FINANCIAL CALCULATOR
Simple and Compound Interest Calculator
Estimate interest earned on a starting balance, compare simple and compound interest, and see the projected final amount. Enter a principal, annual rate, and time period; for compound interest, you can also compare different compounding schedules.
Interest Calculator
Modify the values and click Calculate to update the estimate.
Results reflect your last calculation.
Where Your Money Goes
Principal compared with interest earned.
Interest Summary
Key figures from your calculation.
Estimated Growth Over Time
Move your pointer across the chart to see the exact balance and interest.
Growth Table
Estimated balance and accumulated interest over time.
| Year | Estimated Balance | Interest Earned |
|---|---|---|
| 0 | $10,000.00 | $0.00 |
| 1 | $10,500.00 | $500.00 |
| 2 | $11,000.00 | $1,000.00 |
| 3 | $11,500.00 | $1,500.00 |
| 4 | $12,000.00 | $2,000.00 |
| 5 | $12,500.00 | $2,500.00 |
How This Interest Calculator Works
An interest calculation estimates how much a balance may grow over a chosen period. The result depends on the initial principal, annual interest rate, duration, and whether interest is calculated only on the original amount or also on previously earned interest. Use the calculator to compare scenarios with consistent assumptions.
This page models simple interest and compound interest on one starting balance. For borrowing with regular repayments, see the loan payment calculator. If you are estimating home financing, the mortgage payment estimator covers principal-and-interest payments over a mortgage term.
How to Use the Calculator
- Choose simple interest or compound interest.
- Enter the initial principal or starting balance.
- Enter the annual interest rate as a percentage.
- Enter the time period in years; decimals represent part of a year.
- For compound interest, choose annual, semiannual, quarterly, monthly, or daily compounding.
- Select Calculate Interest to refresh the estimate, charts, and year-by-year table.
To estimate an investment with an initial balance and recurring contributions, try the compound growth calculator. It is better suited to scenarios that include regular deposits.
Simple Interest Formula
Simple interest uses the original principal throughout the full term. Earned interest is not added to the amount used for later interest calculations.
I is interest, P is principal, r is the annual rate as a decimal, and t is time in years. For example, a 5% rate is represented as 0.05.
Compound Interest Formula
Compound interest adds earned interest to the balance at each compounding period. Future interest can therefore be calculated on both the starting principal and interest already accumulated.
A is the final amount, P is the principal, r is the annual rate as a decimal, n is the number of compounding periods per year, and t is time in years. The formula assumes a fixed rate and no additional deposits or withdrawals.
Simple vs. Compound Interest
Simple interest increases in a straight line when the principal and rate stay constant. Compound interest can grow faster because earned interest may itself earn interest. The difference generally becomes more noticeable over longer periods and at higher rates.
| Method | Interest earned | Final amount | Assumption |
|---|---|---|---|
| Simple interest | $2,500.00 | $12,500.00 | Original principal only |
| Compound interest | About $2,833.59 | About $12,833.59 | Compounded monthly |
These are illustrative estimates before taxes, fees, and account adjustments. Actual financial products may use different rates, compounding rules, or calculation conventions.
Worked Example: $10,000 at 5%
Suppose you deposit $10,000 at a fixed annual rate of 5% for 5 years. With simple interest, the calculation is $10,000 × 0.05 × 5 = $2,500, so the final amount is $12,500.
With monthly compounding under the same assumptions, the final amount is approximately $12,833.59, including about $2,833.59 in interest. The difference is caused by interest being added to the balance during the term.
Factors That Affect Interest
- Principal: A larger starting balance generally earns more interest when the rate and term are unchanged.
- Annual rate: A higher rate generally increases the amount earned over the same period.
- Time: A longer period gives interest more time to accumulate.
- Compounding frequency: The schedule can affect the final amount when compound interest is used.
- Transactions: Deposits and withdrawals change real balances but are not included in this single-principal estimate.
How Compounding Frequency Changes Growth
Annual compounding adds interest once a year. Semiannual, quarterly, monthly, and daily schedules apply it more frequently. With a positive fixed rate and otherwise identical assumptions, more frequent compounding generally results in a slightly higher final balance. This mathematical effect is not a guarantee of investment performance.
Understanding Your Results
The result panel shows the principal, estimated interest, final amount, rate, calculation method, and time period. The breakdown chart compares principal with interest, while the growth chart and table show how the modeled balance changes over time. The displayed results reflect the last calculation until you select the calculate button again.
Interest Calculations for Borrowing
These formulas are useful for basic estimates but do not reproduce every loan contract. Many loans use scheduled repayments, amortization, fees, or specific rules for accrued interest. Use the personal loan calculator to estimate payment-based borrowing costs. For revolving balances, the credit card payoff tool can help estimate a payoff timeline.
Assumptions and Limitations
Results are mathematical estimates based on the values entered. The calculation assumes a fixed rate and a single starting principal; it does not account for taxes, fees, inflation, recurring deposits, withdrawals, changing rates, or investment risk. A bank or lender may use daily balances, a different day-count convention, or terms defined by the account agreement.
For take-home pay estimates, use the salary and paycheck calculator. If you are comparing housing costs, the rent-versus-buy comparison can help organize the financial trade-offs.
Frequently Asked Questions
What does an interest calculator do?
An interest calculator estimates the interest earned on a starting balance and the resulting total using the principal, annual interest rate, time period, and selected calculation method.
How do simple and compound interest differ?
Simple interest is calculated on the original principal. Compound interest adds previously earned interest to the balance, allowing later interest to be earned on the principal and accumulated interest.
What is the simple interest formula?
Simple interest is I = P × r × t, where P is the principal, r is the annual rate expressed as a decimal, and t is time in years. The final amount is P + I.
What is the compound interest formula?
For a single deposit without additional contributions, the compound amount is A = P(1 + r/n)^(nt). P is principal, r is the annual rate as a decimal, n is compounding periods per year, and t is time in years.
Does compounding frequency change the result?
Yes. With the same positive rate and time period, more frequent compounding generally produces a slightly higher balance. The difference depends on the rate and schedule.
Can I calculate interest on a loan?
This tool can estimate basic interest, but many loans use scheduled payments, amortization, fees, or daily-interest rules. A payment-based loan calculator is more suitable for those estimates.
Can I include recurring monthly deposits?
No. This calculator models a starting principal without recurring deposits or withdrawals. Use a compound-interest calculator that supports regular contributions to model those transactions.
Are the results guaranteed returns?
No. Results are mathematical estimates based on the inputs. Actual earnings may differ because of changing rates, fees, taxes, withdrawals, account terms, and investment risk.