Mortgage Types and Interest Rates

Core Idea

Mortgages differ primarily in how the interest rate behaves over time. A fixed-rate mortgage locks in one rate for the entire loan term, providing payment predictability. An adjustable-rate mortgage (ARM) starts with a lower introductory rate that resets periodically (e.g., a 5/1 ARM adjusts annually after five years), introducing uncertainty but potentially saving money if rates stay flat or decline. The choice between 15-year and 30-year terms involves a direct tradeoff: 15-year loans have higher monthly payments but substantially lower total interest and typically come with rates 0.5-0.75% lower. Discount points let you prepay interest at closing (one point = 1% of loan amount) to buy a lower rate, which only pays off if you keep the loan long enough to recoup the upfront cost. Rate locks guarantee a quoted rate for a set period during the closing process, protecting against rate increases.

How It's Best Learned

Use an online mortgage calculator to compare total interest paid on a $300,000 loan across scenarios: 30-year fixed vs. 15-year fixed, and then a 5/1 ARM assuming rates rise by 1% at each adjustment. Add a scenario where you pay one discount point and calculate the break-even month. Seeing six-figure differences in total interest makes the tradeoffs concrete.

Common Misconceptions

• ARMs are always risky; if you plan to sell or refinance within the introductory period, an ARM's lower initial rate can save thousands with minimal rate risk — the danger is staying past the fixed period in a rising rate environment.
• A 30-year mortgage is better because the lower payment gives you more flexibility; while true for cash flow, many people never invest the payment difference, turning theoretical flexibility into actual overpayment of interest.
• Paying points always saves money; if you sell or refinance before the break-even point (often 5-7 years), you lose money on points — they are only worthwhile when you expect to hold the mortgage for a long time.

Explainer

You already understand the basics of how mortgages work. Now the question is: which mortgage? The choice begins with whether your interest rate should be fixed or adjustable. A fixed-rate mortgage locks in one rate for the entire loan term — your principal and interest payment will be identical in month 1 and month 360. This predictability has real value: you can budget confidently knowing exactly what you owe. An adjustable-rate mortgage (ARM) starts with a lower rate for an introductory period, then adjusts periodically based on a market index. A "5/1 ARM" is fixed for 5 years, then adjusts once per year. The lower initial rate genuinely saves money during the introductory period, but the uncertainty afterward is the risk — if rates rise significantly, your payment could jump hundreds of dollars per month.

The compound interest mechanics you've studied explain why loan term matters so dramatically. On a $300,000 loan at 7%, the 30-year option has a monthly payment around $1,996 — but you'll pay roughly $418,000 in interest over the life of the loan, nearly 140% of the original principal. The 15-year option has a monthly payment around $2,696 — $700 more per month — but total interest paid drops to about $185,000. That $700/month difference produces $233,000 in savings. Lenders typically charge 0.5–0.75% lower rates on 15-year loans, widening the gap further. The 15-year isn't objectively better — the $700/month freed by a 30-year loan could be invested and potentially outperform — but the comparison must be made explicitly, not assumed.

Discount points are a form of prepaid interest. One point costs 1% of the loan amount ($3,000 on a $300,000 loan) and typically reduces the rate by about 0.25%. The key calculation is the break-even period: divide the upfront cost of the points by your monthly savings from the lower rate. If one point costs $3,000 and saves $50/month, you break even in 60 months (5 years). Points only make financial sense if you're confident you'll keep the loan longer than the break-even period. If you sell, refinance, or pay off the loan before then, you've paid money upfront that you never recovered. Most people refinance or move within 7–10 years, so high-point deals deserve scrutiny.

A rate lock is simply a lender's promise to honor a quoted interest rate for a set period — typically 30 to 60 days — while your loan processes toward closing. Mortgage rates change daily with bond markets, and a rate that looks good today could be meaningfully higher by closing without a lock. Rate locks typically cost nothing for standard windows but may carry a fee for extensions. The practical implication: once you've found a rate you're comfortable with and you're confident the deal will close, locking in protects you. Waiting in hopes of a rate improvement is speculation — rates move in both directions.