Core answer: a trustworthy mortgage calculator passes six tests — current LPR rate basis, equal-payment and equal-principal modes, prepayment simulation, housing-fund and combo loans, payment breakdown, and visible formulas. Sanity test: ¥1M, 30 years, 3.0%, equal payment must return ¥4,216/month and ¥518k total interest.
Six feature checklist
| Feature | Pass | Fail signal |
|---|---|---|
| Rate basis | LPR tiers + custom | hard-coded old benchmark |
| Repayment modes | one-click switch/compare | only one mode |
| Prepayment | both plans | missing entirely |
| Loan types | commercial / fund / combo | commercial only |
| Breakdown | payment split, total interest, schedule | one bare number |
| Formula | shown with steps | black box |
The ¥1M sanity test
¥1M, 30 years (360 payments), 3.0% annual (0.25% monthly):
| Mode | Monthly | Total interest | Trait |
|---|---|---|---|
| Equal payment | ¥4,216 fixed | ¥518k | interest-heavy early years |
| Equal principal | ¥5,278 first, −¥6.9/month | ¥451k | ¥67k less interest, heavier start |
Any calculator off by more than ¥1 on this case has a formula or rounding problem — walk away.
Choosing a repayment mode
| Situation | Choice |
|---|---|
| Payment already > 40% of income | Equal payment — protect cash flow |
| High, stable income | Equal principal — less interest |
| Prepaying within 5-10 years | Equal principal compounds the saving |
| Housing-fund loan at 2.85% | Little difference — follow cash flow |
Example: three rounds for a first home
¥1M, 30 years, 3.0%. Round one: equal payment ¥4,216 sits within 35% of the ¥12k household income. Round two: equal principal starts at ¥5,278 — affordable, but a baby is coming, so cash flow wins. Round three: prepay ¥150k in year three; "shorten the term" saves about ¥40k more interest than "lower the payment". Three rounds settle mode, plan and prepayment strategy.
Example: a combo loan for upgraders
Need ¥1.8M: ¥800k housing-fund at 2.85% + ¥1M commercial at 3.0%. The combo branch computes each part: ¥3,308 + ¥4,216 ≈ ¥7,524/month. A pure commercial ¥1.8M loan costs ¥7,589 — the combo saves about ¥65/month and roughly ¥23k over 30 years, and idle fund quota is wasted money.
Common mistakes
- "Lower payment means better deal": a lower payment usually means a longer term and more total interest — ¥1M at 3.0% costs about ¥180k more interest over 30 years than over 20.
- "My payment drops the day LPR drops": existing loans reprice only on the reset date (commonly Jan 1 or the loan anniversary).
- "Prepay as early as possible": with equal principal, late-term prepayment saves little; check prepayment penalty clauses first.
- "Fund loans are cheap, so borrow the max": quota depends on account balance and city caps — verify policy before planning.