Use this simple interest calculator to find total interest and ending balance from a starting amount, an interest rate, and a time length. You can also switch modes to solve for the principal, rate, time, or ending balance.
Advanced options
How to use our Simple Interest Calculator
- Choose what you want to solve for in "What do you want to solve for?"
- Enter the numbers you already know (principal, interest rate, time, or ending balance) based on your mode.
- Pick a "Time unit" (years, months, or days) and enter the "Time length".
- Optional: If you use days, open Advanced options and choose a "Day-count method (for days)".
- Optional: If your rate is given per month or per day, open Advanced options and set "Rate given as" and "Rate period (if not annual)".
- Optional: Choose "Rounding for money outputs" to match how you want dollars and cents shown.
- Optional: Turn on "Show schedule table" to see the balance grow by period (simple interest grows in a straight line).
- Click Calculate.
- Read "Time converted to years" to double-check the unit conversion, and check "Notes and warnings" for common mix-ups (like APR vs APY).
Definitions
Simple interest: Interest calculated only on the original principal, not on past interest [2].
Principal (P): The starting amount of money (what you deposit or borrow).
Interest rate (APR): A percent per year that tells how much interest happens each year; this tool treats it as simple (no compounding) [1].
Time: How long interest is earned or charged. If you enter months or days, the calculator converts that time into years.
Total interest (I): The total dollars of interest over the whole time period.
Ending balance (A): Principal plus total interest.
Day-count method: A rule for turning days into a fraction of a year (for example, Actual/365 or 30/360).
APY: A yearly rate that includes compounding (interest-on-interest), which is different from simple interest [1].
Methodology
What this calculator assumes
This calculator uses simple interest, which means interest is based only on the original principal and grows linearly over time [2]. It is useful for quick estimates and some simple loans, but many real bank accounts and loans use compounding or amortized payments.
Step 1: Parse and validate inputs
Numbers can include commas (example: 1,000). The calculator removes commas and spaces, then checks that required fields for your selected mode are not blank and are valid numbers. If a required value is missing or a divide-by-zero would happen (like solving for a rate with principal = 0), the solved value is shown as N/A and the reason is listed in Notes and warnings.
Step 2: Convert time into years
The simple-interest formula needs time in years. If you choose years, time_in_years = time_length. If you choose months, time_in_years = months / 12. If you choose days, the calculator uses your selected day-count method: Actual/365 uses time_in_years = days / 365, and 30/360 uses time_in_years = days / 360.
Step 3: Convert the rate into an annual simple rate
If your rate is entered as an annual percent rate, rate_decimal_per_year = (rate_percent_per_year / 100). If you instead choose a periodic rate (per month or per day) in Advanced options, the calculator converts it linearly to an annual simple rate (periodic_rate_decimal times periods_per_year), then uses that annual simple rate for all outputs. This is a simple-interest conversion and is not the same as APY, which includes compounding [1].
Step 4: Calculate interest and ending balance
Total simple interest is I = P * r * t_years. Ending balance is A = P + I.
Step 5: Solve-for modes
Depending on your mode, the calculator rearranges the same simple-interest relationship. Solve for principal: P = I / (r * t_years). Solve for annual rate: r = I / (P * t_years), then rate_percent_per_year = 100 * r. Solve for time in years: t_years = I / (P * r). If you are solving using ending balance, the calculator first finds total interest as I = A - P, then uses the formulas above as needed. If A is less than P, I becomes negative and a warning is shown because the inputs are inconsistent for typical positive-interest situations.
Step 6: Extra outputs you can use to sanity-check
Interest per year is P * r. Interest per month is (P * r) / 12 (a simple monthly breakdown, not a loan payment). Simple interest over the whole period (percent of principal) is 100 * (I / P) when P is greater than 0; otherwise it is N/A to avoid division by zero.
Schedule table (optional)
If you enable the schedule, the calculator lists period-by-period balances by adding the same amount of interest each full period (linear growth). If your time is not an even number of periods (example: 2.5 years), it adds a final partial period row using the fraction of the period.
Rounding
The calculator keeps full precision during calculations and only rounds displayed money outputs using your selected rounding option (usually nearest cent).