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When you make a regular loan payment, only part of it reduces what you actually borrowed. The rest covers interest — the lender's fee for that month. Early in a loan that split is lopsided: on a typical 30-year mortgage, the first payment can be mostly interest.

An extra payment changes the math. Because it isn't owed as interest, every dollar of it lands straight on the principal balance. A smaller balance means next month's interest charge is smaller too, so a little more of your normal payment starts working for you. The effect builds on itself month after month.

A quick example

Take a $280,000 loan at 6.5% over 30 years. Adding $100 a month — less than $25 a week — can shorten the loan by roughly three to four years and save tens of thousands of dollars in interest. Push that to $250 a month and the loan can finish around seven years early.

When extra payments matter most

The strategy is strongest when two things are true: the interest rate is meaningful, and there is a lot of time left on the loan. Paying extra in year one saves far more than paying the same amount in year 25. It matters less on very low-rate debt, where your money might do more in savings or investments instead.

APR stands for Annual Percentage Rate. APY stands for Annual Percentage Yield. The names are nearly twins, which is exactly why they cause confusion.

APR is the yearly cost of borrowing, and on loans it usually folds in certain fees alongside the interest rate. It is the number to compare when shopping for a mortgage, car loan, or credit card. A quoted APR generally does not account for compounding within the year.

APY is the yearly rate of earning, and it does account for compounding. When a savings account or CD pays interest monthly, each month's interest begins earning interest of its own. APY captures that snowball, so it is always equal to or slightly higher than the simple rate.

Why it matters

When you borrow, a lower APR is better. When you save, a higher APY is better. Comparing a loan's APR against a savings account's APY is comparing two different measurements — always line up like with like.

Mortgage affordability usually starts with a lender's formula, but the more useful question is what fits your life without straining it.

The 28/36 guideline

A long-standing rule suggests keeping your housing payment under about 28% of gross monthly income, and total debt payments — housing plus car loans, student loans and credit cards — under about 36%. Lenders may allow more, but these thresholds leave breathing room.

The costs the headline payment hides

Principal and interest are only part of homeownership. Budget also for property taxes, homeowners insurance, possible mortgage insurance, HOA dues, utilities and maintenance — a common estimate sets aside 1% to 2% of the home's value per year for upkeep alone.

Work backward from a comfortable payment

Instead of asking "what is the most I can borrow," decide what monthly payment you would feel calm about, then find the price that produces it. That is the logic behind the Affordability calculator: set the payment, see the price.

Compound interest is simply interest earning interest. You earn a return on your original money, then a return on those returns — and the cycle repeats.

Time does the heavy lifting

The longer money compounds, the more dramatic the curve becomes. Growth looks almost flat for years, then bends sharply upward. That bend is why compounding is often called the most powerful force in personal finance.

The early-start advantage

Picture two savers. One invests for ten years starting in their twenties, then stops adding money entirely. The other waits, then invests steadily for thirty years. Because the early starter's money had decades to compound, they can finish with a comparable — sometimes larger — balance, despite contributing far less in total.

What you can control

Three levers drive compound growth: how much you contribute, the rate of return, and time. You have the most control over the first and the third. Starting now, even with a modest amount, hands the math more time to work.