The basic formula for monthly interest

To find out how much interest you earn in a month, multiply your account balance by the annual percentage yield (APY), divide by 12, then multiply by the number of days in that month divided by the number of days in a year. The formula looks like this:

Monthly Interest = (Account Balance × APY ÷ 12) × (Days in Month ÷ 365)

Most banks use this daily balance method: they calculate interest on the exact balance you hold each day, add those daily amounts together, then pay you once a month. This means your interest compounds — you earn interest on your interest — though the effect is small at typical savings rates.

The simplest version, if your balance stays the same all month, is: multiply your balance by the APY, divide by 12. If you have $10,000 and your APY is 4.5%, you earn roughly $37.50 that month ($10,000 × 0.045 ÷ 12).

Key Takeaways

  • Monthly interest = (balance × APY ÷ 12) × (days in month ÷ 365), though most banks simplify by dividing the annual rate by 12 for a monthly rate.
  • Your actual interest depends on your exact daily balance throughout the month, not just the balance on the first or last day.
  • Banks post interest once a month, usually on the last day or the first day of the next month, and it becomes part of your balance immediately.
  • The APY your bank advertises already accounts for compounding, so you do not need to calculate compound interest separately — the bank does that work.
  • Interest rates change, so the amount you earn this month may differ from next month if your bank adjusts its APY.

Why banks use daily balance instead of a fixed monthly rate

If you deposit $5,000 on the 15th of the month, you should not earn interest on that money for the first half of the month — you did not have it in the account. Banks track this by calculating interest on your balance each day, then adding all those daily interest amounts together at month's end.

This is called the daily balance method, and it is the standard across most savings accounts. On day one, if your balance is $10,000, the bank calculates one day's worth of interest. On day two, if your balance is still $10,000, it calculates another day's worth. If you withdraw $2,000 on day 15, the daily interest calculation drops for the remaining days of the month.

A few banks use the average daily balance method instead, which adds up all your daily balances and divides by the number of days in the month. The result is usually very close to the daily balance method, but it can save you a small amount if your balance fluctuates a lot.

How to track your interest month by month

Your bank statement shows the interest posted each month in a line item labeled "Interest Paid" or "Interest Earned." This is the actual amount the bank calculated and added to your account. You can use this number to reverse-engineer the APY your bank used, or simply to verify the bank's math.

To check the bank's work: take the interest posted, multiply by 12, then divide by your average balance that month. This gives you the APY the bank applied. If it does not match the rate your bank advertised, contact them — errors happen, though they are rare.

If you want to forecast next month's interest before it posts, use your current balance and the current APY. Multiply balance × APY ÷ 12. This assumes your balance stays flat and the rate does not change, so it is an estimate, not a may provide. The actual amount will differ if you deposit or withdraw money, or if the bank changes the rate.

What happens when interest rates change

Banks adjust savings rates frequently, sometimes weekly. When your APY drops, your monthly interest drops with it. When it rises, you earn more. The change takes effect on the date the bank announces it, and you see the new amount in your next month's interest posting.

If you are comparing accounts, do not rely on today's rate — check whether the bank has a history of cutting rates when the Federal Reserve pauses rate hikes. Some banks hold high rates for a few months to attract deposits, then lower them. Others maintain competitive rates longer. Your bank's rate history is not published in one place, but you can call and ask what the rate was three months ago, six months ago, and a year ago.

The difference between APY and APR in savings accounts

APY (annual percentage yield) is what you see on savings accounts. It includes the effect of compounding — interest paid on your interest. APR (annual percentage rate) is used for loans and credit cards and does not include compounding.

For savings, APY is the number you use in the monthly interest formula. The bank has already done the compounding math for you. You do not need to calculate it separately. If a bank advertises 4.5% APY, that 4.5% already reflects the fact that you earn interest on your interest each month.

Why your actual interest may differ from your calculation

If you calculate monthly interest and it does not match what the bank posted, the most common reasons are: the bank changed the rate mid-month, your balance changed on days you forgot about, or the bank uses a slightly different day-count method (some use 360 days instead of 365).

Withdrawals and deposits also shift your daily balance. If you had $10,000 for 20 days and $8,000 for 11 days, your average daily balance is not $9,000 — it is closer to $9,467. The bank calculates this precisely, which is why your interest may be a few cents higher or lower than a simple estimate.

Rounding also plays a small role. The bank calculates interest to many decimal places, then rounds to the nearest cent when it posts to your account. Over many months, this rounding is negligible, but in a single month it can account for a penny or two of difference.

Using online calculators versus doing it by hand

Most banks offer a savings calculator on their website. You enter your balance, the APY, and how long you plan to keep the money, and it shows you the projected interest. These calculators are useful for comparing accounts or planning, but they assume your balance stays flat and the rate does not change.

If you want to calculate by hand, the formula is simple enough that a basic calculator works. For a more detailed month-by-month breakdown, a spreadsheet is faster: create columns for date, balance, daily interest (balance × APY ÷ 365), and running total. This lets you see exactly how much each deposit or withdrawal affects your interest.

The bank's own calculation is always the source of truth. Your estimate may be off by a few cents, and that is normal. If it is off by dollars, contact the bank.

Frequently Asked Questions

Do I earn interest on interest in a savings account?

Yes. The APY your bank advertises already includes compounding. Interest posts monthly and becomes part of your balance, so next month you earn interest on that interest too. The effect is small at typical rates — on $10,000 at 4.5% APY, compounding adds roughly $2 per year — but it does happen.

What if my bank posts interest on a different day each month?

Most banks post on the last day of the month or the first day of the next month, and the timing does not affect the amount you earn. The bank calculates based on your daily balance throughout the entire month, regardless of when it actually credits your account. The day it posts only matters if you are about to withdraw the money and want to know when it becomes available.

Can I calculate interest if my balance changes every day?

You can estimate it, but the bank's calculation will be more precise. Multiply each day's balance by the daily interest rate (APY ÷ 365), add all those daily amounts together, and you get close to what the bank will post. For exact numbers, wait for your statement — the bank has access to every transaction and calculates to many decimal places.

Does the interest I earn count as income for taxes?

Yes. Interest earned on savings accounts is taxable income. If you earn $600 or more in interest in a year, the bank sends you a 1099-INT form for tax filing. Even if you earn less, you still owe tax on it. Keep your monthly statements to track total interest earned.

Why is my interest lower than the APY suggests?

The most common reason is that your balance was lower than you expected during part of the month. If you had $10,000 for only half the month and $5,000 for the other half, your average daily balance was $7,500, not $10,000. Interest is calculated on your actual daily balance, not on what you think you had.