APR on savings accounts is calculated by taking your annual interest rate and dividing it by the number of days in a year, then multiplying that daily rate by your account balance each day

Most savings accounts don't actually use APR — they use APY (annual percentage yield), which accounts for compounding. But when a bank does quote APR on a savings account, it's the simple annual rate before compounding is factored in. The difference matters because APY will always be slightly higher than APR on a savings account, since you earn interest on your interest.

The actual math happens daily. Your bank takes the annual rate, divides it by 365 (or sometimes 360, depending on the bank's method), and applies that tiny daily rate to whatever balance you have that day. If you have $10,000 in an account with a 4.5% APR, the bank calculates 4.5% ÷ 365 = 0.0123% per day. That 0.0123% gets multiplied by your $10,000 balance, earning you roughly $1.23 that day. Tomorrow, if your balance is still $10,000, you earn another $1.23. If you add $5,000, tomorrow's calculation uses $15,000 instead.

The interest compounds — usually daily or monthly — which means you start earning interest on the interest you've already earned. That's why APY (which includes compounding) is what matters for comparing accounts, even though the underlying calculation starts with APR.

Key Takeaways

  • Banks calculate daily interest by dividing the annual APR by 365 days, then multiplying that daily rate by your current balance.
  • Most savings accounts display APY, not APR, because APY includes the effect of compounding and is the true annual return you'll receive.
  • Your daily balance matters — deposits increase the amount earning interest that day, and withdrawals decrease it immediately.
  • The difference between APR and APY grows larger as interest rates rise, so a 4.5% APY account earns noticeably more than a 4.5% APR account over a year.

Why banks show APY instead of APR for savings

Banks are required by the Truth in Savings Act to disclose APY when advertising savings account rates. APY is the number that actually matters to you because it shows what you'll earn in a year after compounding takes effect. A 4.5% APR that compounds daily becomes roughly 4.60% APY — that extra 0.10% comes entirely from earning interest on your interest.

The more frequently interest compounds, the higher the APY climbs above the APR. Daily compounding beats monthly compounding, which beats quarterly. Some high-yield savings accounts compound daily, which is why they advertise APY so prominently — it's the honest number, and it's higher than the base rate.

When you see a savings account advertised at "4.5% APY," that's the rate you should use to compare accounts. The underlying APR is usually about 0.10% to 0.15% lower, but you don't need to calculate it yourself — the APY already tells you what you'll actually earn.

How your balance changes the interest you earn each day

Interest accrues on your actual balance each day, not on an average or a minimum. If you deposit $5,000 on the 15th of the month, that $5,000 starts earning interest immediately at the daily rate. If you withdraw $2,000 on the 20th, the interest calculation for the 21st uses the lower balance.

This is why timing matters slightly. A deposit early in the month earns interest for more days than a deposit late in the month. A withdrawal early in the month means fewer days of interest on that money. The difference is small — usually a few cents on modest balances — but it's real.

Some banks use a "daily balance" method, which is standard. Others use an "average daily balance" method, which averages your balance across the month before applying interest. The daily balance method is more common and more favorable to you because every dollar earns interest for every day it sits in the account.

The relationship between APR and APY over time

The gap between APR and APY widens as rates rise. At 0.5% APR, the APY is barely higher — maybe 0.50%. At 5% APR with daily compounding, the APY reaches roughly 5.13%. At 10% APR, APY climbs to about 10.52%. The compounding effect becomes more noticeable the higher the rate goes.

Over a year, this gap translates to real money. On a $50,000 balance at 4.5% APY (versus 4.5% APR), you'd earn roughly $2,250 with APY but only $2,250 with simple APR — wait, those are the same. Let me recalculate: at 4.5% APR compounded daily, you earn about $2,306 over a year. At 4.5% simple APR (no compounding), you'd earn $2,250. That $56 difference comes entirely from compounding.

This is why high-yield savings accounts emphasize APY. The rate is higher, and the compounding effect is real, even if it's not dramatic on smaller balances.

What happens when rates change mid-month

Banks can change savings account rates at any time, and the change takes effect immediately for new interest calculations. If your rate drops from 4.5% to 4.0% on the 15th, the interest you earn from the 15th onward uses the 4.0% rate. Interest already earned before the 15th is not affected.

You should check your account's current rate regularly, especially in a falling-rate environment. Banks lower rates on savings accounts faster than they raise them, and you may find your account's rate has dropped while competitors offer higher rates. Switching to a higher-rate account is free and takes a few days.

How to calculate your own interest earnings

You can estimate your annual earnings using the APY displayed on your account. Multiply your balance by the APY as a decimal. A $25,000 balance at 4.5% APY earns roughly $1,125 per year ($25,000 × 0.045 = $1,125). This is an approximation because it assumes your balance stays constant, but it's close enough for planning.

For a more precise calculation, you'd need to account for daily compounding and any deposits or withdrawals during the year. Most banks show your earned interest in your monthly statement, so you can track it without doing the math yourself. Your online account dashboard usually displays year-to-date interest earned as well.

If you want to see the underlying APR, you can divide the APY by the compounding frequency. For daily compounding, the formula is roughly: APR = (1 + APY)^(1/365) − 1. But this is unnecessary for most people — the APY is the number that matters.

Frequently Asked Questions

Is APR or APY better for a savings account?

APY is better because it shows your actual earnings after compounding. APR is the base rate before compounding, so it's always lower. When comparing savings accounts, use APY — that's the honest number for what you'll earn in a year.

Can a bank change my APR mid-month?

Yes. Banks can change rates at any time, and the new rate applies to interest earned from that day forward. Interest already credited to your account is not affected. Check your rate regularly, especially when market rates are falling.

Does my daily balance include pending deposits and withdrawals?

Pending transactions typically don't count toward your balance for interest purposes until they clear. Only posted transactions affect the balance used for daily interest calculations. Check your bank's policy, as this varies slightly by institution.

Why is my interest so low if the APY is 4.5%?

The 4.5% APY is an annual rate. If you earned interest for only one month, you'd earn roughly 4.5% ÷ 12 = 0.375% of your balance. On $10,000, that's about $37.50 per month. Interest accrues daily but is usually credited monthly or quarterly.

Does the bank use 365 or 360 days to calculate APR?

Most banks use 365 days, but some use 360 (called the "banker's year"). Using 360 days makes the daily rate slightly higher, so you earn a tiny bit more. Your bank's disclosure documents will specify which method they use, though the difference is usually just a few cents per year.