Mortgage Calculator: How to Calculate Monthly Payments, Interest Costs, and Loan Affordability

A mortgage is the biggest financial commitment most people ever make. Knowing your monthly payment, total interest cost, and how much house you can actually afford before you start house hunting saves you time, stress, and potentially thousands of dollars. The key numbers are your loan amount, interest rate, and loan term — small changes in any of these significantly affect your monthly payment.

Use the Mortgage Calculator on TodayCalculator to model different scenarios and find the loan that fits your budget.

How Monthly Mortgage Payments Are Calculated

Your monthly payment consists of four components, known as PITI:

• Principal (P) — The loan amount you borrowed
• Interest (I) — The cost of borrowing, determined by your interest rate
• Taxes (T) — Property taxes, typically 0.5-2.5% of home value annually
• Insurance (I) — Homeowners insurance plus PMI if down payment is under 20%

The mortgage payment formula is: M = P × [r(1+r)^n] / [(1+r)^n – 1], where M is the monthly payment, P is the principal, r is the monthly interest rate (annual rate divided by 12), and n is the total number of payments.

Real-World Scenarios

Home Price	Down Payment	Rate	Term	Monthly Payment	Total Interest
$300,000	$60,000 (20%)	6.5%	30 years	$1,517	$246,310
$300,000	$60,000 (20%)	5.5%	15 years	$1,961	$82,990
$300,000	$15,000 (5%)	6.5%	30 years	$1,801	$280,840

Notice how a 1% lower rate saves over $160,000 in interest over 30 years. And a 15-year term halves the interest paid but raises the monthly payment by about $444. The Mortgage Calculator lets you compare all these scenarios instantly.

The 28/36 Rule for Affordability

Lenders generally follow the 28/36 rule:

• 28% — Your monthly housing costs (PITI) should not exceed 28% of your gross monthly income
• 36% — Your total monthly debt payments (including mortgage, car loans, student loans, credit cards) should not exceed 36% of your gross income

For a household earning $8,000/month gross, that means a maximum housing payment of $2,240 and total debt payments of $2,880. If you have $600 in existing debt payments, your max housing payment drops to $2,240.

Tips for Getting the Best Mortgage Rate

1. Improve your credit score to 740+ for the best rates
2. Save a 20% down payment to avoid PMI
3. Compare rates from 3-5 lenders within a 14-day window (counts as one hard pull)
4. Consider buying points: paying 1% of the loan amount upfront typically lowers the rate by 0.25%
5. Lock your rate when you find a good deal — rates can change daily

Experiment with different down payments, rates, and terms using the Mortgage Calculator before talking to lenders, so you know exactly what works for your budget.