Hold the assumptions steady first
For the same balance and positive interest rate, paying the loan back over fewer months increases the required payment and decreases the interest paid over the full schedule. Our built-in comparison holds the rate steady to isolate this effect. It is an experiment, not a claim that lenders offer the same rate on both terms.
Then use your actual offers
Save the 15-year offer with its own rate and closing costs. Save the 30-year offer the same way. Compare cash required at closing, required monthly housing cost, and cost over the period you expect to keep the loan. A lifetime total by itself is less informative if you expect to sell or refinance earlier.
Compare flexibility as well as arithmetic
A longer loan with voluntary extra principal can produce a faster payoff while leaving a smaller required payment in months when money is tight. But that result depends on making the extras. A shorter required schedule removes that choice. Neither the calculator nor an interest total can decide which commitment fits your household.
Remember what is not modeled
Opportunity cost, tax deductions, future refinancing and home-price changes are not assumed. The comparison treats principal as equity rather than an expense and counts housing costs after the loan has been repaid for the remainder of your selected horizon.
Educational information, not personalized advice. See the model assumptions and limitations.