How Banks Calculate Interest on High-Yield Savings Accounts

The daily balance method is how most banks calculate your interest

Banks calculate interest on high-yield savings accounts by taking your account balance at the end of each day, adding those daily balances together for the month, dividing by the number of days in that month, and then explore the annual percentage yield (APY) to that average. The result is the interest you earn that month. This method—called the average daily balance method—is the standard across nearly all online banks and many traditional banks.

Here's a concrete example. Say your APY is 4.50% and you have these balances:

June 1–10: $10,000
June 11–20: $15,000 (you deposited $5,000)
June 21–30: $12,000 (you withdrew $3,000)

The bank multiplies: (10 days × $10,000) + (10 days × $15,000) + (10 days × $12,000) = $370,000. Divide by 30 days: $12,333.33 average daily balance. Then multiply by the daily rate (4.50% ÷ 365 days = 0.01233% per day) and by 30 days. You earn roughly $45.62 in interest that month.

Some banks use a simpler method called the ending balance method, where they only look at what you have in the account on the last day of the month. This is less common for savings accounts but matters if you're comparing banks—it means deposits late in the month earn nothing that month, while the average daily balance method counts them from day one.

Key Takeaways

Most banks use the average daily balance method, which counts every day's balance and divides by the number of days in the month before explore your APY.
Interest is calculated daily but usually paid monthly, so a deposit made on the 15th of the month starts earning interest that same day under the average daily balance method.
The APY you see advertised already includes the effect of compounding, so you do not need to calculate compound interest yourself—the bank does that math.
Withdrawals lower your average daily balance for the entire month, so timing large withdrawals near the end of the month preserves more interest than withdrawing early.
Some banks use the ending balance method instead, which only counts what you have on the last day of the month, making late deposits worthless for that month's interest.

Why the calculation happens daily, but interest posts monthly

Banks calculate interest daily because account balances change constantly—deposits, withdrawals, transfers. If they only looked once a month, they would miss the interest you earned on money that sat in the account for part of the month. Daily calculation is more accurate and fairer to you.

But interest does not post to your account daily. Instead, the bank adds up all those daily calculations and deposits the total once a month, usually on the first business day of the next month. So if you earned $45.62 in June, you see that $45.62 hit your account on July 1 or 2. This is why your balance might jump slightly at the start of each month.

The timing matters if you are comparing banks. A bank that calculates daily but posts monthly is standard. A bank that calculates only monthly is less common and usually pays less interest because it ignores the timing of your deposits and withdrawals within the month.

How APY already includes compounding in the advertised rate

The APY (annual percentage yield) you see advertised—say, 4.50%—is not the same as the interest rate. The APY includes the effect of compounding, which means interest earns interest. When your bank pays you $45.62 in interest in July, that $45.62 becomes part of your balance in August and starts earning interest too.

The bank has already done this math for you. When they advertise 4.50% APY, they are telling you that if you leave money untouched for a full year, you will earn 4.50% of your starting balance, including all the compounding that happens month to month. You do not need to calculate it yourself or worry that the advertised rate is misleading.

This is why APY is more useful than the interest rate alone. Two banks might both have a 4.50% interest rate, but if one compounds daily and one compounds monthly, the APY will be slightly different—the daily-compounding bank will advertise a marginally higher APY because your interest earns interest more often.

What happens to interest if you withdraw money mid-month

Under the average daily balance method, a withdrawal lowers your interest for that entire month because it reduces the average. If you withdraw $5,000 on June 15, the bank counts that lower balance for the remaining 15 days of the month, which pulls down your monthly average and your interest payment.

This is why the timing of withdrawals matters. Withdrawing on June 30 costs you less interest than withdrawing on June 1, because the lower balance only affects one day instead of 30. If you know you will need money mid-month, withdrawing as late as possible preserves more of your interest.

Some banks offer a grace period—they might not update your balance for a day or two after a withdrawal—but this is rare and not something to count on. Assume the withdrawal takes effect when ready.

How deposits affect your interest calculation

A deposit made on any day of the month counts toward your average daily balance starting that day. If you deposit $5,000 on June 15, the bank counts that $5,000 for the remaining 15 days of June (plus all of July, August, and beyond). This is one reason to deposit money as early in the month as possible—it earns interest for more days that month.

The difference is small in a single month but adds up over time. A $5,000 deposit on June 1 earns roughly 30 days of interest in June. The same deposit on June 30 earns one day of interest in June. Over a year, depositing early in each month can add $20 to $40 to your interest, depending on the APY and the size of your deposits.

Why APY changes and how it affects your interest

High-yield savings accounts have variable APY, meaning the rate can change. Banks raise or lower the rate based on what the Federal Reserve does with its benchmark interest rate. When the Fed raises rates, banks usually raise APY within days or weeks. When the Fed cuts rates, banks often cut APY more slowly, but they do cut it eventually.

If your APY changes mid-month, the bank calculates interest using the rate that was in effect for each day. Say your APY was 4.50% for the first 15 days of June, then dropped to 4.25% on June 16. The bank calculates interest on the first 15 days at 4.50% and the last 15 days at 4.25%, then adds them together. You see one interest payment in July that reflects both rates.

This is why checking your APY occasionally matters. If your bank's rate has fallen significantly below competitors, moving money to a higher-paying account can add hundreds of dollars a year in interest, especially on larger balances.

The difference between stated interest rate and APY

The interest rate (sometimes called the annual percentage rate or APR) is the raw percentage the bank pays on your balance. The APY is that rate plus the effect of compounding. For savings accounts, APY is always higher than the interest rate because compounding adds extra earnings.

Banks must disclose both numbers, but they advertise the APY because it is the number that matters to you. If a bank advertises 4.50% APY, that is the real return you will get if you leave money untouched for a year. The underlying interest rate might be 4.49%, but the compounding makes up the difference.

The gap between interest rate and APY is usually small—less than 0.01%—but it is real. On a $100,000 balance, that tiny difference is worth $10 a year.

Frequently Asked Questions

Does interest start earning on the day I deposit money?

Yes, under the average daily balance method, which is standard. Your deposit counts toward your balance starting the day it clears, and that balance is included in the daily calculations for the rest of the month. The interest from that deposit posts the following month.

What if I have multiple savings accounts at the same bank?

Each account is calculated separately. The bank does not combine balances across accounts. If you have $10,000 in one savings account and $5,000 in another, the bank calculates interest on each one independently using its own daily balance.

Can I lose money if the APY drops?

No. A drop in APY only means you earn less interest going forward. Your existing balance stays the same. If your APY drops from 4.50% to 4.25%, you will earn less interest next month, but you will not lose any of the principal you have saved.

Why do some banks advertise APY that changes after a few months?

Some banks offer a promotional rate for new customers—say, 5.00% APY for the first three months, then 4.50% after that. The advertised rate is usually the promotional rate, but the fine print explains when it drops. Read the terms carefully, because the long-term rate is what matters for most of your money.

Is the interest I earn taxable?

Yes. Interest earned on a savings account is taxable income. Your bank will send you a 1099-INT form at the end of the year if you earned $10 or more in interest, and you report that amount on your tax return. This is separate from how the bank calculates the interest itself.

This guide is for general information only and is not medical, financial, legal, or other professional advice. For decisions specific to your situation, consult a qualified professional. See our Editorial Policy.