Core answer: personal finance needs five calculators — compound interest (growth), DCA (regular investing), deposit (safe returns), NPV/IRR (project appraisal), FIRE (financial independence). Verify any compound-interest tool with ¥100k at 5% for 10 years: it must return ¥162,889; anything else has a formula problem.
The five essentials
| Tool | Answers | Classic use |
|---|---|---|
| Compound interest | what does money become? | ¥100k at 5%/10Y → ¥162,889 |
| DCA | what does monthly saving build? | ¥1k/month at 5%/20Y → ≈¥411k |
| Deposit | what does the bank pay? | ¥100k 3-year at 1.25% → ¥3,750 interest |
| NPV/IRR | is this project worth it? | IRR above your hurdle = go |
| FIRE | when can I stop working? | annual spend ¥120k → target ≈¥3M |
Sanity-check numbers
| Case | Correct result |
|---|---|
| ¥100k, 5%, 10Y annual compounding | ¥162,889 |
| Rule of 72 at 6% | doubles in ≈12 years (exact 11.9) |
| ¥1k/month, 5%, 20Y DCA | ≈¥411k total |
| FIRE at 4% rule, ¥10k/month spend | target ¥3.0M |
Rate traps to avoid
| Trap | Reality |
|---|---|
| Installment “0.6%/month fee” | true APR ≈13.8% (12 periods) — fees apply to the full principal even as it repays |
| “7-day annualized” yield | a backward-looking snapshot, not a promise |
| Nominal 8% with 3% inflation | real growth ≈5% — purchasing power is what compounds |
| Front-load fees | a 1.5% subscription fee means day-one −1.5% |
Example: a ten-year DCA plan
Goal: ¥500k in 10 years. The DCA tool inverts it: at 5%, you need about ¥3,220/month. Sensitivity check: at 3% you need ¥3,580; at 7%, ¥2,890. The plan survives a ±2% return miss only if you budget ¥3,600 — so set the contribution there and let outperformance finish early.
Example: testing a wealth product
A bank product advertises “4.2% annualized, 3 years”. Compare against the deposit ladder: 3-year deposits at ~1.25% are deposit-insurance covered; the product is not principal-guaranteed. The IRR view: if the 4.2% materializes, ¥100k returns ¥113.2k vs ¥103.8k — the ¥9.4k gap is the price of the risk you carry. Decide with numbers, not adjectives.
Common mistakes
- “Annualized = guaranteed”: annualized yields are historical annualizations; money-market 7-day figures can halve within months.
- Ignoring inflation: 3% inflation halves purchasing power in ~24 years — a “safe” 2% return is a real loss.
- Forgetting fees: a 1.5% subscription fee plus 0.5%/year management drag turns a nominal 6% into a real ~4% over a decade.
- “FIRE means never working”: the 4% rule assumes ~30-year horizons and US-market history; early retirees need bigger margins or flexible spending.