How to Calculate Your Monthly Mortgage Payment (With Real Examples)

The Mortgage Payment Formula (Plain English)

Every fixed-rate mortgage in the United States and the United Kingdom uses the same underlying maths. It is called the amortization formula, and it answers one question: what payment keeps the balance shrinking by exactly enough to hit zero on the final month?

The formula itself looks intimidating but the logic is simple:

M = P × [ r(1 + r)n ] ÷ [ (1 + r)n − 1 ]
Where M = monthly payment, P = loan amount (principal), r = monthly interest rate (annual rate ÷ 12), and n = total number of monthly payments (years × 12).

You do not need to memorise it. The formula is built into our mortgage calculator — enter your numbers, get the payment in under a second. But understanding what it does helps you spot a bad deal.

Worked Example 1: $400,000 US Home at 6.5% for 30 Years

Let's say you buy a house in the US for $500,000 with a 20% down payment. Your loan is $400,000. The current 30-year fixed rate is 6.5% APR.

• Principal (P) = $400,000
• Monthly rate (r) = 6.5% ÷ 12 = 0.541666%
• Number of payments (n) = 30 × 12 = 360

Plug in: M = $2,528 per month for principal and interest. Over the full 30 years you will pay approximately $910,000 — that's $510,000 in interest on top of the original $400,000 loan.

Worked Example 2: £250,000 UK Home at 4.5% for 25 Years

UK mortgages are typically 25-year terms rather than 30. On a £250,000 loan at 4.5% over 25 years:

• Principal (P) = £250,000
• Monthly rate (r) = 4.5% ÷ 12 = 0.375%
• Number of payments (n) = 25 × 12 = 300

Monthly payment = £1,390. Total repaid over 25 years = £417,000. UK rates are typically lower than US rates because most UK mortgages are fixed for only 2–5 years, after which they revert to a variable rate.

How to Estimate Your Total Monthly Housing Cost (PITI)

The formula above only covers principal and interest. Your actual monthly housing cost includes four things — the lender abbreviation is PITI:

• Principal and Interest — the core loan payment
• Taxes — US property tax averages 1.1% of home value per year ($5,500 on a $500,000 house = $458/mo). UK homeowners pay council tax instead (£1,800 average per year = £150/mo)
• Insurance — homeowners insurance averages $1,800/yr in the US, and buildings insurance £300/yr in the UK

Add PMI (private mortgage insurance) if your US down payment is below 20%. PMI costs 0.5–1.5% of the loan per year.

Why a 0.5% Rate Difference Costs £30,000

On the $400,000 US loan above, raising the rate from 6.5% to 7.0% increases the monthly payment from $2,528 to $2,661 — only $133 more. But over 360 months, that's $47,880 of extra interest. Shopping for 3–5 lender quotes before committing is the single highest-value hour you can spend. Online brokers like LendingTree and Credible return multiple quotes in minutes.

15-Year vs 30-Year: The Cash Flow Trade-Off

On that same $400,000 loan at 6.5%, a 15-year fixed costs $3,484 per month instead of $2,528. The monthly payment is 38% higher, but the total interest drops from $510,000 to $227,000 — a saving of $283,000.

Which is right for you? If you can afford the higher payment and want to retire debt-free sooner, go 15. If you want the flexibility to invest the difference (the S&P 500 averages 10% annual returns), the 30-year can come out ahead mathematically — but only if you actually invest the difference rather than spend it.

How to Shave Years Off Your Mortgage (Without Refinancing)

Make one extra monthly payment per year. On a $400,000 loan at 6.5% over 30 years, that single extra payment knocks 5 years and 3 months off the term and saves roughly $115,000 in interest. Most lenders let you split it into 12 small additions of $210/mo — essentially invisible in your budget.

Our mortgage payoff calculator shows the exact impact of extra payments on your specific loan.

Frequently Asked Questions

Is the mortgage payment formula the same in the US and UK?

Yes. The amortization formula is universal. What differs is how long the rate is fixed for: most US borrowers lock in 30 years; most UK borrowers lock in 2, 3, or 5 years, then refinance. That's why UK payments look cheaper but carry reset risk.

What is a good mortgage rate in 2026?

In the US, anything below 6.5% is competitive for a 30-year fixed. In the UK, anything below 4.5% on a 5-year fix is solid. Your exact offer depends on credit score (US) or credit file (UK), deposit size, and lender.

How much house can I afford on my salary?

The conventional rule: keep total housing costs (PITI) below 28% of gross monthly income. Our house affordability calculator applies this automatically.