How Mortgage Payments Work: Amortization, Rates and Terms
"What will my monthly payment be?" is the first question buyers ask. But the payment is only an outcome — what really determines your cost is the loan amount, the annual rate and the term. Understanding how they interact is what lets you judge whether a deal is good.
The amortized payment formula
Most mortgages use level payments (amortization): the payment is fixed, but the split between interest and principal shifts every month. The formula is:
Payment = P × r ÷ (1 − (1 + r)^−n), where r = annual rate ÷ 12 and n = years × 12.
For a 700,000 loan at 4.2% over 30 years: r = 0.0035, n = 360, payment ≈ 3,423. In month one, interest is 2,450 and principal only about 973 — that is why early payments feel like "all interest".
Rate matters more than term
On the same 700,000 over 30 years: raising the rate from 4.2% to 5.2% lifts the payment from ~3,423 to ~3,844 — 421 more per month, about 150,000 extra over the term. Shortening the term from 30 to 25 years raises the payment to ~4,180 but saves roughly 200,000 in interest.
The takeaway: the rate sets the order of magnitude, the term sets the monthly pressure. If cash flow allows, a shorter term usually saves more than chasing a lower payment; if rates may fall, refinancing matters more.
What a down payment actually does
A 30% versus 20% down payment is not just "borrowing 100k less". On a 1,000,000 home at 4.2% over 30 years: 20% down means an 800,000 loan, ~3,912/month and ~608,000 total interest; 30% down means 700,000, ~3,423/month and ~532,000 interest. Putting down 100,000 more saves ~76,000 in interest and ~489 per month.
But more down is not always better: it ties up cash, and if that cash could earn more than the mortgage rate, prepaying is a loss. The test is simple — compare the interest saved against what the money could earn elsewhere.
Is prepaying worth it?
With an amortized loan there are two ways to prepay: keep the payment and shorten the term, or keep the term and lower the payment. The first saves more interest because principal falls faster.
A practical rule: if your mortgage rate is higher than the after-tax return you can reliably earn, prepaying is effectively a risk-free investment at that rate and usually wins. Otherwise it does not. Also check for prepayment penalties and where you are in the schedule — the earlier you prepay, the more interest you save.
Three common mistakes
- Watching the payment, not the total interest: stretching to 30 years flatters the monthly figure while total interest can approach half the principal.
- Treating 0.1% as trivial: on 700,000 over 30 years, 0.1% is roughly 14,000 in total interest.
- Ignoring carrying costs: maintenance, insurance, fees and taxes sit outside the "payment" but still affect affordability.
This article provides general information and calculation methods only. It is not medical, investment or legal advice; consult a qualified professional for your situation.