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For a quick estimate when your balance and annual rate stay the same, multiply the balance by the annual interest rate and by the fraction of the year the money was on deposit: estimated interest = balance × annual rate × days ÷ 365. Use the rate as a decimal (4% = 0.04). That gives a simple-interest estimate, not necessarily the exact amount your bank will pay. For a precise figure, you need the account’s daily balances, applicable rates, and interest-calculation terms.
What information do you need?
Before calculating, gather the details for the period you want to estimate. The account agreement or disclosures explain the bank’s calculation method and conditions.
- Beginning balance and the dates covered.
- The annual interest rate in effect during the period. Use the interest rate—not APY—when estimating daily accrual from a nominal rate.
- Deposits and withdrawals, including the dates they changed the balance.
- Whether the rate changed, and whether the account uses balance tiers or caps.
- The method for computing interest, compounding and crediting schedule, minimum-balance requirements, day-count convention, and rounding rules.
These conventions vary by account. The U.S. rules discussed here are examples under Regulation DD, not a universal standard for accounts in every country.
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For a fixed balance and rate over a short period, use:
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Interest ≈ principal × annual interest rate × days ÷ 365
For example, $10,000 at a 4% annual interest rate for 30 days gives approximately $32.88: $10,000 × 0.04 × 30 ÷ 365. This is arithmetic under those assumptions, not a bank quote or a promise of actual earnings. The bank’s day-count method, compounding, and other account terms may produce a different amount.
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For a simplified fixed-principal, fixed-rate compound-growth estimate, use A = P(1 + r/n)nt. Here, P is principal, r is the annual rate as a decimal, n is the number of compounding periods per year, and t is the number of years. Estimated interest is A − P. This model is useful for illustrating compounding; it does not represent an account whose balance changes through transactions during the period.
How do you calculate interest when deposits or withdrawals change the balance?
Daily balance method
The CFPB defines the daily balance method as applying a daily periodic rate to the full amount of principal in the account each day. In simplified terms, multiply each day’s eligible balance by the daily rate, then add the daily amounts for the period. A deposit or withdrawal affects the calculation when it changes the balance under the account’s terms.
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Average daily balance method
Add the principal balance for each day in the period, then divide by the number of days:
Average daily balance = sum of each day’s balance ÷ days in period
The CFPB defines this method as applying a periodic rate to that average daily balance. The applicable periodic rate and precise procedure are determined by the institution’s terms.
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What is the difference between an interest rate and APY?
An interest rate is the annual rate without the effect of compounding; annual percentage yield (APY) reflects the total interest for a year based on the interest rate and compounding frequency. Regulation DD defines APY as “a percentage rate reflecting the total amount of interest paid on an account, based on the interest rate and the frequency of compounding for a 365-day period and calculated according to the rules in appendix A of this part.” See the CFPB’s Regulation DD definitions.
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For a savings account without a stated maturity, the disclosure APY calculation uses an assumed 365-day term. It assumes that principal and interest stay deposited and that no other transactions occur during the term. So APY is useful for comparing annual yields, but it does not guarantee the exact dollars earned when your balance or rate changes.
If the same balance remains untouched for a full year and the APY applies throughout, a quick estimate is balance × APY. For a month’s actual earnings, do not automatically divide annual APY by 12: use the account’s activity and calculation terms. The CFPB’s Appendix A to Part 1030 explains the APY and APY-earned formulas and their assumptions.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How can you check your estimate against a statement?
Compare like periods and use the account’s actual activity. When the credited interest differs from your estimate, check these items against the statement and account disclosure:
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- Daily balances, including when deposits and withdrawals affected them.
- The interest rate that applied on each day, especially if it changed or had balance tiers.
- Whether the account uses daily balance or average daily balance, and the periodic rate applied.
- When interest compounds and when it is credited to the account.
- Minimum-balance conditions, day-count convention, and rounding.
For U.S. accounts, CFPB guidance permits a daily periodic rate greater than one-365th of the interest rate, such as one-360th, if it is applied 365 days a year. This is one reason a simplified calculation may not match the statement. See the CFPB’s official interpretation of § 1030.7 for minimum-balance treatment and compounding and crediting policies.
A CFPB example of average daily balance
In a regulatory worked example for a 30-day statement period, the account has $1,500 for 15 days and $500 for the other 15 days. Its average daily balance is $1,000; with $5.25 in interest earned, the example’s APY earned is 6.58%. These figures illustrate the CFPB’s calculation and are not typical consumer results or current market rates. Appendix A provides the example and the APY-earned formula.
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