Is the principal amount uploaded?

No. Principal, rate, term, and contribution are all processed by JavaScript in your browser; nothing leaves your device. Safe for personal financial planning.

What's the formula for monthly contributions?

Principal: P × (1 + r)^n. Contribution part (ordinary annuity): PMT × ((1+i)^N − 1) / i, where i = r/12 and N = 12n. The future value is the sum of those two. Annual contributions are the same formula with i = r and N = n.

Are contributions modeled as start-of-period or end-of-period?

End of period (ordinary annuity). The result is slightly smaller than an annuity-due model, but matches the timing of most automatic monthly investments like Tsumitate NISA and iDeCo.

What rate should I use?

Common long-term references: ~7% for the S&P 500, 5–6% for global stocks, and 3–5% for Japanese stocks. Past performance is not a guarantee — try a conservative scenario at 3–4% and an optimistic one at 7–8%.

Are taxes and fees included?

No. We don't subtract capital gains tax (20.315% in Japan), trust fees, or trading costs. For tax-advantaged accounts (Tsumitate NISA, iDeCo) the future value is the take-home figure. For taxable accounts, approximate the take-home as principal + interest × (1 − 0.20315).

Can I adjust for inflation?

Not directly. Enter a 'real rate' (nominal rate − expected inflation, e.g. 5% − 2% = 3%) to see results in today's purchasing power.

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Compound interest calculator — lump-sum & monthly contribution

Project the future value of an investment compounded over time. Supports three modes: principal only, monthly contributions, or annual contributions. Great for simulating Tsumitate NISA / iDeCo / certificates of deposit. Year-by-year balances can be exported as CSV. Principal, rate, and term stay in your browser — nothing is uploaded.

financecalculator

How to use

Pick a contribution style (principal only, monthly, or annual), then enter the principal, annual rate, and term (and contribution amount for the contribution modes). Press Calculate to see the future value, total contributed, and interest earned. Toggle the yearly breakdown to inspect each year's balance, or download it as CSV. The Sample button fills in a typical Tsumitate NISA scenario (¥1M lump sum + ¥30K/month @ 5% / 20 years). All values stay on your device — nothing is uploaded.

FAQ

Is the principal amount uploaded?: No. Principal, rate, term, and contribution are all processed by JavaScript in your browser; nothing leaves your device. Safe for personal financial planning.
What's the formula for monthly contributions?: Principal: P × (1 + r)^n. Contribution part (ordinary annuity): PMT × ((1+i)^N − 1) / i, where i = r/12 and N = 12n. The future value is the sum of those two. Annual contributions are the same formula with i = r and N = n.
Are contributions modeled as start-of-period or end-of-period?: End of period (ordinary annuity). The result is slightly smaller than an annuity-due model, but matches the timing of most automatic monthly investments like Tsumitate NISA and iDeCo.
What rate should I use?: Common long-term references: ~7% for the S&P 500, 5–6% for global stocks, and 3–5% for Japanese stocks. Past performance is not a guarantee — try a conservative scenario at 3–4% and an optimistic one at 7–8%.
Are taxes and fees included?: No. We don't subtract capital gains tax (20.315% in Japan), trust fees, or trading costs. For tax-advantaged accounts (Tsumitate NISA, iDeCo) the future value is the take-home figure. For taxable accounts, approximate the take-home as principal + interest × (1 − 0.20315).
Can I adjust for inflation?: Not directly. Enter a 'real rate' (nominal rate − expected inflation, e.g. 5% − 2% = 3%) to see results in today's purchasing power.

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