Compound Interest Calculator
Calculate how your investments grow over time with compound interest
Results
Future Value
$0.00
Total Principal
$0.00
Total Interest
$0.00
| Year | Balance | Contributions | Interest |
|---|---|---|---|
| Enter values to see yearly breakdown | |||
Compound interest is the engine behind nearly every long-term financial goal, and it rewards time more than it rewards cleverness. Enter a starting balance, an optional monthly contribution, an expected annual return, and a horizon, and this calculator projects your future balance, separates the money you put in from the money the growth produced, and lays out a year-by-year table you can inspect.
How to Use
- Principal: What you are investing today. The default is $10,000.
- Monthly contribution: What you add at the end of each month. Use 0 for a one-off lump sum.
- Annual rate: Your expected yearly return, expressed as a percentage such as 7.
- Years: The horizon, from 1 to 50 years. The default is 10.
- Compounding frequency: Daily (the default), monthly, quarterly, or annually.
- Read the results: Future value, total principal (your deposit plus all contributions), total interest, and a table of balance, contributions, and interest for each year.
All fields recompute as you type, so you can compare a 5% case against a 7% case in seconds without reloading the page.
Formulas Behind the Answers
For money that sits untouched, the future value is the textbook compound-interest formula:
A = P × (1 + r / n)^(n × t)
where P is the principal, r is the annual rate as a decimal, n is the number of compounding periods per year, and t is the number of years. A $10,000 lump sum at 7% for 10 years has a simple-interest value of $17,000, but compounding produces more: $19,671.51 with annual compounding and $20,136.18 with daily compounding.
Recurring deposits turn the problem into an ordinary annuity. With i = r / n and m = n × t total periods, the contribution portion is:
FV_contrib = PMT_period × (((1 + i)^m − 1) / i)
Because you enter a monthly figure, the tool converts it to a per-period deposit of monthly × 12 / n so the annual total always equals monthly × 12. It then adds the lump-sum growth and the annuity growth. Total interest is simply future value minus principal minus total contributions. For a quick sanity check, the Rule of 72 estimates doubling time: 72 divided by the rate gives roughly 9 years at 8% and about 10.3 years at 7%.
Worked Examples
A steady long-term saver. $10,000 to start, $500 added every month, 7% annual return, 10 years, monthly compounding. The future value is $106,639.02, made up of a $10,000 starting balance, $60,000 of contributions, and $36,639.02 of interest. After year one the balance is already $16,919.19; by year five it is $49,972.70, and the final year alone adds $6,967.79 of interest. Switching the frequency to daily raises the result only slightly, to $107,017.68, while annual compounding drops it to $102,570.20 — the real driver is the monthly deposit, not the compounding interval.
A lump sum across frequencies. $5,000 invested for 20 years at 6%. Quarterly compounding yields $16,453.31, annual compounding $16,035.68, and simple interest only $11,000. Frequency matters, but far less than the 20-year runway does.
Starting a decade earlier. Contribute $300 a month at 7% with no lump sum. From age 25 to 65, the balance reaches roughly $787,444. Beginning at 35 and stopping at 65 produces about $365,991. The first saver contributes $72,000 more but ends with more than twice the money, because the earliest dollars compound for an extra decade.
Choosing a Compounding Frequency
| Frequency | Periods per year | Use it when |
|---|---|---|
| Daily | 365 | High-yield savings accounts, some money-market funds |
| Monthly | 12 | Most standard projections, brokerage accounts, many loans |
| Quarterly | 4 | Bonds and some dividend schedules; a conservative planning default |
| Annually | 1 | Simple back-of-envelope estimates and index-return comparisons |
If you are not sure, use monthly for planning and rerun the numbers annually to keep the rate honest. The difference between monthly and daily is small; the difference between a realistic and an optimistic rate is enormous.
Common Mistakes
- Using a best-case rate. A single 10% assumption flatters the result. Model 4% to 6% alongside it.
- Forgetting inflation. Nominal wealth grows faster than purchasing power, so raise contributions with income.
- Double-counting deposits. Do not add your contributions to the principal field and the monthly field — they are tracked separately.
- Assuming a fixed rate. Real markets do not deliver steady returns, and a bad early decade is not recovered by the average.
- Ignoring withdrawals. The model assumes nothing is taken out before the horizon ends.
Accuracy and Limits
Results are nominal and rounded to the cent. The model assumes a constant annual rate, no taxes, no fees, no inflation, and no withdrawals, and it credits interest before each deposit rather than continuously. Contributions are entered monthly only, and the horizon is capped at 50 years. Treat the output as a planning estimate, not a promise, and use the lower-case scenario when a goal matters.
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Frequently Asked Questions
When does the calculator credit interest versus my monthly contribution?
Within each compounding period the tool credits interest on the current balance first, then adds your deposit. That is the ordinary-annuity convention used by most bank and brokerage statements. The order matters only marginally, but it means a deposit made at the end of a period does not earn interest for that period.
Why is the daily-compounding result higher than the monthly result?
Each compounding period multiplies the balance by a slightly smaller but more frequent factor, so interest starts earning interest sooner. On $10,000 growing at 7% for 10 years, daily compounding finishes about $378 ahead of annual compounding, and monthly compounding finishes about $4,069 ahead once $500 monthly deposits are included.
Is the future value shown in today's dollars?
No. Every figure is nominal, meaning it is not adjusted for inflation. At roughly 3% inflation, a dollar 30 years from now buys about what 41 cents buys today. To see purchasing power, subtract your inflation assumption from the return rate before entering it, or divide the result by (1 + inflation) raised to the number of years.
Do the results include taxes, fund fees, or expense ratios?
No. The calculator models a single nominal rate with no tax drag, advisory fees, or fund expense ratios. Those costs reduce real returns substantially over long horizons, so either lower the rate you enter or subtract expected annual costs before comparing scenarios.
What annual rate should I use for a long retirement projection?
A diversified stock index has historically returned roughly 7% to 10% per year before inflation, and about 4% to 7% after it. Conservative plans often assume 5% to 6% nominal. Whatever you choose, model a lower case as well, because a single optimistic rate hides sequence risk.
Why does the total principal figure grow over time?
The field labeled total principal combines your original deposit with every monthly contribution you make, so it climbs each year even though your starting principal is fixed. The yearly table separates these: the contributions column shows that year's deposits, while the balance column shows principal plus all interest and deposits to date.
Reviewed by the HandyTools Hub team · Last updated