Financial Calculators → Compound Interest
Compound Interest Calculator: The Complete Guide
Learn how compound interest works, how to use the calculator, how to choose the right inputs, and how to apply the results to your TFSA, RRSP, 401(k), Roth IRA, or investment account.
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What Is Compound Interest?
Compound interest is interest calculated on both the original principal and all previously accumulated interest. Unlike simple interest — which is calculated only on the original deposit — compound interest earns interest on its own interest. The result is exponential, not linear, growth.
A $10,000 deposit at 7% simple interest earns exactly $700 every year. At 7% compound interest (annually), it earns $700 in year 1, $749 in year 2, $801 in year 3 — growing every year because each interest payment becomes part of the principal for the next period.
Over 30 years, $10,000 at 7% compound interest becomes $76,123. At 7% simple interest, it becomes only $31,000. That $45,123 gap is the entire value of compounding. Time and consistency are the only inputs required to capture it.
→ Full guide: What Is Compound Interest?
The Compound Interest Formula
The standard compound interest formula for a lump-sum deposit:
A = P × (1 + r/n)^(n × t)
- A — Future value (the final amount)
- P — Principal (starting amount)
- r — Annual interest rate as a decimal (e.g., 7% = 0.07)
- n — Compounding periods per year (12 = monthly, 365 = daily)
- t — Time in years
For regular monthly contributions, the annuity formula is added:
FV = PMT × [(1 + r/n)^(nt) − 1] / (r/n)
The combined formula — starting balance plus regular contributions — is what the BankDeMark compound interest calculator runs with every input change.
→ Full guide: Compound Interest Formula
How to Use the Compound Interest Calculator
The calculator takes five core inputs and returns the projected future value, total contributions, and total interest earned.
Starting Balance (Principal)
The amount you have invested today. Enter $0 if starting from scratch. Each $10,000 in starting balance at 7% produces approximately $76,000 after 30 years — with no additional contributions. Existing savings are a powerful foundation.
Monthly Contribution
How much you add per month. Use your current planned contribution. Every $100/month increase at 7% over 30 years adds approximately $121,000 to the final portfolio — regardless of where you start on the contribution scale.
Annual Return Rate
The expected average annual return on your portfolio. Common assumptions:
- Conservative (all-bond or balanced): 4%–5.5%
- Moderate (60/40 equity/bond): 6%–7%
- Growth (diversified equity index): 7%–8%
Always run projections at multiple rates. The difference between 6% and 8% over 30 years on a $25,000 portfolio with $500/month contributions is approximately $378,000.
Compounding Frequency
Monthly is the standard for investment projections. Daily produces marginally more (less than 0.1% difference over 30 years vs. monthly). Annual is meaningfully less. Choose monthly for most planning purposes.
Time Horizon
Years until your target date. Time is the single most powerful variable in compound growth. Starting 10 years earlier at $400/month produces $683,000 more at 65 — for only $48,000 in additional contributions.
→ Open the Calculator
→ Guide: Compound Growth Calculator (existing portfolio + contributions)
Compounding Frequency: Daily vs. Monthly vs. Annual
Compounding frequency determines how often interest is calculated and added to the principal. More frequent compounding means a slightly higher effective annual rate.
On a $25,000 investment at 7% for 30 years, the difference between daily and annual compounding is approximately $2,364 — less than 1.2% of the final portfolio value. Compounding frequency is a second-order variable. Return rate, contribution amount, and time horizon are first-order.
Where compounding frequency genuinely matters: high-interest debt. At 19.99% (standard credit card rate), daily compounding produces an effective annual rate of approximately 22.1% — nearly 2% above the nominal rate.
Regular Contributions and Compound Growth
Most investors are not working with a single lump sum — they are adding money monthly from income. The compound interest calculator models this with the annuity formula, layered on top of any starting balance.
Key projection benchmarks at 7% annual return, monthly compounding:
- $100/month, 40 years: $262,481 (total contributed: $48,000)
- $500/month, 30 years: $609,970 (total contributed: $180,000)
- $1,000/month, 25 years: $813,000 (total contributed: $300,000)
In every scenario, compound interest generates more wealth than the investor contributed — often 2–4× more over long horizons.
Compound Interest in Canadian Registered Accounts
For Canadian investors, the account structure matters as much as the investment. The right account eliminates tax drag and dramatically increases effective compound growth over time.
TFSA — Tax-Free Compound Growth
All growth inside a TFSA — interest, dividends, and capital gains — is completely tax-free. Withdrawals at any time, for any reason, trigger no income tax. 2026 contribution room: $7,000/year new room, $95,000 cumulative.
RRSP — Tax-Deferred Compound Growth
RRSP contributions are tax-deductible; growth is tax-deferred. The deduction multiplier (reinvesting the annual refund into the RRSP) can add hundreds of thousands to your 30-year balance. 2026 limit: 18% of earned income, max $31,560.
FHSA — First Home Savings Account
Deductible contributions + tax-free qualifying withdrawals for a first home purchase. Annual limit: $8,000. Lifetime limit: $40,000.
→ Full guide: Compound Interest in Canada (TFSA, RRSP, FHSA)
Compound Interest for U.S. Investors
American investors have three primary tax-advantaged compound growth vehicles:
- Roth IRA: After-tax contributions, completely tax-free growth and withdrawals. 2026 limit: $7,000 (under 50), $8,000 (50+). The closest U.S. equivalent to the Canadian TFSA.
- 401(k): Pre-tax contributions, tax-deferred growth. 2026 limit: $23,500 (under 50), $31,000 (50+). Always contribute at least enough to capture any employer match — it is a guaranteed immediate return.
- HSA: Triple tax advantage (deductible contributions, tax-free growth, tax-free medical withdrawals). 2026 limits: $4,300 individual, $8,550 family. One of the most overlooked compound growth vehicles in the U.S. tax code.
The Rule of 72
The Rule of 72 estimates doubling time: divide 72 by your annual return rate.
- At 6%: doubles every 12 years
- At 7%: doubles every 10.3 years
- At 8%: doubles every 9 years
- At 19.99% (credit card): doubles every 3.6 years
A $50,000 TFSA at 7% becomes $100,000 in 10 years, $200,000 in 20 years, and $400,000 in 30 years — with no additional contributions. Three doublings in 30 years is the power of the Rule of 72 in practice.
How to Maximize Compound Interest
Six principles that maximize your compound interest outcome:
- Start as early as possible. Ten years of additional compounding time is worth more than doubling your monthly contribution later.
- Use the right account. Tax-free compounding (TFSA/Roth IRA) eliminates the drag that would otherwise reduce your effective rate by 1%–2% annually.
- Keep fees low. A 1% annual fee on a 7% return reduces your effective rate to 6%. Over 30 years, this fee costs approximately 15%–20% of your final portfolio value.
- Automate contributions. Irregular investing breaks the compound chain. Automation ensures every month contributes to the annuity calculation.
- Reinvest all distributions. Dividends and interest taken as cash rather than reinvested remove capital from the compounding base permanently.
- Stay invested through volatility. Selling during a downturn converts a temporary loss into a permanent one and resets the compounding clock on that capital.
→ Best Compound Interest Investments (Canada and USA)
Real-World Compound Interest Examples
The best way to understand compound interest is through specific, calculated scenarios:
- Starting at 22 vs. 32 with $400/month: $683,000 difference at 65 for $48,000 more invested
- $25,000 TFSA at 7% for 20 years: $300,887 — with $170,887 in tax-free growth
- $5,000 credit card balance at 19.99% (unpaid): over $13,000 in 5 years
- $500/month in a Roth IRA for 40 years: $1,312,405 — completely tax-free
- RRSP contribution with refund reinvested: 43% more final value vs. ignoring the refund
→ Full guide: 15 Compound Interest Examples
Compound Interest Resource Library
Every article in the compound interest cluster is designed to answer one specific question at depth. Use the calculator tool alongside these guides for the complete picture.
Build Your Compound Interest Dashboard
Running one projection at a time in a calculator is useful for a single question. But tracking all your accounts, seeing every projection update live, and measuring progress toward your targets requires a dashboard — not a spreadsheet.
BankDeMark Command connects your accounts and shows your compound growth picture across every registered and investment account in one place.
Compound Interest Calculator FAQ
What is a compound interest calculator?
A compound interest calculator projects how money grows when interest is earned on both the original principal and all previously accumulated interest. You enter a starting balance, interest rate, compounding frequency, time horizon, and optional regular contributions — the calculator returns the future value and total interest earned.
What is the compound interest formula?
The standard formula is A = P(1 + r/n)^(nt), where A is the future value, 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. For regular contributions, the annuity formula FV = PMT × [(1 + r/n)^(nt) − 1] / (r/n) is added.
How often should I compound interest?
For long-term investment projections, monthly compounding is the standard assumption. Daily compounding produces marginally more than monthly. Annual compounding is meaningfully less. The difference between monthly and daily compounding over 30 years is typically under 0.1% of the final portfolio value — far less impactful than return rate or contribution amount.
Does compound interest apply to a TFSA?
Yes. All investment growth inside a TFSA compounds completely tax-free. There is no annual tax drag on interest, dividends, or capital gains earned within the account. This makes the TFSA one of the most powerful compound interest vehicles available to Canadians.
What return rate should I use in the calculator?
For long-term diversified equity portfolios, 6%–7% is a commonly used assumption after fees. Conservative plans use 5%. Plans built on assumptions above 9% carry meaningful risk of underperformance. Always run projections at multiple return rates — conservative, moderate, and optimistic.
What is the Rule of 72?
The Rule of 72 estimates how long it takes to double your money: divide 72 by your annual return rate. At 7%, money doubles in approximately 10.3 years. At 10%, it doubles in 7.2 years. At 19.99% (credit card rate), a debt doubles in approximately 3.6 years.
How does compound interest affect debt?
Compound interest runs in reverse on debt. A $5,000 credit card balance at 19.99% compounding daily grows to over $13,000 in 5 years without additional charges. The same mathematical force that builds investment wealth accelerates debt when balances are carried.
How is compound interest different in Canada vs. the U.S.?
The mathematics are identical. The difference is in the account structures available. Canadians have the TFSA (tax-free), RRSP (tax-deferred), and FHSA (deductible + tax-free for home). Americans have the Roth IRA (tax-free), 401(k) (tax-deferred), and HSA (triple tax advantage). The account choice determines the after-tax compound growth rate.
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