Daily Simple Interest Cost CalculatorFree online financial calculator — instant results, no sign-up
Calculate simple interest using a selected annual day-count denominator.
Estimates only, based on the inputs and assumptions shown. This is not financial, investment, tax, or legal advice. Prices, rates and local rules may change; crypto scenarios do not predict returns. Confirm important decisions with a qualified professional.
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About the Daily Simple Interest Cost Calculator
The Daily Simple Interest Cost Calculator estimates the interest cost on a fixed principal over a specified number of elapsed days using an annual rate and a selected day-count denominator. You choose the denominator used to convert the annual rate into a daily rate, rather than assuming every calculation uses 365 days. With the default inputs, the calculator shows daily interest of $3.29 and accrued interest of $147.95.
- Principal ($)
- The principal is the balance in dollars on which interest is calculated, with a default of $10,000.
- Annual rate (%)
- The annual rate is the yearly simple interest rate expressed as a percentage, with a default of 12%.
- Elapsed days (days)
- Elapsed days is the number of days for which interest accrues, with a default of 45 days.
- Day-count denominator (days)
- The day-count denominator is the number of days used as the annual basis when converting the annual rate to a daily rate, with a default of 365 days.
Formula and method
D = P × (r / 100) / B; I = D × t = P × (r / 100) × (t / B). With the defaults: D = 10000 × (12 / 100) / 365 = $3.287671… per day, displayed as $3.29; I = 10000 × (12 / 100) × (45 / 365) = $147.945205…, displayed as $147.95.
- PPrincipal in dollars.
- rAnnual interest rate entered as a percentage; r / 100 converts it to a decimal.
- BSelected annual day-count denominator in days.
- tElapsed interest-accruing days.
- DDaily simple interest cost in dollars per day.
- ITotal simple interest accrued over the elapsed days, in dollars.
Convert the annual percentage rate to a decimal, divide it by the selected day-count denominator, and multiply by the principal to obtain daily interest. Multiply the unrounded daily interest by elapsed days to obtain accrued interest. Round the displayed dollar outputs to two decimal places; do not round daily interest before calculating the accrued total.
Worked example
These are the calculator's starting values and the results it actually produces for them.
Inputs
- Principal10000 $
- Annual rate12 %
- Elapsed days45 days
- Day-count denominator365 days
Results
- Daily interest$3.29
- Accrued interest$147.95
How to use the Daily Simple Interest Cost Calculator
- 1Enter the principal balance in dollars; the default is $10,000.
- 2Enter the annual simple interest rate as a percentage; the default is 12.
- 3Enter the elapsed interest-accruing days; the default is 45.
- 4Select or enter the day-count denominator specified by your agreement; the default is 365.
- 5Read the daily and accrued interest outputs: the default inputs produce $3.29 per day and $147.95 over 45 days.
Reading your result
Daily interest is the cost for one day at the entered principal, annual rate, and denominator, displayed to the nearest cent. Accrued interest is the total interest for the entered elapsed days, calculated using the unrounded daily amount. For the defaults, these rows show $3.29 and $147.95 respectively. Accrued interest excludes the original principal and is not a payment amount or a total payoff quote.
Limitations
This calculation assumes a constant principal, a constant annual rate, and simple interest without compounding. It does not account for payments, new borrowing, fees, rate changes, or lender-specific rounding rules. The denominator must be greater than zero. Choosing a denominator does not implement a complete contractual day-count convention: you must supply the correct elapsed days, including any rules about start and end dates. A single denominator may not match agreements that split accrual across calendar years or use adjusted month lengths.
Common questions
Why does the calculator ask for a day-count denominator separately from elapsed days?+
Elapsed days specifies how long interest accrues, while the denominator specifies the annual basis used to derive the daily rate. Use the basis stated in your agreement; a smaller denominator produces more daily interest when the principal and annual rate stay the same.
Why should I not multiply the displayed $3.29 by 45 to check accrued interest?+
The displayed $3.29 is rounded. The calculator multiplies the unrounded daily amount, $3.287671…, by 45 and then rounds the result to $147.95, avoiding intermediate rounding differences.
Does selecting a 365-day denominator automatically handle leap years?+
No. The calculator uses the denominator you supply for the entire elapsed period. It does not inspect dates or automatically change the basis for a leap year; follow the day-count rules in your agreement.
Can I use one calculation if I make a principal payment during the 45 days?+
Not accurately if the payment changes the interest-bearing balance. Calculate each constant-balance period separately using its applicable principal and elapsed days, then combine the interest amounts according to your lender's rounding rules.
Does the calculator save my information?+
Calculations run in your browser. While signed in, inputs and results are automatically recorded in account history; saving a result creates a separate saved record.
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Results are estimates for informational purposes and are not financial, medical or legal advice.