What Is a Mortgage Calculator?
A mortgage calculator computes your estimated monthly housing payment and the total cost of your home loan over its full term. By entering the home price, down payment, interest rate, loan term, and optionally property taxes and insurance, you can see your exact monthly obligation, the split between principal and interest, and the complete amortization schedule showing how every payment chips away at your loan balance.
How Mortgage Payments Are Calculated
The monthly principal and interest payment (P&I) uses the standard loan amortization formula:
Monthly Payment = P × [r(1+r)^n] / [(1+r)^n − 1]
Where P = loan amount, r = monthly interest rate (annual rate ÷ 12), n = total payments (years × 12).
How Down Payment Affects Your Mortgage
| Home Price | Down Payment | Loan Amount | Monthly P&I (6.75%, 30yr) | Total Interest Paid |
|---|---|---|---|---|
| $400,000 | 3.5% ($14,000) | $386,000 | $2,503 | $515,100 |
| $400,000 | 10% ($40,000) | $360,000 | $2,335 | $480,580 |
| $400,000 | 20% ($80,000) | $320,000 | $2,075 | $427,022 |
| $400,000 | 30% ($120,000) | $280,000 | $1,816 | $373,770 |
Fixed vs. Adjustable Rate Mortgages
| Type | Rate Behaviour | Best For | Risk |
|---|---|---|---|
| Fixed-Rate (30yr) | Stays constant for life of loan | Buyers planning to stay 7+ years; those who value payment certainty | Miss out if rates drop significantly |
| Fixed-Rate (15yr) | Stays constant; lower rate than 30yr | Buyers with strong income who want to build equity fast | Higher monthly payment |
| 5/1 ARM | Fixed 5 years, then adjusts annually | Buyers who plan to sell or refinance within 5 years | Payments can rise sharply after adjustment period |
| 7/1 ARM | Fixed 7 years, then adjusts annually | Medium-term owners who want a lower initial rate | Rate uncertainty after year 7 |
Understanding Your Amortization Schedule
The amortization schedule reveals an important truth about mortgages: in the early years, almost all of your monthly payment goes to interest. On a $300,000 loan at 6.75% for 30 years, your first monthly payment of ~$1,946 breaks down as approximately $1,688 interest and only $258 principal. By year 25, this flips — most of each payment is principal. This is why paying extra in the first 10 years saves a disproportionate amount of total interest.