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Growth (year by year)
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How to use the compound interest calculator, and how compounding works
"What does saving 30,000 yen a month at 5% for 20 years come to?" cannot be answered with simple multiplication, because interest earns interest. Enter an initial amount, a monthly contribution, an assumed return and a term, and this tool separates the future total into contributions and investment gains.
How to use it
Put a lump sum in the initial amount field, regular saving in the monthly contribution field — or use both together. The assumed return is an annual percentage, compounded once a year.
How far simple and compound interest diverge
Simple interest pays only on the principal; compound interest pays on the interest as well. One million yen at 5% for 30 years reaches 2.5 million with simple interest, but about 4.32 million compounded. The gap widens sharply with time, which is why compounding is described as putting time on your side.
The rule of 72
72 ÷ the annual rate gives a good estimate of the years needed to double your money: 24 years at 3%, 12 years at 6%, 9 years at 8%. Read backwards, doubling within 10 years needs about 7.2% a year — a quick sanity check before doing the real arithmetic.
Remember tax and inflation
The figures here are before tax. In Japan investment gains are normally taxed at about 20.315% (15% income tax, 0.315% special reconstruction tax and 5% resident tax). Tax-free schemes such as NISA exempt gains but cap annual contributions. And with 2% inflation, a nominal 5% return is about 3% in real terms — what matters is how much you can buy, not the headline number.
An assumed return is not a promised one
The projection assumes your chosen rate repeats exactly every year. Real markets move both ways and can lose money. Treat the output as a planning guide. For the borrowing side, see the loan calculator.
Frequently asked questions
What is compound interest vs simple interest?+
This tool compounds monthly.
How is the future value calculated?+
At 0% it equals the total contributions.
Does the contribution timing change the result?+
Use it as a guide.
Are taxes and fees included?+
It assumes a constant rate, and real returns vary.
Are my inputs stored?+
・Future value = initial × (1+i)ⁿ + monthly × ((1+i)ⁿ − 1) ÷ i (i = monthly rate = annual ÷ 12, n = months = years × 12, end-of-period contributions)
・At 0%: initial + monthly × n. Total contributions = initial + monthly × n; gain = future value − contributions.
・An estimate assuming a constant rate and no taxes or fees, based on the standard future-value and annuity formulas.
・Rule of 72: years to double ≒ 72 ÷ annual rate(%) — a rule of thumb for quick estimates
・Income Tax Act, Local Tax Act and the Special Measures Act for securing reconstruction funding (Japan): gains and dividends on listed shares are taxed at 20.315% in total (15% income, 0.315% reconstruction, 5% resident). Figures here are pre-tax.
・Act on Special Measures Concerning Taxation (NISA): gains within a tax-exempt account are untaxed, subject to an annual contribution cap.
・Real return ≒ nominal return − inflation rate
This site does not represent any financial institution and does not solicit any product or provide investment advice. The result does not guarantee future returns. Real investments fluctuate and may lose principal, and taxes and fees apply. Make investment decisions at your own responsibility.