On a 30-year mortgage above ~5.5%, total interest roughly equals — then exceeds — the amount borrowed.
At 6.5% over 30 years you repay about 2.3× the principal. Extra principal payments early on attack exactly this.
Only if you keep it for the full 30 years and never overpay. Most borrowers move or remortgage long before then, and the interest is front-loaded, so the total actually paid is nothing like the headline figure. On a 15-year term at the same rate interest is well under half the principal.
At 6% the rule says 200,000 and the exact answer is 231,676 — 14% low. At the point most people use it the shortcut is already outside its own 10% tolerance. It only holds between 4.9% and 5.8%, which is not where it gets used.
This one holds up. Total interest passes the amount borrowed at 5.30% on a 30-year term, so the rule’s “above about 5.5%” is accurate and, if anything, cautious — by 6% you are paying 116% of the house in interest, and by 8% it is 165%. The rule is drawn here as a flat line at the purchase price precisely so you can see where the real curve crosses it. Below 5% the claim stops being true, which is the half most people lived through and now do not.
The rule against the exact answer, computed across the range. Inside the shaded band the shortcut is close enough to use; outside it, reach for the calculator.
Estimate with the rule, then check it against the calculator that models it properly.
Open Mortgage Calculator →On a 30-year mortgage above ~5.5%, total interest roughly equals — then exceeds — the amount borrowed. At 6.5% over 30 years you repay about 2.3× the principal.