Interest Calculator: How to Calculate Simple & Compound Interest

An interest calculator helps you figure out how much money you'll earn on savings accounts, CDs or investments—or how much you'll owe on loans, credit cards and mortgages. You input the principal (starting amount), annual interest rate, compounding frequency and time period, then the calculator runs the math for you.

You need an interest calculator whenever you compare bank accounts, evaluate loan offers, or forecast investment growth. The two core types are simple interest (rare today) and compound interest (the standard for nearly every financial product).

Banks, brokers and lenders nearly always quote an annual percentage rate (APR) and a compounding frequency (daily, monthly, quarterly, annually). That combination determines your real return or cost, called the annual percentage yield (APY) for savings or the effective annual rate for loans.

An online interest calculator automates the formulas below and lets you toggle variables to see how small rate changes, longer terms or higher principal shift the final number. You can use spreadsheet formulas, financial calculators or [free tools](/free-tools) to run the same calculations yourself.

I = P × r × t

• I = interest earned or owed
• P = principal (initial amount)
• r = annual interest rate (as a decimal)
• t = time in years

Your final balance is A = P + I.

Simple interest is rare. You might see it on short-term promissory notes, some auto-loan calculations or promotional savings bonds.

Compound interest uses:

A = P × (1 + r/n)^(n×t)

• A = final amount
• P = principal
• r = annual rate (decimal)
• n = compounding periods per year
• t = years

Example: You invest $10,000 at 6 % compounded monthly for 5 years. A = 10,000 × (1 + 0.06/12)^(12×5) A = 10,000 × (1.005)^60 A ≈ $13,488.50.

Daily compounding (n = 365) produces slightly higher returns than monthly (n = 12) or annual (n = 1). The difference grows with larger principals and longer time horizons.

1. 1Enter the principal. This is the starting balance in your savings account, CD, loan or credit card.
2. 2Input the annual interest rate. Convert percentages to decimals (5 % = 0.05) if your calculator requires it; most auto-convert.
3. 3Select the compounding frequency. Daily is typical for savings; monthly for mortgages and credit cards.
4. 4Set the time period. Use years or months depending on the calculator; most accept either.
5. 5Add regular contributions or payments (optional). Many calculators let you model monthly deposits into a high-yield savings account or extra principal payments on a mortgage.
6. 6Read the output. Look for total interest, final balance and—if available—a year-by-year breakdown or amortization schedule.

When you use an interest calculator for high-yield savings accounts, certificates of deposit (CDs) or money-market accounts, you care about APY. APY reflects compounding and shows the true annual return.

For index funds, bonds or dividend stocks, the calculator estimates growth assuming you reinvest all distributions. Real investment returns vary by market performance, fees and tax treatment.

For auto loans, personal loans, student loans and mortgages, an interest calculator shows total interest paid over the loan term. Input the loan amount (principal), APR, term in months and any extra payments.

Credit cards typically compound daily on the average daily balance. A card at 20 % APR with daily compounding has an effective rate near 22.13 %.

Ignoring compounding frequency. A 5 % rate compounded daily beats 5 % compounded annually. Always check how often interest posts.

Confusing APR with APY. APR is the nominal rate; APY includes compounding. For loans, APR may exclude fees; read the fine print.

Using the wrong time unit. If the calculator expects years but you enter months, your answer will be off by 12×. Double-check labels.

Overlooking inflation. A 3 % savings rate loses purchasing power if inflation runs 4 %. Real return = nominal return minus inflation.

You can calculate simple interest by hand in seconds: multiply principal × rate × time. For compound interest, you need a scientific calculator or spreadsheet because of the exponent.

In a spreadsheet, use the formula =P*(1 + r/n)^(n*t) in any cell. Replace P, r, n and t with cell references or hard-coded numbers.

Use an interest calculator whenever you:

• Compare savings-account APYs at different banks.
• Decide between a 15-year and 30-year mortgage.
• Estimate credit-card payoff timelines at different monthly payments.
• Forecast investment growth for retirement, college or a down payment.
• Evaluate CD ladders or bond maturity dates.

You can also reverse the calculation to find the rate required to reach a goal. For example, to grow $50,000 into $100,000 in 10 years with annual compounding, you need roughly 7.2 % per year.

Most bank and brokerage sites offer free calculators tuned to their products. Independent tools give unbiased results and let you compare multiple scenarios.

Look for calculators that:

• Accept monthly contributions or payments.
• Show year-by-year breakdowns or amortization tables.
• Let you toggle compounding frequency.
• Display APY alongside APR for savings products.
• Export results to PDF or CSV for record-keeping.

What is the difference between simple interest and compound interest?

Simple interest applies the rate only to the original principal, so you earn or owe the same dollar amount each period. Compound interest reinvests all accrued interest, so each period's calculation includes previous gains.

How do I calculate compound interest daily?

Use the formula A = P × (1 + r/365)^(365×t), where 365 is the number of compounding periods per year. Divide your annual rate by 365, raise the result to the power of total days, then multiply by principal.

Why does my savings-account interest seem lower than the APY?

Can an interest calculator predict stock-market returns?

No. An interest calculator assumes a fixed rate; stock and bond returns fluctuate daily.

How does compounding frequency affect my loan payments?

More frequent compounding raises the effective cost of a loan. A mortgage at 4 % compounded monthly costs slightly more than one at 4 % compounded annually, because interest on unpaid interest accrues faster.

Explore our [money and debt](/money-and-debt) guides for deep dives on paying off credit cards, refinancing mortgages and building emergency savings. If you need one-on-one help with complex scenarios—estate planning, tax-advantaged accounts or business credit—check our [find a pro](/find-a-pro) directory for vetted financial planners and CPAs.

Compounding works for you when you save and against you when you borrow. Understanding the math puts you in control of both.