Take a 300,000 loan over 30 years with a fixed rate. Everything below uses the standard amortization formula and covers principal and interest only, so taxes, insurance and any mortgage insurance would sit on top. The rates are examples to show how the numbers move, not predictions or quotes.
At 6.5%, the payment is 1,896.20 a month. Over 360 payments you hand over about 682,600 in total, which means 382,633 of it is interest. The interest alone is about 28% more than the amount you borrowed. Now see what happens when only the rate changes.
Look at the gaps. Each half point adds roughly 95 to 105 a month, and roughly 33,000 to 37,000 in lifetime interest. A full point, say 6% to 7%, costs about 197 more every month and 71,000 more over the loan. That is the price of a decent used car, hiding inside a number most people only glance at on the way to the closing table.
Flip the question. Suppose you have decided you can afford 2,000 a month in principal and interest. How much can you borrow?
Between 5% and 8% your borrowing power falls by about 100,000, or roughly 27%, on an unchanged monthly budget. This is the mechanism behind a familiar housing pattern: when rates climb, prices do not fall to compensate right away, so buyers either stretch or step out of the market. If you are weighing what to chase, the house affordability calculator lets you start from a monthly budget instead of a price, which keeps you honest about this effect.
If you cannot control the rate, you can control the pace. On the 6.5% loan, here is what a fixed extra amount added to every payment does:
Notice that an extra 200 a month on a 6.5% loan lands you at about the lifetime interest of a plain 5% loan (279,767), though you are paying considerably more per month to do it. There is a trade here and it is not free: extra payments move money out of your emergency fund and other investments. They make most sense when your mortgage rate is higher than what you would earn elsewhere without taking risk. Run your own figures in the mortgage payoff calculator before committing.
A 15-year loan at 6.0% on the same 300,000 costs 2,531.57 a month, which is 635 more than the 30-year loan at 6.5%. In exchange, total interest drops to about 155,700, compared with 382,600. Part of that gain is the lower rate lenders usually give on shorter terms, but most of it is simply that you carry the debt for half as long. The catch is the monthly obligation: a payment about a third higher leaves less room if your income dips.
A rate drop of one point looks like an obvious reason to refinance, and often it is, but the break-even point depends on closing costs. If refinancing from 7% to 6% on this loan saves about 197 a month and costs 4,000 to close, you recover the cost in roughly 20 months, assuming the balance and term stay comparable. If you expect to move in a year, you lose. The refinance calculator does this comparison, and it is worth running before you fall for the headline rate.
Lenders often offer a lower rate in exchange for an upfront fee called a discount point. One point costs 1% of the loan amount, so 3,000 on this loan. Suppose, as an example, that one point lowers the rate from 6.5% to 6.25%. The payment falls from 1,896.20 to about 1,847.15, a saving of roughly 49 a month. Divide the 3,000 cost by that saving and you reach a break-even point of about 61 months, a little over five years. Sell or refinance before then and the point lost you money. Stay for the full 30 years and it saves about 17,700 in payments, minus the 3,000 you paid. The honest question is not whether points are good or bad, it is how long you will keep this particular loan.
The same logic applies in reverse to "no-closing-cost" loans, where the lender covers your fees in exchange for a slightly higher rate. That can be a sensible trade if you expect to move or refinance within a few years, and an expensive one if you stay a long time.
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