Mortgage Calculator — Payment, Interest, Amortization, Early Payoff

Real-world use cases

• Decide whether you can actually afford the apartment you saw

  The agent said "the monthly payment is around 7,000". Plug the price, your 30% down, 30 years, and the bank's quoted 4.0% rate. The calculator returns the real monthly payment, the income ratio band (green / yellow / red), and the total interest over 30 years. If the ratio is yellow, you know you can stretch but you'll be house-poor; if it's red, you walk.

• Compare equal-installment vs equal-principal head-to-head

  Toggle between 本息 and 本金 with the same loan size and rate. Equal-installment keeps the monthly fixed, equal-principal starts higher and tapers. The total-interest line shows you how much extra you pay for the smoother cash flow. For a 1M / 30y / 4% loan, equal-installment costs about 116,000 more in total interest than equal-principal — the calculator surfaces that gap in one click.

• Test "what if I pay off 100k extra at month 12"

  Run the base loan, then enter an extra 100,000 prepayment at month 12. The schedule reflows: term shrinks by roughly 3.5 years, total interest drops by 80,000-100,000. Try the same prepayment at month 60 and the savings nearly halve. Now you know whether to use that year-end bonus on the mortgage or on something else.

• Stress-test the loan against a rate hike

  Run the loan at the rate the bank quoted. Then bump the rate 0.5% to simulate a future rate cycle. Watch the monthly payment and total interest move. If a +0.5% move pushes your income ratio from green to red, you've got too little buffer — either a bigger down payment or a smaller property is the right move.

• Plan the down payment with a fixed monthly budget

  Lock the monthly payment you can comfortably afford (e.g. 25% of household income). Adjust the down payment up and down until the calculator returns roughly that monthly. That tells you the maximum property price you should be shopping for, before you fall in love with one above your real budget.

Common pitfalls

• Using the LPR base rate alone. Banks add a city-specific premium and a first-vs-second-home delta; ask your loan officer for the actual rate you'll get and plug that in.

• Forgetting that prepayment penalties exist in the first 1-3 years at many banks. The calculator shows the savings but the bank may charge a few months of interest as a fee — check the contract.

• Treating the income-ratio band as the only check. 30% is a safe number for a stable two-income household; a single earner or commission-heavy income should run the math at a tighter 20-25%.

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