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Daily Interest Calculator

Enter the principal and annual interest rate to find the daily interest.

How it is calculated

The tool uses simple interest on a fixed balance with a 365-day year: yearly = principal × annual_rate / 100, daily = yearly / 365 and monthly = yearly / 12. The monthly figure is one twelfth of the year, not 30 or 31 times the daily amount, so 30 × daily comes out slightly below it. Nothing compounds: interest is not added back to the principal, and leap years and actual month lengths are ignored. Both inputs may be zero, in which case every result is zero. Lenders that use a 360-day year or compound daily will show somewhat different amounts, so check the day-count basis in your contract.

Worked example

You hold $10,000 at 4.5% a year. Yearly interest: 10,000 × 4.5 / 100 = $450.00. Divide by 365 days: 450 / 365 = $1.23 per day (1.2329 before rounding). Monthly interest is 450 / 12 = $37.50. If you only need the interest for a 20-day stretch, multiply the unrounded daily figure by 20, which gives about $24.66 rather than 20 × 1.23.

Try this example →

Common mistakes

  • Entering the rate as a daily or monthly rate: the field expects the annual percentage, so 4.5 means 4.5% a year.
  • Multiplying the rounded daily amount by many days: 1.23 × 365 is $448.95, not $450, so use the yearly figure or an unrounded daily value.
  • Expecting compounding: a card charging interest on interest each day will cost more than this simple figure.
  • Assuming every lender divides by 365; some use 360, which makes the daily amount slightly larger.
  • Using the monthly figure for a billing cycle of 28 or 31 days when the interest should be prorated by the actual days.

About this calculator

The daily periodic rate is simply the annual interest rate divided by 365, and multiplying it by the principal gives the amount of interest that accrues on a balance for a single day. Lenders and card issuers use this rate every day: credit cards typically calculate interest daily on the outstanding balance, mortgages compute per diem interest for closing statements, and many loans use it to prorate interest for a partial period.

The calculator also shows the yearly interest (the daily figure times 365) and a monthly equivalent equal to one twelfth of the year, which is useful for comparing a daily accrual against a stated monthly or annual cost. Because interest compounds in practice, this simple daily-accrual method is an approximation — it is most accurate for short periods or accounts that do not compound daily.

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