How Savings Accounts Work: Interest & APY Explained

Knowing what a savings account is is one thing. Knowing how the interest actually gets calculated day to day — and what happens to your money in between deposits and withdrawals — is a different question, and it's the one that determines what you actually earn.

Here's what happens mechanically once you open a savings account and start using it.

Where Your Money Goes

When you deposit money into a savings account, it doesn't just sit untouched in a vault. Banks operate on a fractional-reserve model: they hold a portion of deposits on hand and are generally free to lend out or invest much of the rest, within regulatory limits. That lending activity — mortgages, auto loans, business credit — is part of how a bank generates the revenue it uses to pay you interest in the first place.

From your side, none of that changes how the account behaves. You can still access your balance on demand (subject to your bank's own transfer policies), and federal deposit insurance protects your principal regardless of what the bank does with the funds internally.

How Interest Actually Accrues

Interest doesn't wait for the end of the month to start counting. Under Regulation DD, interest must begin accruing no later than the business day the bank receives credit for your deposit, and it continues accruing until the day funds are withdrawn.

Banks use one of two methods to calculate the balance that interest applies to:

The daily balance method applies a daily periodic rate to whatever your actual balance is at the end of each day.

The average daily balance method adds up your principal for every day in the period and divides by the number of days in the period, then applies the periodic rate to that average.

For most accounts, these two methods produce the same total interest over a period, since they're really just two ways of arriving at the same daily-weighted calculation. Where they can matter is around minimum balance requirements: some banks that use the daily balance method will simply skip paying interest on days your balance dips below the minimum, while others using the average daily balance method may withhold interest for the whole period if the average falls short. Your account's disclosure — required under Regulation DD — will state which method applies and how minimum balances are handled.

A Worked Example

Say you keep $5,000 in a savings account for the first 10 days of a 30-day cycle, then deposit another $1,000 on day 11, bringing your balance to $6,000 for the remaining 20 days. With a 4.50% APY:

Average daily balance: (5,000 × 10 + 6,000 × 20) ÷ 30 = $5,666.67

Interest earned that period: $5,666.67 × (0.045 ÷ 365) × 30 ≈ $20.96

If the bank instead applies the daily balance method — the actual daily rate against each day's real balance — you land at the same $20.96 in this case, because there's no minimum-balance rule kicking in. The two methods only diverge when a threshold like a minimum balance changes which days actually earn interest.

Compounding Frequency vs. Crediting Frequency

These two terms get mixed up often, but they're not the same thing.

Compounding frequency is how often interest gets added to your balance so that it starts earning its own interest. Many savings accounts compound daily.

Crediting frequency is how often that accrued interest actually shows up as a posted transaction you can see in your account — commonly monthly, even when compounding happens daily behind the scenes.

Regulation DD doesn't require any particular frequency for either — it only requires banks to disclose whatever schedule they use. A bank can compound interest daily internally while only crediting (posting) it to your visible balance once a month. The APY figure already accounts for the actual compounding frequency, so you don't need to track this separately to know your real annual return — it's built into the number.

Tiered and Stepped Rate Accounts

Not every savings account pays one flat rate on the entire balance. Some use tiered rates, where different portions of your balance earn different rates — for example, the first $2,500 might earn one rate and any amount above that earns another. Others use stepped rates, where the rate that applies to your entire balance changes once you cross a threshold.

Regulation DD has specific rules for how banks must calculate and disclose APY on these accounts, since a single flat percentage can't accurately describe a blended, tiered structure. If your account uses tiered pricing, the disclosure will typically show the effective APY range across those tiers rather than one single number.

What Happens When You Deposit or Withdraw

Every deposit and withdrawal changes the balance that interest is calculated against going forward, starting from the day the transaction posts. There's no "waiting period" built into the interest calculation itself beyond the timing rules already described — a deposit generally starts earning from the day the bank credits it, and a withdrawal stops that portion from earning starting the day it's taken out.

This is also why the timing of contributions matters for anyone trying to maximize what a savings account earns: money deposited earlier in a cycle spends more days included in that cycle's balance calculation than money deposited later.

Statement Cycles

Regulation DD requires banks to send periodic statements that disclose, among other things, the amount of interest earned and the annual percentage yield actually earned for that statement period — not just the advertised APY, but what your account specifically earned given your actual balance activity. This is what lets you check that the bank applied its own stated method correctly, rather than taking the advertised rate on faith.

Frequently Asked Questions

Does my balance need to stay the same for interest to accrue properly?

No. Interest accrues against whatever your actual (or average) balance is each day, and it adjusts automatically as your balance changes.

If interest compounds daily, why do I only see it posted once a month?

Compounding and crediting aren't the same thing. A bank can calculate and add interest to the internal balance daily while only posting a visible transaction to your account monthly. Your account disclosure states both.

Does withdrawing money reduce interest I've already earned?

No, interest already credited to your balance stays yours. A withdrawal only affects interest accruing from that point forward, since it lowers the balance the calculation applies to.

What's the difference between the daily balance and average daily balance methods?

Both apply a daily rate to your balance, but the daily balance method looks at each day's actual balance while the average daily balance method uses one averaged figure for the whole period. They usually produce the same result unless a minimum-balance rule is involved.

Do all savings accounts use the same interest calculation method?

No. Regulation DD allows either method, and the specific choice — along with how minimum balances are treated — is up to the bank and must be disclosed in your account agreement.

Key Takeaways

A savings account's advertised APY tells you the annual result, but the mechanics behind it — when interest starts accruing, which balance-calculation method your bank uses, how compounding and crediting frequency differ, and how tiered rates apply — determine how that number plays out in practice for your specific balance and activity.

None of this is guesswork on the bank's end: Regulation DD requires all of it to be disclosed in your account agreement, which is the place to check if you want to know exactly how your particular account calculates what you earn.

Sources

This article is for general educational purposes only and does not constitute financial, investment, tax, or legal advice.

Last updated: August 2026