複利計算機

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Compound Interest

What is "Compound Interest"?

Compound interest is when interest is calculated on both the initial principal and the accumulated interest from previous periods. This 'interest on interest' causes exponential growth over time. Albert Einstein reportedly called it the 'eighth wonder of the world' due to its powerful wealth-building effect.

Example: $10,000 at 7% compound interest for 30 years → $76,000 (7.6x the principal)

初期投資額￥1,200,000

月間積立額￥60,000

年間利率（%）

0%10%20%

投資期間（年）

1y25y50y

Last Year Rate Assumption (%)

Used for the "this year vs last year" simulation.

複利頻度Results update in real time when you move the rate/term sliders.

総投資額

￥15,600,000

1,560万円

獲得利息合計

+2,050万円

最終金額 🚀

(3,610万円)

成長チャート

You vs Average

Benchmark: U.S. long-term average return assumed at 7%

Final gap: +￥0

This Year vs Last Year

Compares one-year outcomes using current and last-year return assumptions.

End-of-year difference: +￥16,156

💰

Compound Interest Result

￥36,102,086

✨ Estimated interest: ￥20,502,086

Expected value after 20 years (7% annual return)

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💸What if I keep this for retirement?

👉 Calculate my retirement asset and monthly pension

複利とは？

複利とは、元本だけでなく、過去の期間に蓄積された利息にも利息が計算される仕組みです。単利が元本のみに計算されるのとは異なり、複利は時間とともにお金をより速く成長させます。

複利計算式

A = P(1 + r/n)^nt

A = P(1 + r/n)^(nt) ここで、Aは最終金額、Pは元本、rは年間利率、nは年間の複利回数、tは年数です。

72の法則

72の法則は、投資が2倍になるまでの期間を簡単に推定する方法です。年間収益率で72を割ると、おおよその年数が得られます。例えば、年間6%の収益率の場合、投資は約72÷6=12年で2倍になります。

At 7% return, your investment doubles in about 10.3 years.

How to Read the Comparison Features

"You vs Average": See whether your assumptions outperform or underperform a U.S. long-term baseline.
"This Year vs Last Year": Quantify how a rate change shifts your one-year ending value under the same contribution plan.
Rate and term sliders update charts in real time so you can test scenarios quickly.

複利活用のコツ

✓複利効果を最大化するために早めに投資を始める
✓定期的に積み立てる、少額でも積み重なります
✓収益を再投資して成長を加速させる
✓長期的な視点で最良の結果を得る

Last updated: 2025-01

📝 How to Use

Enter Initial Investment

Input your starting principal amount.

💡 Starting small is okay - consistency matters!

Set Annual Return Rate

Enter expected annual return rate (%).

💡 Stock funds average 7-10%, savings 2-4%.

Choose Investment Period

Enter how many years you plan to invest.

💡 Compound effect accelerates after 10 years!

Add Monthly Deposits (Optional)

If you plan to add money monthly, enter the amount.

💡 Regular contributions maximize compound growth.

🎯 Who is this for?

Useful for various situations

👶

College Fund for Kids

Start when your child is born to comfortably cover tuition costs.

Long-termEducation

🏠

Down Payment Savings

Calculate how long it takes to save for your dream home.

HomeSavings

🧓

Retirement Planning

Starting in your 30s makes comfortable retirement possible.

PensionRetirement

💎

Investment Simulation

Compare expected returns from ETFs, funds, and savings accounts.

ComparisonETF

💡

Expert Tip

The key to compound interest is 'time'. If you start investing $300/month at 30, someone starting at 40 can't catch up even with $1,000/month. The best time to invest was 10 years ago. The second best time is now.

— Warren Buffett's Investment Philosophy

❓ Frequently Asked Questions

How do I use the compound interest calculator?

Just enter your initial investment, annual return rate, and investment period. Optionally, you can add monthly contributions. The calculator shows your final amount, total interest, and yearly growth chart.

💡 Adding monthly deposits supercharges your compound growth!

What is the Rule of 72?

Divide 72 by your annual return rate to estimate how long it takes to double your money. For example, at 8% return: 72÷8=9 years to double your investment.

💡 Our calculator automatically shows you the Rule of 72 result!

What is the difference between simple and compound interest?

Simple interest applies only to principal, while compound interest applies to principal + accumulated interest. $10,000 at 5% for 20 years: simple interest = $20,000, compound = $26,500. The longer you invest, the bigger the difference.

💡 This is why compound interest is called "interest on interest"!

What is a realistic annual return rate?

It varies by investment: Savings 2-4%, Bonds 3-5%, Stock ETFs 7-10%, Individual stocks vary widely. For long-term planning, 7% is a conservative and realistic estimate.

💡 Remember: higher returns usually mean higher risk!

How early should I start investing?

The earlier, the better! Starting at 25 with $200/month at 7% gives you ~$550,000 by 65. Starting at 35 with the same amount yields only ~$250,000. A 10-year difference makes more than 2x difference!