Mortgage Payments: What You're Actually Paying Each Month
A mortgage is an amortising loan secured against property. Your monthly payment (PITI in the US — Principal, Interest, Taxes, Insurance) is the largest financial commitment most people make. Understanding how each component works lets you make better borrowing decisions.
Monthly Principal + Interest Formula
M = L × [r(1+r)ⁿ] ÷ [(1+r)ⁿ − 1]
L = loan amount (purchase price − down payment)
r = monthly rate (annual rate ÷ 12)
n = months (e.g. 25 years = 300)Example: £250,000 mortgage at 4.5% over 25 years
r = 0.045 ÷ 12 = 0.00375
n = 300
M = £1,389/month
Total repaid: £416,700 | Total interest: £166,700Impact of a Larger Down Payment
- 10% down (£27,778 on £250k): mortgage £222,222 → £1,234/month
- 20% down (£55,556): mortgage £194,444 → £1,079/month
- 20%+ often unlocks better rates, saving even more
Fixed vs Adjustable Rate
A fixed rate locks your payment for the full term — predictable but typically higher at the start. An adjustable rate mortgage (ARM) starts lower but resets periodically, introducing payment uncertainty. In rising rate environments, fixed rates are usually worth the premium for long-term financial planning.
Extra Payments: The Most Powerful Lever
Adding just £100/month to the example above reduces the term by ~3 years and saves approximately £15,000 in interest. Early in the mortgage term, extra principal payments have the greatest effect because they reduce the outstanding balance on which all future interest is calculated.
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