Interest Calculator
Calculate how money grows with compound interest — daily, monthly, quarterly, or annually — with optional monthly contributions.
Future Value
$20,096.61
$10,000.00 invested at 7% compounded monthly for 10 years grows to $20,096.61. Total interest earned: $10,096.61.
Total Principal
$10,000.00
Total Interest
$10,096.61
Ending Balance
$20,096.61
Final balance breakdown
- Starting Amount: $10,000.00
- Interest Earned: $10,096.61
Balance growth over time
Year-by-Year Breakdown
| Year | Start Balance | Interest Earned | End Balance |
|---|---|---|---|
| 1 | $10,000.00 | $722.90 | $10,722.90 |
| 2 | $10,722.90 | $775.16 | $11,498.06 |
| 3 | $11,498.06 | $831.20 | $12,329.26 |
| 4 | $12,329.26 | $891.28 | $13,220.54 |
| 5 | $13,220.54 | $955.71 | $14,176.25 |
| 6 | $14,176.25 | $1,024.80 | $15,201.06 |
| 7 | $15,201.06 | $1,098.89 | $16,299.94 |
| 8 | $16,299.94 | $1,178.32 | $17,478.26 |
| 9 | $17,478.26 | $1,263.51 | $18,741.77 |
| 10 | $18,741.77 | $1,354.84 | $20,096.61 |
What is a Compound Interest Calculator?
A compound interest calculator shows how a lump sum — and optional regular deposits — grows over time when interest is earned not just on the original principal but on the accumulated interest as well. Unlike simple interest (which only applies to the original amount), compound interest snowballs: each period's interest becomes part of the balance that earns interest in the next period.
The compounding frequency matters: daily compounding produces a slightly higher balance than monthly, which in turn beats annual compounding. For most savings accounts and investments the difference is small, but over decades on large balances it becomes meaningful. The year-by-year table above makes the acceleration visible — later years contribute far more growth than earlier ones at the same rate.
The Compound Interest Formula
For a lump sum with no additional contributions, compound interest is calculated as:
- A — final amount (future value)
- P — principal (starting amount)
- r — annual interest rate (as a decimal)
- n — number of compounding periods per year
- t — time in years
When regular monthly contributions are included, the calculator adds each deposit at the end of the month and applies sub-period compounding before the next contribution, giving an accurate result regardless of whether the compounding frequency matches the contribution frequency.
Why Compounding Frequency Matters
The stated annual rate and the effective annual yield (APY) are only identical when interest compounds once a year. With more frequent compounding, the APY exceeds the nominal rate because each period's interest earns its own interest before the year ends. At 7% nominal:
- Annual compounding → 7.000% APY
- Monthly compounding → 7.229% APY
- Daily compounding → 7.250% APY
The gap between monthly and daily is small (~0.02%), which is why most savings products advertise monthly compounding rather than daily — the marketing benefit of "daily" barely shows up in the actual numbers. The bigger lever is always the rate and the time horizon.
The Power of Regular Contributions
Adding a fixed monthly contribution to a compound interest account often contributes more to the final balance than the initial lump sum — especially over 20+ years. For example, investing $10,000 at 7% for 30 years with no contributions grows to about $76,100. Adding just $200/month pushes the final balance past $260,000 — the $72,000 in contributions earned over $177,000 in interest on top of the initial lump sum's $66,000.
The key insight is that contributions made early get more time to compound than later ones. This is why starting early, even with a small amount, consistently outperforms starting late with a larger amount. A 25-year-old investing $200/month will almost always end up with more than a 35-year-old investing $500/month, even though the 35-year-old contributes more per month.
The Rule of 72
A quick mental shortcut: divide 72 by the annual interest rate to estimate how many years it takes to double your money. At 6%, money doubles in roughly 72 ÷ 6 = 12 years. At 9%, it doubles in about 8 years. At 4%, roughly 18 years.
The rule works because of the natural logarithm underlying exponential growth — ln(2) ≈ 0.693, and 72/rate is a close approximation that works well for rates between 2% and 20%. At very low rates (under 2%) the rule slightly overestimates; at very high rates it slightly underestimates. For exact results, use the calculator above.
Example — Your Current Inputs
$10,000.00 invested at 7% compounded monthly for 10 years grows to $20,096.61. Total interest earned: $10,096.61.
Interest as a percentage of the final balance: 50.2%. Your money more than doubled — it grew 2.0×.
Additional Example — Retirement Savings
A 30-year-old invests $5,000 as a lump sum in a retirement account earning 7% annually, compounded monthly, and adds $300/month consistently until age 65 (35 years). At the end:
- Starting amount: $5,000
- Total contributions: $126,000 ($300 × 420 months)
- Interest earned: approximately $388,000
- Final balance: approximately $519,000
More than 75% of the final balance came from compound interest — not from the money actually put in. This is the core argument for starting early: time is the most powerful variable in the compound interest formula, not the rate or the contribution amount.
About These Parameters
- Starting Amount
- The initial lump sum deposited at time zero. It is the base on which the first period's interest is calculated. Larger starting amounts give compound interest more to work with from day one, but the time horizon and rate often matter more for the final outcome than the starting amount alone.
- Annual Interest Rate
- The nominal annual rate. This is the rate before compounding effects are applied. It differs from the APY (Annual Percentage Yield), which reflects the effective return after compounding within the year. Banks typically advertise the APY on savings products; for investments, use the expected annual return as the nominal rate.
- Investment Length
- How long the money is invested, expressed in years and months. Time is the most powerful lever in compound interest because growth is exponential — not linear. Doubling the time does not double the final balance; it can more than quadruple it at typical investment return rates. Even one extra year at the end of a long horizon can add substantial dollar amounts.
- Compound Frequency
- How often interest is computed and credited to the balance. Daily and monthly compounding are the most common in banking products. The difference between monthly and daily compounding is usually negligible (fractions of a percent annually), but the difference between annual and monthly compounding at high rates or over long periods can be thousands of dollars.
- Monthly & Annual Contribution
- Optional fixed amounts added every month and/or once a year, on top of the initial investment. This models regular contributions to a savings account, 401(k), IRA, or investment account — for example a monthly payroll deduction plus an annual bonus deposit. The year-by-year table shows each year's contribution total and how much of the balance growth came from contributions versus interest. Leave both at $0 for pure lump-sum growth.
- Contribute At (Beginning / End)
- Controls whether each contribution is added before or after that period's interest is calculated. A contribution made at the beginning of the period earns interest for the whole period; one made at the end earns nothing until the next period. Over many years and contributions, beginning-of-period timing produces a modestly higher final balance.
- Tax Rate
- Your marginal tax rate on interest income, for modeling a taxable brokerage or savings account rather than a tax-deferred one like a 401(k) or Traditional IRA. When set above 0%, the calculator deducts that percentage from each period's interest before it compounds, so the effective growth rate is lower than the nominal rate you entered. Leave at 0% for tax-deferred or tax-free accounts.
- Inflation Rate
- Your assumed average annual inflation rate, used only to translate the final nominal balance into today's purchasing power — it does not change the nominal ending balance itself. A 3% long-run inflation assumption is a common default; over long horizons even modest inflation meaningfully erodes what a given ending balance can actually buy.
Frequently Asked Questions
What is the difference between compound interest and simple interest?
Simple interest is calculated only on the original principal: $10,000 at 7% simple interest earns exactly $700 every year, forever. Compound interest applies interest to the growing balance: in year 1 you earn $700, in year 2 you earn $749 (7% of $10,700), in year 3 you earn $802, and so on. After 20 years, simple interest produces $24,000 ($10,000 + $14,000 interest) while compound interest (monthly) produces $40,065. Consumer loans use compound interest, as do most savings and investment accounts.
How does the APY differ from the stated interest rate?
The Annual Percentage Yield (APY) is the effective annual return after compounding is applied. Formula: APY = (1 + r/n)ⁿ − 1, where r is the nominal rate and n is the compounding frequency. At 6% nominal: APY = (1 + 0.06/12)¹² − 1 = 6.168%. Banks are required to disclose the APY on deposit accounts (in the US, under Regulation DD). When comparing savings accounts, compare APY, not the nominal rate.
Does inflation affect compound interest calculations?
The Ending Balance is always nominal growth — the dollar amount before adjusting for inflation. Enter a value in the Inflation Rate field to also see "Buying Power After Inflation," which divides the ending balance by (1 + inflation rate)^years to show what that future dollar amount is worth in today's purchasing power. If inflation averages 3% per year and your account earns 7% nominal, your real return is roughly 7% − 3% = 4% (or more precisely, 1.07/1.03 − 1 ≈ 3.88%).
What return rate should I use for stock market investments?
The US stock market (S&P 500) has historically returned approximately 10% annually on average before inflation, or about 7% after inflation. However, individual years vary wildly — returns have ranged from about −37% to +38% in a single year. For retirement planning, 7% nominal (or 4% real) is a common conservative assumption for a diversified equity portfolio. For bonds or cash, use current yield rates. Never assume historical averages guarantee future performance.
Why does interest earned accelerate so much in later years?
Because each year's interest is calculated on a larger balance than the year before. In year 1 of a $10,000 investment at 7%, interest is $700. By year 20, the balance has grown to about $38,700, so the same 7% rate generates $2,709 in interest that year — nearly four times as much despite the same rate. By year 30, the single year's interest ($5,332) exceeds the entire original principal. This acceleration is the defining characteristic of compound interest and why time is said to be its most powerful ingredient.