Understanding Loan Amortization

理解贷款摊销

If you have ever taken out a mortgage, a car loan, or a personal loan, you have probably noticed that the monthly payment stays exactly the same for the entire life of the loan — yet the amount of each payment that goes to interest versus principal changes every month. This schedule is called amortization, and understanding it can save you thousands of dollars over the life of a loan.

如果你办过房贷、车贷或个人贷款，你可能注意到：每月还款额在贷款期限内始终相同，但每月还款中利息和本金的比例每个月都在变。这张时间表叫"摊销表"，理解它能在整个贷款周期内帮你省下不少钱。

The monthly payment formula

月供公式

The standard amortization formula gives a fixed monthly payment that fully repays a loan over a chosen term. It looks like this:

标准的摊销公式用于计算在固定期限内还清贷款的等额月供：

M = P · r · (1+r)^n / ((1+r)^n − 1)

Where:

其中：

P — the principal (amount borrowed)
P —— 本金（借款金额）
r — the monthly interest rate (annual rate ÷ 12, expressed as a decimal)
r —— 月利率（年利率 ÷ 12，以小数表示）
n — the total number of payments (years × 12)
n —— 总期数（年数 × 12）

Plug those numbers into our loan calculator to see your monthly payment, total interest, and a full schedule.

把这些数字代入我们的贷款计算器，可以查看月供、总利息和完整还款表。

How each payment is split

每期还款如何拆分

Each month, the lender multiplies your remaining balance by the monthly interest rate. That number is the interest portion of your payment. Whatever is left over goes toward paying down the principal. In the first month of a 30-year mortgage, for example, a huge fraction of the payment is interest — sometimes 70% or more — because the balance is at its highest.

每月，贷款机构会用剩余本金乘以月利率，得到当月利息部分。剩下的金额才用来偿还本金。以 30 年期房贷为例，第一个月的还款中可能有 70% 以上是利息 —— 因为此时本金最高。

As the balance shrinks, each subsequent interest charge shrinks too. A larger share of every later payment goes to principal. By the final year, the same monthly payment is almost entirely principal. The schedule is mathematically guaranteed to reach zero balance at the end of the term, assuming a fixed rate and on-time payments.

随着本金减少，每个月的利息也在减少，更多比例的还款用于偿还本金。最后几年，同样的月供几乎全部是本金。在固定利率且按时还款的情况下，时间表保证在到期日余额归零。

Why extra payments save a lot of money

为什么提前还款能省很多钱

Because early payments are mostly interest, paying even a small amount of extra principal early in the loan has an outsized effect. Example: on a $300,000 30-year mortgage at 6%, an extra $100 per month toward principal cuts the term by about 4 years and saves over $60,000 in interest. The reason is simple: every dollar of extra principal reduces the balance on which all future interest is calculated.

由于前期还款大部分是利息，贷款初期哪怕只多还一点本金，效果也会被显著放大。举例：30 万美元、30 年期、6% 利率的房贷，每月额外还 100 美元本金，能把贷款期限缩短约 4 年，并省下超过 6 万美元的利息。原理很简单：每减少 1 美元本金，未来所有利息的计算基础都跟着缩小。

Different kinds of loans, different rules

不同贷款，不同规则

Not every loan follows a strict amortization schedule. Some common variants:

并非所有贷款都遵循严格的摊销表。一些常见的变体：

Interest-only loans — the borrower pays only the interest for a set period, then the principal becomes due (often as a balloon payment).
纯利息贷款 —— 借款人在一定期限内只还利息，到期一次性偿还本金（通常作为气球贷）。
Adjustable-rate mortgages (ARMs) — the interest rate is fixed for an initial period, then adjusts periodically. The payment recalculates based on the new rate and remaining balance.
可调利率房贷（ARM） —— 利率在初始期内固定，之后定期调整。月供按新利率和剩余本金重新计算。
Credit cards — generally not amortized. Minimum payments are usually a small percentage of the balance, and the rate is variable. Carrying a balance can result in paying far more than the original purchase price.
信用卡 —— 通常不摊销。最低还款一般是余额的一个较小比例，利率浮动。长期欠款会让实际支付远超原始消费金额。

Practical advice

实用建议

Always compare loans by total cost (APR × term), not just the monthly payment. A longer term means lower payments but more interest.
比较贷款时一定要看总成本（年利率 × 期限），不能只看月供。期限越长，月供越低，但总利息越高。
If you can afford it, make one extra payment per year toward principal. It can shave years off the loan.
如果经济允许，每年多还一次本金，能显著缩短贷款期限。
Refinancing is worth modeling when rates drop by at least 0.75–1.0 percentage points — but factor in closing costs.
当利率下调至少 0.75–1.0 个百分点时，可以考虑再融资 —— 但要把手续费算进去。
For credit cards, pay the full statement balance each month to avoid interest entirely.
信用卡尽量每月全额还清，避免任何利息。

Try it yourself

自己试试

Open the loan calculator to see how a real loan would look under your numbers. The amortization table in the result shows you exactly how each payment is split, including a sample of the first 12 months.

打开贷款计算器，用你自己的数字查看真实贷款的样子。结果中的摊销表会精确显示每期还款如何拆分，并展示前 12 期的明细。