Compound Interest Calculator
Calculate how your savings grow over time with compound interest and regular contributions.
Results
Visualization
How It Works
This calculator shows you how your money grows when you earn interest on your interest—a powerful wealth-building force called compound interest. By entering your starting amount, interest rate, time period, and regular contributions, you can see exactly how much you'll have in the future, how much you actually contributed, and how much came from interest alone. This tool is designed for both quick estimates and detailed planning scenarios. Results update instantly as you adjust inputs, making it easy to compare different approaches and understand how each variable affects the outcome. For best accuracy, use precise measurements rather than rough estimates, and consider running multiple scenarios to establish a realistic range of expected results.
The Formula
Variables
- P — Initial Deposit — the amount of money you start with, deposited at the beginning
- r — Annual Interest Rate — the percentage rate per year (for example, 5% for a savings account earning 5 annually)
- n — Compounding Frequency — how often interest is calculated and added to your balance (daily, monthly, quarterly, or annually)
- t — Number of Years — the total time period you're saving or investing for
- PMT — Monthly Contribution — the fixed amount you add to your account each month
- A — Future Value — the total amount you'll have at the end of the period, including all interest earned
Worked Example
Let's say you open a savings account with $5,000 and your bank offers 4.5% annual interest compounded monthly. You also commit to adding $200 every month for 10 years. The calculator first grows your initial $5,000 by the compound interest formula, applying the 4.5% rate each month (0.375% per month) for 120 months. Simultaneously, it grows each monthly $200 contribution by the time remaining after you deposit it. Your initial $5,000 grows to roughly $7,764, your 120 monthly contributions of $200 total $24,000 and grow to about $27,447 with interest. Your final balance is approximately $35,211—meaning you contributed $29,000 total, and earned about $6,211 in free interest. This demonstrates how regular contributions amplify the compounding effect.
Practical Tips
- Increase your compounding frequency when possible—daily compounding earns slightly more than monthly or annual compounding at the same interest rate, because interest gets calculated and added to your balance more often.
- Monthly contributions matter more than you might think: in the example above, your monthly $200 contributions actually generated more total interest ($3,447) than your initial deposit, even though the initial amount was higher.
- Higher interest rates have an exponential impact over long periods; a difference of just 1% annually can add thousands of dollars over 20+ years, so shop around for the best rates on savings accounts, CDs, and investment accounts.
- Start saving as early as possible, even with small amounts—a 25-year-old saving $100/month at 6% interest will have roughly $100,000 by age 65, while a 35-year-old saving the same amount accumulates only about $51,000 due to lost compounding time.
- Be consistent with contributions; skipping months or irregular deposits disrupt the compounding rhythm and reduce your final balance, so automate your monthly savings to stay on track.
Frequently Asked Questions
What's the difference between simple interest and compound interest?
Simple interest is calculated only on your original principal amount, while compound interest is calculated on your principal plus all previously earned interest. For example, $1,000 at 10% simple interest earns $100 per year forever, but compound interest earns $100 the first year, then $110 the second year (interest on the original $1,000 plus the $100 earned), creating exponential growth. This is why compound interest is much more powerful over time.
Why does compounding frequency matter if the annual interest rate is the same?
Compounding frequency determines how often interest is added to your balance and begins earning its own interest. Daily compounding calculates interest 365 times per year, while monthly does it 12 times—so money earns interest on interest more frequently. At 5% annual interest, daily compounding will give you slightly more money than monthly compounding, though the difference is small on typical savings accounts (often just a few dollars per year on moderate balances).
How long does it actually take to double my money with compound interest?
A quick rule of thumb called the 'Rule of 72' estimates this: divide 72 by your annual interest rate. At 6% interest, your money doubles in roughly 12 years (72 ÷ 6 = 12). At 8%, it's about 9 years. This rule works well for interest rates between 1% and 10%, though actual results depend on compounding frequency and whether you're making regular contributions.
Does compound interest work the same way for loans and debt?
Yes, but in reverse—compound interest works against you on debt. Credit card balances often compound daily, meaning you owe interest on your unpaid interest, which is why credit card debt grows so quickly. A $5,000 balance at 20% APR (typical for credit cards) compounding daily will cost you thousands in interest if you only make minimum payments, because the interest keeps compounding on an ever-growing balance.
Should I prioritize a larger initial deposit or consistent monthly contributions?
Both matter, but for most people, consistent monthly contributions have a bigger long-term impact because you're continuously feeding the compound machine. However, if you have a windfall, putting it in early maximizes compounding time. Ideally, maximize both: start with whatever initial amount you can, then set up automatic monthly contributions you'll actually maintain—automation ensures you don't skip months and break the compounding cycle.
Sources
- SEC: Investor Alert on Compound Interest
- Federal Reserve: Understanding Interest Rates
- MyMoney.gov: Saving and Investing