bank takes whatever principal is in your account at the end of the day and multiplies it by the daily rate.
Say you have $10,000 in an account paying 4.00%. The daily rate is 4% divided by 365, so you earn about $1.10 that day. If you deposit $500 tomorrow, tomorrow’s interest is calculated on $10,500. If you withdraw $2,000 the day after, that day’s interest is calculated on $8,500. Your interest tracks your balance day by day.
If the bank used 1/360 instead, the same $10,000 would earn about $1.11 a day. Across a full year that works out to roughly $405.56 in simple interest instead of $400. It’s a small difference, but it’s real money in your favor.
The average daily balance method
Here the bank adds up your balance at the end of every day in the statement period, divides by the number of days, and applies a periodic rate to that average.
Picture a 30-day month where you keep $2,000 in savings for the first 15 days, then move in $6,000 and hold $8,000 for the last 15 days. Your average daily balance is $5,000. At 4.00%, the interest for that month comes to about $16.44.
Mathematically, the two methods land in nearly the same place as long as the bank is counting every day. The main practical difference shows up with minimum balance requirements.
The methods that are banned
Regulation DD’s official interpretation spells out a few calculation methods banks can’t use, and they’re worth knowing because they used to be common. A bank can’t pay interest only on your ending balance for the period. It can’t use the “low balance” method, which paid you based on the lowest balance you had on any single day. And it can’t pay interest on only a portion of your money by carving out an amount for the bank’s own reserve requirements.
Under the old low balance approach, the account in the example above would have earned interest on just $2,000 for the whole month, even though you had $8,000 sitting there for half of it. Today that’s not allowed.
Minimum balances and how they interact
Many accounts require a minimum balance to earn interest, and the rules keep that fair. A bank has to use the same method to check the minimum as it uses to calculate interest.
With the daily balance method, the bank can skip interest only on the specific days you fall below the minimum. With the average daily balance method, it can skip interest for the whole period if your average falls short. Banks also can’t make you meet both a daily minimum and an average minimum to earn interest.
Two more protections are easy to miss. When you meet the minimum, the bank must pay the stated rate on your full balance, not just the amount above the threshold. So if the minimum is $300 and you have $500, you earn interest on all $500. And a negative balance is treated as zero, so an overdraft day can’t drag down your average.
Compounding versus crediting
Compounding is when earned interest gets added to your principal so it starts earning interest too. Crediting is when that interest actually lands in your account where you can see and use it. Regulation DD doesn’t require any particular schedule for either one. Banks can compound daily, monthly, quarterly, or on any other basis.
Compounding frequency is the reason your account lists both an interest rate and an annual percentage yield. At a 4.00% rate, monthly compounding produces an APY of about 4.07%, and daily compounding produces about 4.08%. On $10,000 left alone for a year, that’s roughly $407.42 versus $408.08. The gap is small, which is why comparing APYs across banks matters more than obsessing over compounding schedules. APY already folds compounding into one number.
Timing: when interest starts and stops
Interest has to start accruing no later than the business day set by the Expedited Funds Availability Act and Regulation CC for interest-bearing accounts. For cash, that tends to be quick. For a check, it’s tied to when the bank receives credit for the item, which may not be the same day you deposit it.
Interest also runs until the money leaves. The regulation’s own example: if a check is debited from your account on Tuesday, the bank must pay interest on those funds through Monday.
A common question is what happens if you withdraw before interest is credited. The bank can hold the interest you earned on that money until the regular crediting date, but it can’t skip paying it. Closing the account is different. Subject to state law, a bank may keep interest you accrued but haven’t been credited yet if you close the account before the crediting date, provided it disclosed that policy. If you’re about to move your savings, check the crediting date first. Waiting a few days can keep you from giving up interest you already earned.
Why the rate still matters most
All these rules protect you from being shortchanged on the math. They can’t fix a low rate. The FDIC’s national average for savings accounts is about 0.38%, while top online savings accounts, according to Bankrate’s September rankings, pay above 4%. On a $10,000 balance, that’s the difference between about $38 a year and roughly $400.
The calculation method, the compounding schedule, and the crediting date are fine print that decides whether you get every cent. The rate decides how many cents there are. If you understand both, that monthly interest line becomes something you can check instead of something you just trust.