How this page is reviewed
| Risk tier | YMYL |
|---|---|
| Author | Calculover Editorial Team Finance and legal education |
| Editorial owner | Calculover Loans & Housing Desk Loan and housing methodology owner |
| Reviewer | Calculover Editorial Review Source and limitation review |
| Last reviewed | 2026-05-10 |
| Last verified | 2026-05-10 |
| Data effective date | 2026-05-10 |
Methodology
Mortgage Math: How Banks Calculate Your Payment, Interest, and Qualification uses the amortization, escrow, rate, fee, and housing-cost formulas documented on the page, then layers loan-program or property-cost assumptions when the user provides them.
Assumptions
- Mortgage Math: How Banks Calculate Your Payment, Interest, and Qualification relies on the values the user enters and does not independently verify income, balances, legal status, policy terms, or market quotes.
- Loan rates, fees, taxes, insurance, PMI or MIP, HOA dues, and closing costs are planning inputs unless a lender quote is supplied.
- The calculator assumes scheduled payments are made on time and that extra payments are applied according to the selected scenario.
Limitations
- Mortgage Math: How Banks Calculate Your Payment, Interest, and Qualification does not approve a loan, lock a rate, quote closing costs, determine program eligibility, or replace a Loan Estimate from a lender.
- Property taxes, insurance, HOA dues, PMI or MIP, lender overlays, credit score, and local fees can materially change the payment or cash-to-close.
Sources
- Buying a House, Consumer Financial Protection Bureau
- Loan Estimate Explainer, Consumer Financial Protection Bureau
- Mortgage Rates, Freddie Mac
Professional guidance: Mortgage Math: How Banks Calculate Your Payment, Interest, and Qualification is for housing-finance education only and is not mortgage, legal, tax, or underwriting advice. Confirm rates, fees, eligibility, and cash-to-close with a lender or housing professional.
A mortgage is the largest financial commitment most people make, yet few understand the math behind it. This guide breaks down the formulas so you know exactly what you are signing up for.
Step 1: The Payment Formula
M = P × [r(1+r)^n] / [(1+r)^n − 1] P = loan amount, r = monthly rate (annual/12), n = total payments (years × 12).
Example: $350,000 loan at 6.5% for 30 years: r = 0.065/12 = 0.005417, n = 360. M = $2,212/month (principal and interest). With taxes and insurance (PITI), expect $2,800-3,200.
Step 2: Total Interest Over the Loan
Total Interest = ($2,212 × 360) - $350,000 = $446,320. You pay 127% of the home price in interest alone. This is why extra payments and rate shopping matter enormously. Compare with the Mortgage Calculator.
Step 3: Qualification (DTI)
| Loan Type | Max Front-End DTI | Max Back-End DTI |
|---|---|---|
| Conventional | 28% | 36-43% |
| FHA | 31% | 43-50% |
| VA | N/A | 41% |
Calculate your full PITI payment with rate presets and refi analysis
Try the Mortgage Calculator →Key Takeaways
- You will pay more in interest than the home costs on a 30-year loan at current rates.
- DTI under 36% is ideal — above 43% disqualifies you from most conventional loans.
- A 1% lower rate on $350K saves about $250/month and $90K over the life of the loan.
- 15-year mortgages have lower rates and save 60%+ on total interest.