Compound Interest Calculator
See how your money grows over time. The longer your timeline, the more compounding works in your favour.
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Future Value
$300,851
after 20 years
Total Contributed
$130,000
Interest Earned
$170,851
Breakdown
Growth Over Time
What is compound interest?
Compound interest is interest earned on both your original deposit and the interest you have already collected. Simple interest only grows on the starting amount. Compound interest grows on the whole balance, so each period adds a little more than the last.
The result is a curve, not a straight line. Growth feels slow in the early years and then accelerates sharply. That is not magic — it is just the math catching up with itself.
Worked example
$10,000 starting amount · $500/month · 7% annual rate · 20 years
You put in
$130,000
Interest earned
$171,000
Final balance
$301,000
More than half came from interest, not from your deposits. You contributed $130,000 over 20 years — the other $171,000 showed up because the interest had time to compound.
How to use this compound interest calculator
- 1.
Starting amount: What you have today. Zero is fine — monthly contributions alone grow significantly given enough time.
- 2.
Monthly contribution: How much you add each month. Consistency matters more than the size of your initial deposit.
- 3.
Annual interest rate: The expected return per year. Use your savings rate for safe goals; see the rate guide below for investing benchmarks.
- 4.
Time period: Years to let the money grow. Even a few extra years at the end make a surprisingly large difference.
- 5.
Compound frequency: How often interest is added to your balance. Monthly is standard for most savings accounts and funds.
Compound vs simple interest, side by side
Same numbers, different math. $10,000 at 7% for 20 years, no monthly contributions:
Simple interest
$24,000
$10K + $14K interest (7% × 20 years × $10K)
Compound interest
$38,700
$10K growing at 7% compounded annually
At 30 years: simple $31K vs compound $76K. At 40 years: simple $38K vs compound $149K. Time is doing the work.
What rate should you enter?
Use a realistic number for your situation. Here are common benchmarks:
Dutch high-yield savings account
2025 rates, easy to access, low risk
2–3%
Government bonds (NL / EU)
Low risk, fixed income, predictable
3–4%
Balanced fund (60/40 stocks & bonds)
Moderate risk, suitable for 5+ year horizons
5–6%
Global equity index fund
Higher volatility, historical average before inflation
7–8%
Past returns are not a guarantee of future results. Higher expected returns always come with more risk and more volatility along the way.
The Rule of 72
A useful shortcut: divide 72 by your annual return to estimate how many years it takes to double your money.
3%
24 yrs to double
6%
12 yrs to double
8%
9 yrs to double
12%
6 yrs to double
This is why starting at 25 instead of 35 can mean retiring with twice as much money, even with identical contributions.
Frequently asked questions
How does compound interest work?
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Compound interest calculates interest on both the principal and the accumulated interest from prior periods. Each period your balance grows slightly larger, so the next interest payment is slightly larger too. Over time this creates exponential rather than linear growth.
What is the compound interest formula?
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A = P(1 + r/n)^(nt), 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 the number of years. This calculator also adds monthly contributions to the formula for a realistic savings projection.
How often does compound interest compound?
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Monthly compounding is most common for savings accounts and investment funds. Daily compounding is slightly more generous than monthly, but the practical difference is small. This calculator lets you compare monthly, quarterly, and annual compounding.
What is the difference between compound and simple interest?
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Simple interest is calculated only on the original principal — it grows in a straight line. Compound interest is calculated on the principal plus all accumulated interest — it grows exponentially. On $10,000 at 7% over 20 years: simple interest gives $24,000; compound gives $38,700.
What is the Rule of 72?
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Divide 72 by your annual return percentage to get the approximate number of years it takes to double your money. At 8% annual return: 72 ÷ 8 = 9 years to double. It works because ln(2) ≈ 0.693, and 72/r ≈ ln(2)/(ln(1+r)) for small r.
Keep exploring
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→ Read articleSources & methodology
Standard financial mathematics (annuity and compound interest formulas); no external data feeds · Last verified: July 2026
MoneyCho calculators are educational tools. Results are indicative and do not constitute financial advice.