Compound Interest Calculator
Enter your principal, interest rate, time period, and compounding frequency to see how your savings or investment grows over time.
How to use the Compound Interest Calculator
Enter your starting principal amount, the annual interest rate, how many years you plan to save or invest for, and how often interest compounds (monthly, quarterly, etc). Click "Calculate" to see your final balance and total interest earned.
What is compound interest?
Compound interest is interest calculated on both the original principal and the accumulated interest from previous periods. Unlike simple interest, which only grows based on the original amount, compound interest grows faster over time because each period's interest is added to the balance before the next period's interest is calculated — often described as "interest on interest."
How compounding frequency affects returns
The more frequently interest compounds, the faster your balance grows, even at the same annual rate. Monthly compounding will produce a slightly higher return than annual compounding over the same period, because interest is calculated and added to the balance more often.
Frequently asked questions
What's the formula used?
This calculator uses the standard compound interest formula: A = P(1 + r/n)^(nt), where A is the final amount, P is the principal, r is the annual interest rate (as a decimal), n is the number of times interest compounds per year, and t is the time in years.
Can I use this for loan interest instead of savings?
This calculator shows growth on a principal amount, useful for savings and investments. For loan repayments with monthly installments, try the Loan / EMI Calculator instead.
Does this account for regular monthly contributions?
Not currently — this calculator shows growth from a single lump-sum principal. Adding recurring contributions may be added as a future feature.
Is my data stored anywhere?
No. This tool is part of the Toolyard collection of free, browser-based utilities. All calculations run locally in your browser — nothing you enter is uploaded to a server or stored anywhere.
Why compound interest grows faster than people expect
With simple interest, you earn the same dollar amount every period. With compound interest, each period's earnings get added to your principal, so the next period earns interest on a slightly larger base. The difference is small in year one but compounds — literally — over time. A deposit growing at 8% annually roughly doubles in about nine years (a rough rule of thumb: divide 72 by the interest rate to estimate doubling time). This is why starting to save or invest even a few years earlier can matter more than contributing a larger amount later.
Reading your result correctly
The compounding frequency you select — annually, monthly, or daily — changes the outcome even at the same stated interest rate, because more frequent compounding means interest starts earning its own interest sooner. A savings account advertising 5% compounded monthly will out-earn one compounded annually at the same nominal rate, though usually not by a huge margin. Always check whether a bank or investment product quotes an annual percentage rate (APR) or annual percentage yield (APY) — APY already accounts for compounding, while APR does not.
Common uses for this calculator
People use compound interest math to project retirement savings, compare CD or high-yield savings account offers, estimate how a loan balance grows if payments are missed, or simply satisfy curiosity about "what if I saved $50 a month for 20 years." It's also a standard exercise in personal finance and business math courses.