Simple Interest Calculator
Calculate simple interest, principal, rate or time using the J = P × i × t formula.
What is Simple Interest Calculator?
In simple interest, interest is always applied to the principal — there is no "interest on interest". The formula is A = P × (1 + r × t), common in short-term personal loans, simplified installment plans and some penalty fees. For investments and long-term financing, compound interest is usually the right model.
How to use
- Enter the principal (amount lent or invested).
- Set the interest rate and the period (monthly, yearly, etc.).
- See total interest and the final amount (principal + interest).
- Compare with the compound interest calculator to see the gap.
Frequently asked questions
What is the simple interest formula?
Simple interest is J = P × i × t, where P is the principal, i is the interest rate per period (as a decimal), and t is the number of periods.
How is simple interest different from compound interest?
Simple interest is calculated only on the original principal. Compound interest also applies the rate to accumulated interest, growing faster over time.
Can I mix rate per year with time in months?
Yes — pick the unit on each input and the calculator normalizes both to monthly to keep the math consistent.