Interest Per Day Calculator
Watch your money earn, one day at a time
Most calculators only tell you what your savings or loan will be worth in a year. This one breaks it down to a single day, so you can see exactly how much interest lands in your account (or leaves your pocket) between sunrise and sunset — whether you're comparing savings accounts, checking a loan's daily cost, or just curious what your money is doing while you sleep.
Step 1 · Your numbers
Enter your details
Fill in the fields below. Every field has a short note under it explaining exactly what to type and why it matters.
The amount sitting in the account or the loan balance you're earning or paying interest on.
This only changes the symbol shown — pick whatever matches your numbers.
The yearly rate quoted by your bank, lender, or investment — for example, a savings account advertised at 6.5% APY.
Most savings accounts use 365. Some commercial loans use the 360-day "banker's year" convention.
Simple interest is calculated only on your starting amount. Compound interest is recalculated on your growing balance — most modern savings accounts compound daily.
Start date
End date
Pick any custom date range — today isn't your only option. Great for backdated deposits or fixed loan terms.
Step 2 · Your results
Step 3 · The bigger picture
How your balance grows over the period
This chart plots your balance day by day across the time period you selected, so you can see the curve — not just the endpoint.
Worth knowing
Simple vs. compound, side by side
The difference between simple and compound interest looks small over a few days, and then quietly becomes significant over months. Here's what your numbers look like under both methods, using the same rate and time period.
| Method | Daily interest (day 1) | Total interest | Ending balance |
|---|---|---|---|
| Simple interest | $0.00 | $0.00 | $0.00 |
| Compound interest (daily) | $0.00 | $0.00 | $0.00 |
How this works
The formulas behind the numbers
No black box here. This calculator uses the same formulas taught in personal finance and accounting, following the standard day-count conventions used by banks worldwide (ISO 8601 for date handling, and the Actual/365 or Actual/360 conventions common in ISO-aligned financial reporting).
Simple interest per day
Daily interest = Principal × (Annual rate ÷ 100) ÷ Days in year. This amount stays flat every day because it's always calculated on your original principal, never on the interest you've already earned.
Compound interest per day
Balance after n days = Principal × (1 + daily rate) raised to the power of n, where the daily rate depends on your chosen compounding frequency. Each day's interest is added to the balance before the next day's interest is calculated — which is why the daily amount grows slightly over time instead of staying flat.
For a hands-on example: a $10,000 balance at 6.5% simple annual interest earns about $1.78 on day one. Left for a full year, that's roughly $650 in interest. Switch that same balance to daily compounding and the first day looks almost identical, but by month twelve you'll have earned a little more — the exact gap depends on your rate and how often it compounds, which is exactly what the comparison table above shows for your own numbers.
Common questions
Frequently asked questions
How do I calculate interest per day manually?
Take your principal, multiply it by your annual interest rate as a decimal (so 6.5% becomes 0.065), then divide by the number of days in the year — usually 365. For $10,000 at 6.5%, that's 10,000 × 0.065 ÷ 365, which comes to about $1.78 per day.
What's the difference between daily interest and APY?
APY (annual percentage yield) already accounts for compounding over a full year. Daily interest is the actual dollar amount credited or owed on a single day, which this calculator derives by working backward from your rate and compounding choice.
Why does my bank use a 360-day year instead of 365?
The 360-day convention, sometimes called the "banker's year," is a legacy accounting shortcut that's still common in commercial lending and some bond calculations. It slightly increases the daily rate compared to a 365-day basis, so it's worth checking which one your loan agreement actually uses.
Does interest accrue on weekends and holidays?
For most savings accounts and loans, yes — interest accrues every calendar day, including weekends and holidays, even though it might only be credited or posted to your account on business days.
How much daily interest does $10,000 earn?
It depends entirely on the rate. At 4% annually, $10,000 earns roughly $1.10 a day. At 6.5%, that rises to about $1.78 a day. Enter your own rate above to see the exact figure along with weekly, monthly, and yearly totals.
Is compound interest always better than simple interest for savers?
If you're earning interest, yes — compounding works in your favor because each day's interest starts earning its own interest. If you're paying interest on a loan, compounding works against you for the same reason, which is why it's worth checking which method your lender uses.