Compound Interest Calculator
Calculate how your investment or savings grows over time with the power of compounding.
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How to use this calculator
A = final amount, P = principal, r = annual interest rate (decimal), n = compounding frequency per year, t = time in years.
- 1
Enter the principal — the amount you are investing or depositing today.
- 2
Enter the annual interest rate offered by your savings account or investment.
- 3
Set the time period in years you want to project.
- 4
Choose the compounding frequency — monthly is standard for most savings accounts.
Frequently asked questions
What is the difference between simple and compound interest?
Simple interest is worked out on the starting amount only. Compound interest is worked out on the starting amount plus the interest already added, so each period builds on the last. For $10,000 at 8% over 10 years, simple interest adds $8,000 and annual compounding adds $11,589.25.
Exactly what does this calculator compute?
One lump sum, no deposits: A = P × (1 + r ÷ n)^(n × t), where P is the principal, r the annual rate as a decimal, n the compounding periods per year and t the years. The interest line is A − P. Fees, taxes and inflation are not part of it, and the result is an illustration from your inputs, not a forecast.
Can I add monthly contributions?
No. There is a single starting amount and nothing is added or withdrawn during the term. If you add deposits in real life, this tool will understate the end balance, so run it only for the lump sum and work out the deposits separately.
How much difference does the compounding frequency make?
Here, $10,000 at 8% for 10 years ends at $21,589.25 compounded annually, $22,080.40 quarterly, $22,196.40 monthly and $22,253.46 daily. Daily means 365 periods a year, with no leap-year adjustment. Each step up adds less than the one before.
My bank quotes an APY. Which frequency should I choose?
Choose annually. An APY already includes the effect of compounding within the year, so entering it with monthly compounding would count that effect twice. If you have the nominal rate instead, pick the frequency the account states.
What happens with a rate of 0, a time of 0 or a blank box?
A blank box is read as 0. With a rate of 0 or a time of 0 the total equals the principal and the interest is 0.00, and with a principal of 0 everything is 0.00. A negative rate is accepted and shrinks the balance: −3% for 5 years on $10,000 monthly gives $8,605.46.
Can the time be a fraction of a year?
Yes, the exponent n × t takes any number, so 2.5 years works. $10,000 at 8% compounded semi-annually for 2.5 years gives 5 periods and a total of $12,166.53. The years box is not for months, so divide months by 12 first.
How are the results rounded?
The calculation runs on the full-precision numbers and only the three displayed figures are rounded to two decimals. They carry no currency symbol or thousands separators, so $2,500 at 4.5% for 3 years compounded daily shows as 2861.32.
What is compound interest?
How compound interest works
Compound interest is the process by which interest earned on a principal amount is reinvested, so that subsequent interest applies to the accumulated total rather than the original deposit alone. This creates exponential rather than linear growth — the longer money remains invested, the more powerful the effect becomes.
Compound vs simple interest — a real example
On a $10,000 deposit at 8% over 5 years, simple interest returns $14,000. Compound interest (monthly) returns $14,898 — a $898 advantage that grows larger with time. Over 20 years, the same comparison produces a difference of about $23,300 ($49,268 compound against $26,000 simple).
Where compound interest applies in everyday finance
Compound interest applies to savings accounts, certificates of deposit (CDs), bonds, money market accounts, and retirement vehicles like 401(k) and IRA plans. Credit card debt also compounds, usually daily — making it important to pay balances in full each month.
Simple vs compound interest over time: $10,000 at 8%
The gap between simple and compound interest starts small and then widens quickly. Compounding here is monthly, so the formula is A = 10,000 × (1 + 0.08/12)^(12 × years) — for 10 years that is 10,000 × (1.006667)^120 = $22,196.40.
| Years | Simple interest | Compound (monthly) | Extra from compounding |
|---|---|---|---|
| 5 | $14,000 | $14,898 | $898 |
| 10 | $18,000 | $22,196 | $4,196 |
| 20 | $26,000 | $49,268 | $23,268 |
| 30 | $34,000 | $109,357 | $75,357 |
Does compounding frequency matter? $10,000 at 8% for 10 years
More frequent compounding earns more, but with sharply diminishing returns. Moving from annual to quarterly adds about $491, quarterly to monthly adds about $116, and monthly to daily adds only about $57. For most savings decisions, the interest rate matters far more than how often it compounds.
| Compounding | Final amount | Interest earned |
|---|---|---|
| Annually | $21,589.25 | $11,589.25 |
| Quarterly | $22,080.40 | $12,080.40 |
| Monthly | $22,196.40 | $12,196.40 |
| Daily | $22,253.46 | $12,253.46 |
Rule of 72 compared with the exact doubling time
The Rule of 72 (years to double ≈ 72 ÷ rate) is a quick mental estimate. The table shows how close it is to the exact answer for annual and monthly compounding; it is most accurate around 8%.
| Annual rate | Rule of 72 | Exact (annual compounding) | Exact (monthly compounding) |
|---|---|---|---|
| 4% | 18.0 | 17.7 | 17.4 |
| 6% | 12.0 | 11.9 | 11.6 |
| 8% | 9.0 | 9.0 | 8.7 |
| 10% | 7.2 | 7.3 | 7.0 |
| 12% | 6.0 | 6.1 | 5.8 |
Common mistakes when projecting compound growth
Using a nominal rate where the bank quotes an APY (APY already includes compounding, so choose annual compounding to avoid counting it twice). Entering the time in months instead of years. Treating the result as certain: it is an illustration of the numbers you typed, and it ignores fees, taxes and inflation, all of which change what you actually keep. Expecting deposits to be included, when this tool starts from one lump sum and adds nothing along the way. Leaving the time or rate blank, which is read as 0 and returns the principal unchanged.
Learn more from an authoritative source:
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Results are estimates for informational purposes only and do not constitute professional financial, medical, legal, or technical advice. Read full disclaimer →