Calculate simple or compound interest and the total from a principal, rate, and time, right in your browser. Estimates only, not financial advice.
Interest Calculator Online
Calculate simple or compound interest and the total from a principal, rate, and time, right in your browser. Estimates only, not financial advice.
The Interest Calculator runs entirely in your browser. The principal, rate, time, and contribution figures you enter never leave your device and are not uploaded to ArrayKit. Results are estimates only, not financial advice.
Interest Calculator finds the interest earned or owed and the resulting total from a principal, an annual rate, and a time period, in either simple or compound mode. Switch to Simple to apply the classic I = P × r × t formula, where the rate acts on the original principal only, or switch to Compound to apply A = P(1 + r/n)^(n·t) with a choice of compounding frequency — annually, semi-annually, quarterly, monthly, or daily — plus an optional regular contribution. It is useful for comparing a savings account, a certificate of deposit, or a short-term loan quoted as a flat rate against one that compounds. These are estimates only, not financial advice — verify important figures with a qualified professional before acting on them. Everything runs locally in your browser; your numbers are never uploaded.
Features
Toggle between simple interest (I = P × r × t) and compound interest (A = P(1 + r/n)^(n·t))
Compound mode supports annually, semi-annually, quarterly, monthly, or daily compounding
Optional regular contribution in compound mode to model ongoing deposits
Shows interest earned and the final total side by side
Copy button for a plain-text summary of principal, rate, time, interest, and total
Clear error message when the principal, rate, or time is missing or invalid
Compares simple and compound growth on the same numbers so you can see the difference
Runs entirely in your browser with no financial figures sent anywhere
How to use the Interest Calculator
Choose Simple or Compound as the interest type
Enter the principal amount, annual interest rate, and time in years
For compound mode, pick a compounding frequency and an optional contribution
Read the interest earned and the total amount, and copy the summary if needed
Example
Input
$1,000, 5%, 3 yr, simple
Output
Interest: $150
Total: $1,150
Common errors & troubleshooting
The rate was entered as 0.05 instead of 5, giving an interest figure that looks far too small. — Enter the annual rate as a percentage number, such as 5 for 5%, not as a decimal fraction — the Interest Calculator already divides by 100 internally.
Compound mode gives the same result as simple mode. — Confirm the Compound segment is selected. Simple and compound produce the same figure only when the rate or term is very small, since compound interest earns interest on interest.
A regular contribution was typed but the total did not change. — Contributions only apply in Compound mode. Switch to Compound and make sure a compounding frequency is selected before typing the contribution amount.
The total looks lower than a bank statement for the same account. — Most bank accounts compound rather than apply simple interest. Switch to Compound and match the compounding frequency your account states, such as monthly or daily.
Frequently asked questions
What formulas does the Interest Calculator use?
In Simple mode it uses I = P × r × t, applying the annual rate to the original principal only. In Compound mode it uses A = P(1 + r/n)^(n·t), where n is the compounding frequency per year, so interest earns interest over time.
How is simple interest different from compound interest in this calculator?
Simple interest is calculated only on the original principal for the entire term, so it grows in a straight line. Compound interest adds earned interest back into the balance at each compounding period, so growth accelerates the longer the term runs.
Can the Interest Calculator handle regular deposits?
Yes, in Compound mode. Enter an optional regular contribution amount alongside the principal, rate, and compounding frequency, and the calculator adds the future value of those deposits to the compounded principal.
Which compounding frequency should I choose?
Match whatever your account or loan statement specifies — annually, semi-annually, quarterly, monthly, or daily. More frequent compounding produces a higher final total at the same nominal rate.
Is the Interest Calculator's result financial advice?
No. It produces mathematical estimates only, based on the numbers you enter. Verify important savings, loan, or investment decisions with a qualified financial professional before acting on them.
Does the Interest Calculator upload my principal or rate figures?
No. All calculations run locally in your browser. The principal, rate, time, and contribution figures you enter stay on your device and are never uploaded to ArrayKit.
Related tools
Simple Interest Calculator — Calculate simple interest and the final amount from principal, rate and time.
Compound Interest Calculator — Project savings growth with compound interest and optional regular contributions.
Savings Calculator — See how a starting balance plus regular deposits grow with interest over time.
CD Calculator — Calculate the maturity value and APY of a certificate of deposit.
Loan Calculator — Calculate the monthly payment, total interest and payoff of any amortized loan.