Simple Interest Calculator
By Talcart · Last updated July 10, 2026
Simple Interest Calculator
Simple Interest Calculator
This simple interest calculator finds interest earned or owed on a fixed principal with no compounding: $5,000 at 4% for 3 years produces exactly $600 of interest, for a total of $5,600. Enter any principal, annual rate, and term to get the interest, the total repayment amount, and a comparison against compound growth.
Key facts
- $10,000 at 6% simple interest earns exactly $600 every year — $3,000 over 5 years, versus $3,382 with annual compounding.
- Under simple interest, money at 6% takes 16.67 years to double (100 / 6); compounding at 6% does it in 11.90 years.
- Doubling the term exactly doubles simple interest: $5,000 at 4% earns $600 over 3 years and $1,200 over 6 years.
What is the Simple Interest Calculator?
Simple interest is interest computed only on the original principal, never on interest that has already accrued. Because the base never grows, the charge is perfectly linear: the same dollar amount accrues every year, and total interest is proportional to both rate and time. Simple interest governs many short-term personal loans, auto loans (via daily simple-interest accrual), Treasury bills, and some bonds, whereas savings accounts, credit cards, and mortgages compound. It is the fairest method for the borrower over any term.
How does the Simple Interest Calculator work?
The calculator multiplies three inputs — principal P, annual rate r as a decimal, and time t in years — using I = P x r x t, then adds the result to the principal for the maturity amount A = P(1 + rt). Time converts naturally: 18 months enters as t = 1.5, and 90 days as 90/365. Because there is no exponent, doubling either the rate or the term exactly doubles the interest, a linearity compound interest does not share.
What is the Simple Interest Calculator formula?
- P – principal
- r – annual rate (decimal)
- t – time in years
- I – interest
- A – total amount
Simple vs compound interest on $10,000 over 5 years
| Annual rate | Simple interest | Compound interest (annual) | Compounding advantage |
|---|---|---|---|
| 3% | $1,500.00 | $1,592.74 | $92.74 |
| 4% | $2,000.00 | $2,166.53 | $166.53 |
| 5% | $2,500.00 | $2,762.82 | $262.82 |
| 6% | $3,000.00 | $3,382.26 | $382.26 |
| 7% | $3,500.00 | $4,025.52 | $525.52 |
| 8% | $4,000.00 | $4,693.28 | $693.28 |
| 10% | $5,000.00 | $6,105.10 | $1,105.10 |
How do you use the Simple Interest Calculator?
- Enter the principal.
- Enter the annual interest rate.
- Enter the duration in years.
- Read the interest and total amount.
Worked example
| Scenario | $5,000 at 4% for 3 years |
| Calculation | 5,000 × 0.04 × 3 = 600 |
| Result | Interest $600; total $5,600. |
Common use cases
Tips & best practices
Banks usually use compound interest — verify which method applies to your product.
Frequently asked questions
Multiply principal by the annual rate (as a decimal) by the time in years: I = P x r x t. For $5,000 at 4% for 3 years, I = 5,000 x 0.04 x 3 = $600, making the total owed $5,600. For part-years, convert first — 9 months is t = 0.75.
Simple interest is charged only on the original principal, while compound interest is charged on principal plus accumulated interest, so it grows faster. On $10,000 at 6% for 5 years, simple interest totals $3,000 but annual compounding produces $3,382 — a gap of $382 that widens every additional year.
Most commonly on auto loans and other installment loans that accrue interest daily on the outstanding principal, short-term personal loans, Treasury bills, and late-payment penalties. Deposit products are the opposite case: banks almost always compound savings interest daily or monthly, which is why APY exceeds the quoted rate.
Yes — for the same quoted rate and term, a borrower always pays less under simple interest because interest never accrues on interest. Borrowing $10,000 at 8% for 5 years costs $4,000 in simple interest versus $4,693 with annual compounding, a saving of $693, and paying early on a daily simple-interest loan cuts the cost further.
Convert the period to years before applying I = P x r x t. Six months is t = 0.5, so $8,000 at 5% for 6 months earns 8,000 x 0.05 x 0.5 = $200; 90 days on a 365-day convention is t = 0.2466, giving $98.63 on the same principal and rate.
It is the shortcut for principal plus interest in one step: A = P(1 + rt) equals P + Prt. With $5,000 at 4% for 3 years, A = 5,000 x (1 + 0.12) = $5,600. The rt term (here 0.12) is the fraction of the principal added as interest over the whole term.