How the repayments are worked out
Repayment mortgages use the standard annuity formula, which spreads capital and interest into equal monthly payments:
Monthly payment = loan × r ÷ (1 − (1 + r)−n)
r = annual rate ÷ 12, n = years × 12
For £300,000 at 4.5% over 25 years: r = 0.00375 and n = 300, giving £1,667.50 a month. Over the full term you repay £500,249, so the interest comes to £200,249.
Interest-only
On interest-only you pay just the interest: £300,000 × 4.5% ÷ 12 = £1,125 a month. It is cheaper each month, but the £300,000 is still owed at the end, so lenders want a repayment plan.
Income you would need
Lenders usually cap borrowing at about 4 to 4.5 times income, with some going to 5 or 5.5 times for higher earners. To borrow £300,000 you would typically need a household income of £66,667 to £75,000. Check your figures with the mortgage affordability calculator.
Buying a home? Add the stamp duty to your budget, and see how much overpaying could save.
Frequently asked questions
How much is a £300,000 mortgage a month?
A £300,000 repayment mortgage costs about £1,667 a month over 25 years at 4.5% interest, £1,520 over 30 years and £1,420 over 35 years.
What salary do I need for a £300,000 mortgage?
Most UK lenders lend around 4 to 4.5 times income, so a £300,000 mortgage usually needs a household income of about £66,667 to £75,000 a year, depending on your debts and outgoings.
How much interest will I pay on a £300,000 mortgage?
At 4.5% over 25 years you would repay £500,249 in total, of which £200,249 is interest. A shorter term or a lower rate cuts this.
How much is a £300,000 interest-only mortgage a month?
At 4.5%, interest-only payments on £300,000 are about £1,125 a month, but you still owe the full £300,000 at the end of the term.
Last reviewed October 2026. Example rates only; your lender's rate, fees and term decide what you actually pay.