Daily Interest Rate Calculator (APR to Per-Day Rate)
Use this calculator to convert an annual interest rate into a daily rate, then estimate interest per day and total growth over a chosen number of days.
Tip: APR ÷ 365 gives a nominal daily rate. If interest compounds daily, real growth is slightly higher over time.
What is an interest rate per day?
An interest rate per day is the amount your money grows (or costs) each day based on an annual rate. It is commonly used for savings accounts, loans, credit card interest, and per diem interest calculations. Converting APR to a daily rate helps you understand the short-term impact of borrowing and investing decisions.
Even a small daily rate matters because it applies repeatedly. On loans, this can increase your payoff amount every day. On savings, it can steadily build your balance—especially with compounding.
How to convert annual interest to daily interest
Basic daily rate formula
The standard nominal daily conversion is:
Daily Rate = Annual Rate ÷ Days in Year
- Annual rate should be in decimal form (5% = 0.05)
- Days in year is typically 365, but some institutions use 360
Daily interest amount formula
Once you have the daily rate, estimate daily interest with:
Daily Interest Amount = Principal × Daily Rate
Total interest over multiple days
- Simple interest: Principal × Daily Rate × Number of Days
- Daily compounding: Principal × (1 + Daily Rate)Days − Principal
How to use this interest rate per day calculator
- Enter your principal (starting balance or loan amount).
- Enter your annual interest rate (APR).
- Select your number of days.
- Choose 365 or 360 day-count convention.
- Pick simple or daily compounding mode.
- Click Calculate to get daily rate, daily interest amount, total interest, and ending balance.
Example: quick daily interest estimate
Suppose your principal is $10,000 with a 5% APR and a 365-day year:
- Daily rate = 0.05 ÷ 365 = 0.000136986
- Daily interest amount ≈ $10,000 × 0.000136986 = $1.37/day
Over 30 days, simple interest is about $41.10. With daily compounding, the result is slightly higher.
360 vs 365 day-count convention
Some financial products calculate daily interest using 360 days, while others use 365. The 360 method produces a slightly higher daily rate for the same APR. Always check your account terms to match the correct method:
- 365: common for many consumer accounts
- 360: often seen in certain commercial loans or banking products
Simple vs compounded daily interest
Simple interest assumes interest is calculated only on the original principal. Compounded interest adds earned interest back to the balance, so future interest is calculated on a larger amount.
For short periods, the difference may be small. Over longer periods, compounding can become significant.
Common mistakes to avoid
- Mixing APR and APY (they are not the same).
- Forgetting to convert percent to decimal when doing manual math.
- Using 365 when your lender uses 360 (or vice versa).
- Assuming interest compounds when your contract specifies simple interest.
When this calculator is useful
- Estimating credit card interest per day
- Checking personal loan per diem interest
- Projecting short-term savings growth
- Comparing accounts with different APRs
- Planning payoff timing to reduce interest charges
Final thoughts
Understanding your daily interest rate gives you a practical edge. It turns a large annual percentage into a clear daily number you can act on. Whether you are paying down debt or growing savings, this simple daily view helps you make smarter money decisions.
Educational use only. For legal or contractual calculations, rely on your lender’s or institution’s official method and disclosures.