How to Calculate Mortgage Payments — Formula & Examples

Your monthly mortgage payment is the single biggest line item in most household budgets, yet the formula behind it looks intimidating at first glance. Once you understand the three inputs that drive it — loan amount, interest rate, and term — the formula becomes straightforward, and you can see exactly how changing any one of them affects your payment.

The Mortgage Payment Formula

Monthly Payment = P × r × (1 + r)^n ÷ ((1 + r)^n − 1), where P is the loan principal (home price minus down payment), r is the monthly interest rate (annual rate ÷ 12 ÷ 100), and n is the total number of monthly payments (loan term in years × 12).

Worked Example

A $300,000 mortgage at 6.5% annual interest over 30 years: P = 300,000; r = 6.5 ÷ 12 ÷ 100 = 0.005417; n = 30 × 12 = 360. Monthly Payment = 300,000 × 0.005417 × (1.005417)^360 ÷ ((1.005417)^360 − 1) ≈ $1,896 per month. Over 30 years, total payments = $1,896 × 360 ≈ $682,560 — meaning total interest paid is roughly $382,560, more than the original loan amount.

What Changes Your Monthly Payment?

• •Loan amount (principal): a larger down payment directly and proportionally reduces your monthly payment.
• •Interest rate: even a 1% difference in rate meaningfully changes your payment — on the example above, a rate of 5.5% instead of 6.5% drops the payment to roughly $1,703/month.
• •Loan term: a 15-year term has a much higher monthly payment than a 30-year term, but dramatically less total interest paid over the life of the loan.

15-Year vs 30-Year Mortgage

On the same $300,000 loan at 6.5%: a 30-year term gives a payment of about $1,896/month with total interest of about $382,560. A 15-year term at a (typically slightly lower) 6% rate gives a payment of about $2,532/month — higher — but total interest drops to roughly $155,760, saving over $225,000 in interest. The trade-off is simple: pay more per month now, or pay far more in total interest over time.

Does the Payment Stay the Same Every Month?

For a fixed-rate mortgage, yes — the total monthly payment stays constant for the entire term. What changes is the split between principal and interest: early payments are mostly interest, and later payments are mostly principal, a process called amortization. For an adjustable-rate mortgage (ARM), the payment can change at scheduled reset points if the underlying interest rate changes.

Does This Include Taxes and Insurance?

The formula above calculates principal and interest (P&I) only. In many countries, lenders also collect property tax and homeowners insurance monthly into an escrow account, added on top of the P&I payment — so your total monthly housing cost will typically be higher than the P&I figure alone. Always ask your lender for the full "PITI" (Principal, Interest, Taxes, Insurance) estimate for true affordability planning.