Calculate simple interest with the formula I = PRT.
Enter any three values to find the fourth. The calculator solves for the missing variable automatically.
Simple Interest
$0
Total Amount
$0
Formula: I = P × R × T
The Simple Interest Calculator solves the classic interest formula I = P × R × T for any missing variable. Enter any three of principal, rate, time, or interest, and the calculator automatically solves for the fourth. This makes it useful not only for computing interest but also for finding the rate you paid on a loan, the time needed to earn a target interest amount, or the principal required to hit a savings goal.
I = P × R × T
A = P + I = P × (1 + R × T)
Where:
Find interest: You deposit $2,000 at 6% annual interest for 4 years. I = 2000 × 0.06 × 4 = $480. Total = $2,480.
Find rate: You paid $300 interest on a $1,500 loan over 2 years. R = 300 ÷ (1500 × 2) = 0.10 = 10% per year.
Find time: How long does it take $5,000 at 4% to earn $800 interest? T = 800 ÷ (5000 × 0.04) = 800 ÷ 200 = 4 years.
Find principal: You want to earn $600 in 3 years at 5% interest. P = 600 ÷ (0.05 × 3) = 600 ÷ 0.15 = $4,000.
Simple interest is calculated only on the original principal, making it linear: doubling the time doubles the interest. Compound interest is calculated on principal plus accumulated interest, making it exponential: doubling the time more than doubles the interest.
For example, $1,000 at 10% for 3 years:
The $31 difference might seem small, but over 20-30 years it becomes enormous. This is why credit cards (which compound daily) can trap borrowers while simple-interest loans are more predictable. Always check whether a loan uses simple or compound interest before signing.
Simple Interest (I) = Principal (P) × Annual Rate (R) × Time in Years (T). The total amount to repay is A = P + I = P(1 + RT).
Simple interest is calculated only on the original principal. Compound interest is calculated on the principal plus accumulated interest, so it grows faster. A \$1,000 loan at 10% for 3 years costs \$300 in simple interest but \$331 in compound interest compounded annually.
Simple interest is commonly used in short-term loans, car loans, some mortgages, and bonds. Most credit cards and savings accounts use compound interest instead.
Rearrange the formula: Rate = Interest ÷ (Principal × Time). For example, if you paid \$120 interest on a \$1,000 loan over 2 years, the rate = 120 ÷ (1000 × 2) = 0.06 = 6% per year.
The standard formula uses years. For months, divide by 12. For days, divide by 365 (or 360 in some banking conventions).
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