A $400,000 house at 7% interest on a 30-year mortgage costs you $958,036 by the time it's paid off. That's $558,036 in interest alone. You're paying for the house almost two and a half times over.
Most people don't run these numbers before they sign. They focus on the monthly payment, the realtor tells them they're "pre-approved," and they move forward without understanding how amortization actually works against them for the first decade.
How the Monthly Payment Formula Works
The standard mortgage payment formula looks intimidating but it's straightforward once you break it down:
M = P[r(1+r)^n] / [(1+r)^n - 1]
P is the principal (loan amount). r is the monthly interest rate (annual rate divided by 12). n is the total number of payments (years times 12).
For that $400K loan at 7%: r = 0.07/12 = 0.00583. n = 360 months. Your monthly payment comes out to $2,661.
That $2,661 stays the same every month for 30 years. But where that money goes changes dramatically over time.
The Front-Loading Problem
Your first payment of $2,661 breaks down like this: $2,333 goes to interest, $328 goes to principal. That's 87.7% interest on your first payment.
This is the part that catches people off guard. For roughly the first seven years of a 30-year mortgage, more than half of every payment goes to interest. You're barely building equity. If you sell after five years, you've paid about $159,660 total but only reduced your principal by roughly $23,000.
By year 15, the split is approximately even. By year 25, you're finally paying mostly principal. The bank collects most of its money early, which is exactly why refinancing after 15 years into a new 30-year term is such a bad deal. You restart the front-loading cycle all over again.
15-Year vs 30-Year: The Real Comparison
Same $400K at 7%.
A 30-year mortgage: $2,661/month, $558,036 total interest, $958,036 total cost.
A 15-year mortgage: $3,595/month, $247,516 total interest, $647,516 total cost.
The 15-year mortgage costs $934 more per month. But it saves you $310,520 in interest. That's not a typo. Three hundred ten thousand dollars.
The monthly payment difference is real and it's not trivial. But the total cost difference is massive. Whether the higher payment fits your budget is a personal question, but you should at least know what the 30-year term is actually costing you.
What Extra Payments Do
This is where the math gets interesting. On that 30-year $400K mortgage at 7%, paying an extra $200 per month toward principal does three things: it pays off the mortgage 6 years early, it saves you roughly $142,000 in interest, and it builds equity significantly faster in the early years when the amortization schedule is working hardest against you.
Even one extra payment per year (you can do this by paying bi-weekly instead of monthly, which gives you 26 half-payments or 13 full payments annually) shaves about 4 years off a 30-year mortgage and saves around $85,000 in interest.
The reason extra payments are so powerful early in the loan is that every dollar of principal you pay down early eliminates the interest that dollar would have generated for the remaining 25 or 30 years. A $100 extra payment in year 1 saves you far more than a $100 extra payment in year 25.
Points vs Rate
Mortgage points let you buy a lower interest rate. One point costs 1% of the loan amount and typically reduces your rate by 0.25%.
On a $400K loan, one point costs $4,000 and drops your rate from 7% to 6.75%. That reduces your monthly payment by about $68, which means it takes 59 months (just under 5 years) to break even.
If you plan to stay in the house for 10+ years, points usually pay off. If you might move in 3 to 4 years, you're better off keeping the cash. The break-even calculation is simple division: point cost divided by monthly savings equals months to recoup.
When Adjustable Rate Makes Sense
ARMs get a bad reputation from the 2008 crisis, and a lot of that reputation is deserved. But there are narrow situations where a 5/1 or 7/1 ARM is rational.
If you know you're moving in 3 to 5 years (job relocation, growing family, temporary market), a 5/1 ARM with a rate 1 to 1.5% below the fixed rate saves you real money during the fixed period without exposing you to adjustment risk.
The key is being honest with yourself about your timeline. People who took ARMs in 2005 planning to move in 3 years got trapped when housing prices collapsed and they couldn't sell. The risk isn't the rate adjustment itself. It's being unable to exit when you planned to.
Running Your Own Numbers
I built a mortgage calculator at zovo.one that lets you plug in your specific numbers and see the full amortization schedule, extra payment impact, and 15 vs 30 year comparison side by side. Seeing exactly how much of each payment goes to interest versus principal, month by month, tends to change how people think about their loan structure.
The numbers don't lie, but they do hide behind monthly payment amounts that feel manageable. The total cost is what matters, and most people never calculate it until after they've signed.
I'm Michael Lip. I build free developer tools at zovo.one. 350+ tools, all private, all free.
Top comments (0)