ReckonBoxHow the simple interest calculator works
Simple interest is interest charged only on the original principal, never on the interest already accrued. The formula is I = Prt and the balance is A = P(1 + rt). Pick which variable you're missing and this tool solves for it.
Underneath that it asks something no other calculator on this search asks: how the lender counts days.
Every rearrangement, because they exist
CalculatorSoup publishes the full set, and it's worth having them all in one place:
| Solving for | Formula |
|---|---|
| Interest | I = Prt |
| Balance | A = P(1 + rt) |
| Principal | P = A / (1 + rt) |
| Rate | r = (1/t)(A/P - 1) |
| Time | t = (1/r)(A/P - 1) |
Four variables, any three give you the fourth. That's the whole tool.
There's a quiet irony in the field here. Simple interest, the version people need least often, gets complete algebraic transparency. Compound interest with contributions, the version almost everyone is actually using, gets published by nobody.
What compounding would have added
$10,000 at 5% over five years:
| Method | Balance |
|---|---|
| Simple interest | $12,500.00 |
| Compounded monthly | $12,833.59 |
A gap of $333.59, or about 2.7%. Small over five years because simple interest is linear and compounding is exponential, and the two only diverge sharply once you extend the term.
The tool prints both, so the cost of the convention is on screen rather than implied.
The day-count basis, which nobody asks about
A day-count convention sets two separate things: the divisor that turns an annual rate into a daily one, and how many days count as elapsed. Those can differ, and when they do the borrower pays for it.
| Convention | Divisor | Days accrued per year |
|---|---|---|
| 30/360 | 360 | 360 |
| Actual/365 | 365 | 365 |
| Actual/360 | 360 | 365 |
Look at the last row. Actual/360 divides by 360, which makes the daily rate larger, and then still accrues the real 365 days. It takes the borrower-unfavourable half of each convention. On the same terms it charges 365/360 of what 30/360 does, about 1.4% more, every year, forever.
PropertyMetrics puts real numbers on it. A $2.5M loan at 4% over ten years with monthly payments:
| Convention | First month | Total over ten years |
|---|---|---|
| 30/360 | $8,333.33 | $537,354.14 |
| Actual/365 | $8,493.15 | $537,396.13 |
| Actual/360 | $8,611.11 | $547,154.46 |
Nearly $10,000 more on the same loan at the same headline rate. The same source notes the difference has taken several banks to court, where they prevailed in cases where the method was properly disclosed.
Not one simple interest calculator fetched for this build asks which basis applies. Omni's page discusses where simple interest genuinely appears and never mentions day counts. calculator.net takes a term in years and months and does the same.
Where simple interest actually turns up
Rarely, in consumer finance. calculator.net notes that most credit cards and loans use compound interest, which makes simple interest something of a teaching device.
Omni names the genuine cases: car loans, lines of credit, and early payment discounts. Add to that the short-dated end of lending generally, where a single interest period means the distinction collapses anyway.
It favours the borrower and disadvantages the lender, which explains its scarcity better than any pedagogy does.
What this calculator does not do
Leap days are not modelled. An "actual" convention over a term expressed in years is approximated at 365 days a year, and a calculation that needs exact calendar dates needs exact dates rather than a term.
Compound interest is a different page. The comparison figure appears here to show the gap, and the compound interest calculator handles contributions, frequency and the effective rate properly.
Results are arithmetic on your inputs, not financial advice. Loan terms carry fees and conditions this equation never sees, and a quoted rate is only half the story. For decisions about borrowing, speak to a qualified professional. To find a rate you were never told, the interest rate calculator solves for it from the payment.
Frequently asked questions
What is the simple interest formula? I = Prt, where P is the principal, r the annual rate as a decimal and t the time in years. The balance is A = P(1 + rt). Unlike compound interest, the rate only ever applies to the original principal, so the interest amount is the same every year.
How do you solve simple interest for the rate or the time? By rearranging the same equation. The rate is r = (1/t)(A/P - 1) and the time is t = (1/r)(A/P - 1), while the principal is P = A / (1 + rt). CalculatorSoup publishes all of these, which is more algebraic transparency than any compound interest calculator on this search offers.
How much less is simple interest than compound? On $10,000 at 5% over five years, simple interest gives $12,500 while monthly compounding gives $12,833.59, a difference of $333.59. The gap widens with time because compounding is exponential and simple interest is linear.
What is a day-count convention and why does it matter? A day-count convention sets both the divisor that turns an annual rate into a daily one and how many days count as elapsed. 30/360 treats every month as 30 days over a 360 day year. Actual/365 uses real days over 365. Actual/360 mixes them, dividing by 360 for a higher daily rate while still accruing the real 365 days, which makes it the most expensive for a borrower.
How much does the day-count basis actually change? On a $2.5M loan at 4% over ten years with monthly payments, PropertyMetrics puts 30/360 at $537,354.14 in total interest, Actual/365 at $537,396.13 and Actual/360 at $547,154.46. Actual/360 costs close to $10,000 more than 30/360 on the same loan, a gap that has taken several banks to court, where they prevailed when the method was properly disclosed.
Where is simple interest actually used? Omni names car loans, lines of credit and early payment discounts. calculator.net notes that most credit cards and loans use compound interest, which makes simple interest relatively uncommon in modern consumer finance even though it is the version taught first.