Savings account interest is money an account provider adds under the account's rate and balance rules. The displayed annual rate does not simply get multiplied by whatever balance appears on the last day of the month. The provider has to determine which balance earned interest on each day or over the calculation period, accrue the interest, and then credit it.

If your interest looks wrong, trace those events in order. Starting with the advertised percentage usually sends you in circles.

The five events behind one interest credit

  1. Money enters or leaves the account.
  2. The provider records a balance for the relevant day or period.
  3. It applies the account's periodic interest rate to the eligible balance.
  4. The interest accrues, which means it has been earned under the calculation even if it is not visible yet.
  5. The provider credits the accumulated interest to the account on the date stated in the terms.

Interest can be calculated daily and credited monthly. Those descriptions do not conflict. One says how often the provider works out the interest; the other says when the result is added to the account.

A worked daily-balance example

For US consumer deposit accounts covered by Regulation DD, the CFPB describes the daily balance method as applying a daily periodic rate to the full principal balance for each day. Its payment-of-interest rule uses either the daily balance or average daily balance method and a daily rate of at least 1/365 of the stated interest rate, with a leap-year variation permitted.

Use that method only as an illustration outside the United States. Account rules differ.

Suppose an account has:

  • a 2,000 balance for the first 10 days of a 30-day period;
  • a 3,000 balance for the next 20 days after a 1,000 deposit;
  • a constant 3.65% nominal annual rate;
  • a 365-day calculation basis;
  • no withdrawals, conditions, tiers, fees, or tax effects.

For the first 10 days:

2,000 × 0.0365 ÷ 365 × 10 = 2.00

During the remaining 20 days:

3,000 × 0.0365 ÷ 365 × 20 = 6.00

Total interest for the period:

2.00 + 6.00 = 8.00

Because it arrived on day 11, the 1,000 deposit earned interest for only 20 days. The ending balance alone cannot reproduce the result.

The average daily balance reaches the same result here

Here, the average daily balance comes to:

[(2,000 × 10) + (3,000 × 20)] ÷ 30 = 2,666.67

For the full 30-day period, the interest is:

2,666.67 × 0.0365 ÷ 365 × 30 = 8.00

The two methods agree because the same full daily balances and period are used. That does not mean every account's balance rule is identical. Minimum-balance conditions, tiers, transaction cut-off times, and the period used for an average can alter the amount.

Crediting is when compounding becomes visible

At period-end, the provider adds the 8.00 to the account. With no other changes, the new balance is 3,008.00. A later calculation can include that credited interest in the principal.

That is the beginning of compound growth. The compound-interest guide shows the effect across complete monthly and yearly schedules.

Accrued interest may not yet appear in the available balance. If you close or empty an account before the crediting date, the product terms should say what happens to accrued interest.

Interest rate, APY, AER, and interest earned

These figures answer different questions.

The stated or nominal rate

This is the annual rate the account applies under its calculation rules. It may be variable, fixed for a period, tiered by balance, or conditional.

APY

In the United States, Regulation DD says APY is an annualized measure based on the interest rate and compounding frequency. For a typical savings account without a maturity date, its general disclosure calculation assumes principal and interest remain deposited for 365 days with no added deposits or withdrawals.

That assumption makes APY useful for comparing eligible accounts under the same US convention. It does not predict the exact interest on a balance that changes every week.

AER

The UK Financial Conduct Authority describes AER as an annual equivalent rate reflecting a year's interest with compounding, bonuses, and charges taken into account. AER belongs to UK disclosure practice. Other markets use their own measures.

Interest earned

This is the money actually accrued or credited over a statement period. It reflects the balances, dates, applicable rate, and account conditions during that period. It can differ from a simple ending balance × annualized yield ÷ 12 estimate.

Reconcile an interest payment from the statement

Use one line per balance change. Do not start by trying to reverse-engineer APY.

STATEMENT PERIOD:
NOMINAL RATE OR RATES APPLIED:
CALCULATION BASIS: daily / average daily / other
DAY-COUNT BASIS:
CREDITING FREQUENCY:

DATE        TRANSACTION        ELIGIBLE BALANCE        DAYS AT BALANCE
____        ___________        ________________        _______________
____        ___________        ________________        _______________

CONDITIONS MET OR MISSED:
BALANCE TIER:
FEES:
INTEREST ACCRUED:
INTEREST CREDITED:
ROUNDING DIFFERENCE:
QUESTION FOR PROVIDER:

US Regulation DD requires covered account disclosures to state the compounding and crediting frequency, balance-computation method, minimum-balance rules, and relevant fees. Those fields are a good reading checklist elsewhere, but they are not a promise that every jurisdiction uses the US rule.

Why two equal deposits may earn different amounts

Leah transfers 600 on the first day of the month. Arun transfers 600 on the twenty-fifth. Their contribution totals match, but Leah's deposit has more days in the account during that month.

If they are saving for one shared goal, the interest difference says something about timing, not effort or authority. Arun might be paid later. Leah might make the transfer from a household account both partners funded.

A useful ledger keeps the questions apart:

  • How much came from each person, or from the household?
  • Which day did the deposit start earning interest?
  • How much interest did the provider credit?
  • Does the contribution agreement still fit their income dates?

You could say:

"The statement gave our deposits different numbers of earning days. Can we check the dates before we decide whether the transfer plan needs changing?"

Common reasons the credited amount is lower than expected

  • A deposit arrived late in the calculation period.
  • A withdrawal reduced the eligible balance.
  • The account moved into a different balance tier.
  • A monthly deposit, no-withdrawal, linked-account, or other condition was missed.
  • The attractive rate applied only during an introductory period.
  • A fee reduced the net result.
  • The displayed annualized yield was treated as a monthly rate.
  • The estimate assumed every month has the same number of days.

Do not compare accounts from this list alone. How high-yield savings accounts work covers access, conditions, fees, institution checks, and local deposit protection without ranking providers or quoting live rates.

If the provider's current disclosure and your calculation still disagree, ask for the eligible balances, rates, date basis, conditions, and rounding used for that exact statement period. A clean audit trail is more useful than arguing from the headline percentage.