How much interest you really pay on a mortgage

A mortgage is almost always the largest interest bill a household ever pays, yet it is usually quoted as a monthly payment — a number designed to feel manageable, not to reveal its total cost. This study strips that away and asks one question: across the full range of rates and terms a borrower might actually face, how much does the interest come to?

Everything below is normalised to $100,000 borrowed. Scale it to your own loan by multiplying.

The core table. Read a row as “at this rate, borrowing $100,000 over this term costs this much in interest over the life of the loan”. The monthly payment is shown beneath each figure so it stays traceable.

Each monthly payment is rounded to the cent and the final payment adjusted to land the balance on exactly zero, exactly as a loan servicer does — so these totals are what a borrower actually pays, not payment × months.

Stretch a loan long enough, or borrow at a high enough rate, and the interest alone exceeds the amount you borrowed. Here is the exact rate where that line is crossed, for each term:

• On a 15-year mortgage, interest equals the sum borrowed at about 10.6%. Above that rate you repay more than double what you borrowed.
• On a 20-year mortgage, interest equals the sum borrowed at about 8.0%. Above that rate you repay more than double what you borrowed.
• On a 30-year mortgage, interest equals the sum borrowed at about 5.3%. Above that rate you repay more than double what you borrowed.

This is why term length matters as much as rate. A 30-year loan crosses the break-even at a rate 5.3% lower than a 15-year loan does.

A 15-year loan has a higher monthly payment but a dramatically smaller interest bill — you are borrowing the money for half as long. At every rate in the study, the 30-year loan costs roughly 2.3× the interest of the 15-year one on the same balance.

“Extra/mo for 15yr” is how much more the 15-year payment is than the 30-year one, per $100,000 borrowed — the price of that interest saving.

Paying a little more than the scheduled amount goes straight against the principal, where it stops accruing interest for the rest of the term. Modelled as one extra monthly payment spread across the year — the standard “biweekly” effect of 13 payments instead of 12 — here is what it saves on a 30-year loan:

At 7%, an extra $55 a month per $100,000 borrowed clears the loan 6 years, 4 months early and saves $34,142 in interest — a return almost no other use of that money can match risk-free.

To anchor the per-$100,000 figures: a $400,000 home bought with 20% down is a $320,000 loan. At 6.5% over 30 years that is $2,022.62 a month in principal and interest, and $408,141 of interest over the full term — more than the sum borrowed, because 6.5% is above the 5.3% break-even. You can confirm every figure by entering this loan into the mortgage calculator.

The numbers are not estimates or round figures. Each one is produced by the standard amortisation formula — M = P · i / (1 − (1 + i)⁻ⁿ) — the same code that powers the site’s mortgage calculator, building a full month-by-month schedule and summing the interest column. To reproduce any cell, open the calculator, switch off tax, insurance and other costs, and enter the matching loan amount, rate and term.

• Rates covered: 3% to 9% in half-point steps.
• Terms covered: 15, 20, 30 years.
• Interest only — property tax, insurance, PMI and HOA fees are deliberately excluded, since those vary by location and are not interest.
• Fixed rate for the whole term; adjustable-rate loans will diverge once the rate moves.

The figures in this study are free to reuse with attribution (CC BY 4.0) — a link back to this page is all we ask. If you spot an error, the report an error link reaches us directly.