Compound Interest Calculator
Calculate how your savings or investments grow over time with compound interest. Supports daily, monthly, quarterly, and annual compounding with optional regular contributions. Results include a year-by-year growth schedule, an interactive chart comparing compound vs simple interest, and an inflation-adjusted real return.
Calculator
Future Value
$20,096.61
Total Interest
$10,096.61
Total Invested
$10,000.00
Return on Investment
100.97%
Rule of 72: At 7%, your money doubles in approximately 10.3 years.
Daily Compound Interest
Daily (365×/yr)
$20,136.18
Monthly (12×/yr)
$20,096.61
Annually (1×/yr)
$19,671.51
With daily compounding you earn $464.66 more than annual compounding over 10.0 years.
Simple Interest vs Compound Interest
Simple Interest
$17,000.00
Interest earned: $7,000.00
Compound Interest
$20,096.61
Interest earned: $10,096.61
Growth Over Time
Year-by-Year Schedule
| Opening | Interest | Contributions | Closing | |
|---|---|---|---|---|
| 1 | $10,000.00 | $722.90 | $0.00 | $10,722.90 |
| 2 | $10,722.90 | $775.16 | $0.00 | $11,498.06 |
| 3 | $11,498.06 | $831.20 | $0.00 | $12,329.26 |
| 4 | $12,329.26 | $891.28 | $0.00 | $13,220.54 |
| 5 | $13,220.54 | $955.71 | $0.00 | $14,176.25 |
| 6 | $14,176.25 | $1,024.80 | $0.00 | $15,201.06 |
| 7 | $15,201.06 | $1,098.89 | $0.00 | $16,299.94 |
| 8 | $16,299.94 | $1,178.32 | $0.00 | $17,478.26 |
| 9 | $17,478.26 | $1,263.51 | $0.00 | $18,741.77 |
| 10 | $18,741.77 | $1,354.84 | $0.00 | $20,096.61 |
How Compound Interest Works
Compound interest is interest calculated not just on your original principal but also on all previously earned interest. Over time this creates a snowball effect — your returns generate their own returns, and growth accelerates with every passing period.
Consider $1,000 invested at 10% per year. With simple interest you earn $100 every year — a flat line. With compound interest, you earn $100 in year one, but $110 in year two (10% of $1,100), and so on. After 20 years, simple interest gives $3,000. Compound interest gives $6,727 — more than double.
The compounding frequency also matters. The more often interest is applied — daily beats monthly beats annually — the faster your balance grows, because each compounding adds to the base for the next calculation.
Compound Interest Formula
A — Final amount (principal + interest)
P — Principal (starting amount)
r — Annual interest rate as a decimal (e.g., 7% = 0.07)
n — Compounding periods per year (Daily = 365, Monthly = 12, Annually = 1)
t — Time in years
Worked example: $5,000 at 6% for 5 years, compounded monthly
A = 5,000 × (1 + 0.06/12)^(12 × 5)
A = 5,000 × (1.005)^60
A = 5,000 × 1.3489
A ≈ $6,744.25
Daily Compound Interest
Daily compounding (n = 365) is the most aggressive compounding frequency and is used by many high-yield savings accounts. The daily formula applies a tiny rate each day: A = P × (1 + r/365)^(365 × t). While the difference versus monthly compounding is modest in dollar terms, it adds up meaningfully over decades. Use the Daily preset in the frequency dropdown above to see the exact difference for your inputs.
Compound Interest Calculator for Savings
Monthly compounding (n = 12) is the most commonly offered frequency for savings accounts, CDs, and investment accounts. Interest earned each month is immediately added to your balance, so the following month’s interest is calculated on a slightly larger amount. For long-term savings goals like a home down payment or retirement fund, even small differences in compounding frequency and rate compound into significant differences over 10–30 years.
Compound Interest vs Simple Interest
Simple Interest
Interest is calculated only on the principal. Formula: SI = P × r × t. Growth is linear — the same dollar amount is earned every year. Typically used for short-term loans and bonds.
Compound Interest
Interest is calculated on principal plus accumulated interest. Formula: A = P(1 + r/n)^(nt). Growth is exponential. Used by savings accounts, investments, and most loans.
How to Calculate Compound Interest in Excel
In Excel or Google Sheets, use the FV (Future Value) function to calculate compound interest with regular contributions:
Where rate is annual interest rate, n is compounding periods per year, t is years, PMT is periodic contribution, and PV is initial principal. This online calculator gives the same result instantly — no spreadsheet needed.
Rule of 72
The Rule of 72 is a quick mental shortcut for estimating how long it takes to double your money. Simply divide 72 by the annual interest rate. At 8%, your money doubles in approximately 72 ÷ 8 = 9 years. At 6%, it takes about 12 years. At 12%, only 6 years. The rule is an approximation — the calculator above shows the exact doubling time for your specific rate and compounding frequency.
at 4%
18.0 yrs
at 6%
12.0 yrs
at 8%
9.0 yrs
at 10%
7.2 yrs
at 12%
6.0 yrs
at 15%
4.8 yrs
Frequently Asked Questions
Why use our online Compound Interest Calculator?
See how your savings or investments grow over time with compound interest. Adjust principal, interest rate, compounding frequency, and duration to compare scenarios and plan your financial goals.
How to use Compound Interest Calculator
- 1Enter your starting principal
Type the initial amount you are investing or saving. You can leave this as zero if you are starting from scratch with regular contributions only.
- 2Set the annual interest rate
Enter the annual interest rate as a percentage. For a savings account paying 4.5%, enter 4.5. For an investment returning 10% per year, enter 10.
- 3Choose your compounding frequency
Select how often interest is calculated and added to your balance. Daily compounds the most aggressively, while annually is the simplest. Monthly is the most common for savings accounts.
- 4Add regular contributions (optional)
Enter the amount you plan to deposit each compounding period. For monthly deposits, select Monthly frequency. Negative values represent withdrawals. Toggle Start/End to set whether contributions happen at the beginning or end of each period.
- 5Read your results
See your future value, total interest earned, total amount invested, and return on investment — all updating in real time. Scroll down for the year-by-year schedule, growth chart, and simple vs compound comparison.
The mathematics of compounding — why time is the most powerful variable
The compound interest formula A = P(1 + r/n)^(nt) reveals that time (t) appears in the exponent, while principal (P) and rate (r) appear as coefficients. This means doubling the time doesn't double your money — it squares the compounding factor. Doubling the principal does exactly double your money. This asymmetry is why starting early matters far more than investing large amounts.
Example: Two investors each earn 8% annually. Investor A invests $5,000/year from age 25–35 (10 years, $50,000 total) then stops. Investor B invests $5,000/year from age 35–65 (30 years, $150,000 total). At age 65, Investor A has approximately $615,000 — more than Investor B's $611,000, despite investing one-third as much money. This is the power of a 10-year head start.
The Rule of 72 gives an intuitive feel: divide 72 by your return rate to find the approximate doubling time. At 6%: doubles every 12 years. At 8%: doubles every 9 years. At 12%: doubles every 6 years. Each doubling doubles all previous gains — so the 4th doubling adds as much absolute wealth as the first three combined.
Investment return rates — realistic expectations by asset class
Compound interest calculators are only useful if the rate you enter is realistic. Here are evidence-based long-run return estimates by asset class.
US stock market (S&P 500): approximately 10% nominal annual return over the past 100 years, or roughly 7% after inflation. This includes both bull markets and major crashes (Great Depression, 2008, 2020). Individual years vary wildly — the long-run average smooths decades of volatility.
Diversified global equities: approximately 8–9% nominal, 5–6% real, reflecting both US and international market returns.
Savings accounts and CDs: historically 0.5–5% depending on the interest rate environment. In 2024–2025, high-yield savings accounts offer 4–5.5% — the highest in 15 years due to elevated federal funds rates, which will eventually decline.
Bonds (US Treasury, investment grade): approximately 3–5% nominal, often negative real in high-inflation periods. Lower return than equities with lower volatility.
Real estate (rental income + appreciation): approximately 8–12% total return with significant local variation and leverage effects.
For long-term retirement planning, 7% nominal is a conservative and defensible assumption for a diversified equity portfolio.
Compound interest for debt — how it works against you
The same mathematics that grows savings dramatically also grows debt dramatically. Credit card debt in the US carries an average APR of approximately 20–27% as of 2025, compounded daily. At 24% APR, a $5,000 balance left untouched doubles in about 3 years (Rule of 72: 72 / 24 = 3).
The minimum payment trap: credit card minimum payments are typically 1–2% of the outstanding balance or $25–35, whichever is greater. At the minimum payment on a $5,000 balance at 24% APR, it takes approximately 25 years to repay and costs over $8,000 in interest — more than the original balance. Paying the minimum is designed to maximize the total interest paid over the lifetime of the debt.
The compounding frequency matters: daily compounding (24% nominal / 365) produces an effective annual rate (EAR) of approximately 27.1%, compared to 26.8% for monthly compounding. The difference seems small but adds up over years.
Debt avalanche strategy: to minimize total interest paid, direct extra payments toward the highest-APR balance first while paying minimums on all others. Once the highest-rate debt is eliminated, roll its payment amount to the next-highest-rate balance. This is mathematically optimal but psychologically harder than the "snowball" method (targeting smallest balance first).
Frequently Asked Questions
What is compound interest?
- Compound interest is interest calculated on both the initial principal and the accumulated interest from previous periods. Unlike simple interest, which only grows on the original amount, compound interest causes your balance to grow exponentially over time — often called the snowball effect.
How is compound interest calculated?
- Compound interest uses the formula A = P × (1 + r/n)^(n×t), where P is the principal, r is the annual interest rate as a decimal, n is the number of compounding periods per year, and t is time in years. The result A is the total amount including principal and all earned interest.
What is the compound interest formula?
- The standard formula is A = P(1 + r/n)^(nt). When you add regular contributions (PMT), it extends to A = P(1 + r/n)^(nt) + PMT × [((1 + r/n)^(nt) − 1) / (r/n)]. This calculator handles both cases automatically based on whether you enter a contribution amount.
What is the difference between compound interest and simple interest?
- Simple interest only earns interest on the original principal (SI = P × r × t), while compound interest earns interest on both the principal and previously accumulated interest. Over long periods the difference is dramatic — compound interest grows exponentially while simple interest grows linearly.
How often should interest be compounded?
- The more frequently interest compounds, the more you earn. Daily compounding produces slightly more than monthly, which produces more than annual. For savings accounts, daily or monthly compounding is most common. The difference between daily and monthly is small; the rate and time period have a much larger impact.
Does compounding frequency really make a difference?
- Yes, but the difference diminishes at higher frequencies. Going from annual to monthly compounding on a $10,000 deposit at 8% over 20 years adds about $800 in extra earnings. Going from monthly to daily adds only about $50 more. The annual rate and investment duration matter far more than frequency.
What is the Rule of 72?
- The Rule of 72 is a mental math shortcut: divide 72 by your annual interest rate to estimate how many years it takes to double your money. At 8%, money doubles in about 9 years (72 ÷ 8). At 6%, it takes about 12 years. This calculator shows the exact doubling time alongside the Rule of 72 estimate.
How do monthly contributions affect compound interest?
- Regular contributions dramatically accelerate growth because each new deposit also earns compound interest from the moment it is added. Adding $500 per month to a 7% account for 30 years can result in over $600,000 — far more than the $180,000 contributed directly. The contribution amount and frequency matter as much as the interest rate.
What is daily compound interest?
- Daily compound interest means interest is calculated and added to your balance every day (n = 365 per year). This is the most aggressive form of compounding and is commonly used by high-yield savings accounts. The dedicated Daily section of this calculator shows exactly how much more you earn compared to monthly or annual compounding.
Is compound interest good for borrowers or savers?
- Compound interest works in favor of savers and investors — the longer money stays invested, the more it grows. For borrowers, compound interest works against you. Credit card balances, for instance, compound daily, causing debt to grow quickly if not paid off each month. Understanding compound interest helps you make better decisions on both sides.
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