Mortgage Calculator: The Complete Guide

What a mortgage calculator actually calculates

Every fixed-rate mortgage payment comes from one formula: M = P × [r(1+r)^n] / [(1+r)^n − 1], where P is the loan amount (home price minus down payment), r is the annual interest rate divided by 12, and n is the total number of monthly payments (loan term in years × 12). That gives you principal and interest (P&I) — the core payment. A full monthly housing cost, often called PITI, also folds in Property tax, home Insurance, and (when your down payment is under 20%) PMI, plus any HOA dues. Frost Rank's Mortgage Calculator computes both: P&I by default, and full PITI the moment you check "Include property tax, insurance, PMI & HOA."

Down payment size changes more than just your loan amount

A larger down payment does two things at once: it shrinks the amount you're borrowing (and therefore your monthly P&I), and — once it crosses 20% of the home price — it eliminates PMI entirely. On a $400,000 home at 6.5% APR over 30 years:

The jump from 15% to 20% down doesn't just save $126.41 a month in P&I — it also removes PMI altogether, which on this loan size typically adds another $130-$400+ a month depending on your lender and credit profile. That combined effect is why 20% down is treated as a meaningful threshold rather than an arbitrary round number.

Loan term: 15-year vs 30-year

Term length changes both your monthly payment and your total interest — in opposite directions, and by very different magnitudes. On the same $320,000 loan at 6.5% APR:

The 15-year loan demands $764.92 more per month — but pays off in half the time and saves $226,384.52 in total interest, largely because 15-year loans also tend to carry lower rates than 30-year ones. The right choice comes down to whether the higher required payment is comfortably affordable every month, not just whether the total-cost math looks better on paper.

Interest rate: why a single point matters so much

Because interest compounds against the full remaining balance every month for the entire term, even a modest rate change produces a large total-cost swing. On the same $320,000, 30-year loan:

Two points of rate — 5.5% to 7.5% — adds $420.57 to the monthly payment and $151,402.18 to total interest on an identical loan amount. This is exactly why comparing rate offers across a few lenders is worth the effort: the rate has more leverage over your total cost than almost any other single number in the transaction.

Reading an amortization schedule — and why extra payments matter

An amortization schedule shows the principal/interest split of every payment across the loan's life. Because interest is charged on whatever balance remains, that split shifts steadily: early payments are mostly interest, and later payments are mostly principal, even though the payment amount itself never changes. This is exactly why paying extra toward principal is most powerful early in a loan — every extra dollar of principal paid sooner is a dollar that stops accruing interest for the rest of the term. On the same $320,000, 30-year loan at 6.5%, adding a consistent extra $200/month toward principal from the first payment:

Without extra payment:  30.0 years,  $408,142.36 total interest
With +$200/month extra: 23.4 years,  $302,713.69 total interest

Time saved:      6.6 years (79 fewer payments)
Interest saved:  $105,428.67

Before relying on this, confirm with your lender that extra payments are applied directly to principal rather than held toward next month's payment, and check for a prepayment penalty — rare on conventional mortgages, but worth a two-minute call to rule out. You can model your own numbers, including extra scenarios like this, by adjusting the loan amount and term on the Mortgage Calculator and comparing the resulting total-interest figures.

Mortgage glossary

Frequently asked questions