How is a monthly mortgage payment worked out?

A monthly mortgage payment is set by three numbers: the amount borrowed, the interest rate, and the term. On a repayment mortgage it is whatever clears the debt and its interest by the end of the term. On an interest-only mortgage it is the interest alone, and the capital is still owed in full on the last day. Everything else — insurance, service charge, ground rent, council tax, repairs — sits outside that figure.

The three inputs, and how each moves the number

Interest is charged on the balance still outstanding, so the payment is the level amount that clears it by the end of the term.

• Amount borrowed. The payment moves broadly in proportion: borrow 5% more and pay about 5% more a month.
• Interest rate. A higher rate raises the payment, and raises the total paid by more again.
• Term. A longer term lowers the payment and increases total interest, because the balance is carried for longer. A shorter term does the reverse.

With P the amount borrowed, i the monthly rate (annual rate ÷ 12) and n the number of payments:

M = P × i ÷ (1 − (1 + i)^−n)

An illustrative example. Take £200,000 over 25 years at an illustrative 5% a year — round, made-up figures showing the arithmetic, not a rate anyone is offering and not a quotation. The payment comes to about £1,169 a month, roughly £150,800 of the total being interest. Over 30 years it falls to about £1,074, with interest near £186,500; over 20 years it rises to about £1,320, with interest near £116,800. Same debt, three answers.

Repayment or interest-only

A repayment payment has two parts: that month's interest, and a slice of capital. The balance falls every month, so the debt is gone at the end of the term.

An interest-only payment has only the interest — about £833 a month in the example, rather than £1,169. It is lower because nothing is being repaid. After 25 years of it, the £200,000 is still outstanding and falls due as a lump sum.

The regulator treats interest-only as conditional. Under FCA rules a lender may only enter into one where it has evidence the borrower will have in place "a clearly understood and credible repayment strategy", and it "must not accept speculative repayment strategies". Affordability is otherwise assessed "on the basis of both repayment of capital and payment of interest over the term". Interest-only is common on buy-to-let lending — see our buy-to-let checks.

Why the early years are nearly all interest

Because interest is charged on what is still owed, and at the start nearly all of it is.

In the example, the first payment of about £1,169 splits roughly £833 interest and £336 capital. By month 120 it is about £618 and £551; by month 240, about £262 and £907. The payment never changes; its composition inverts. Five years in, about £70,100 has been paid and only £22,800 of the debt cleared.

Overpayments

An overpayment cuts the balance immediately, so every later month is charged interest on a smaller number. Early, that smaller number persists for decades; in the final year it has almost nothing left to work on. The effect is compounding, not a reward.

In the example, adding £100 a month from the start clears the loan in about 21 years six months rather than 25, with interest near £126,200 instead of £150,800 — again, illustrative only.

Two limits. Fixed deals commonly allow only a set amount of overpayment each year before an early repayment charge applies; the allowance and the charge are in the offer and the illustration, and vary by product. FCA rules require any such charge to be a cash value and "a reasonable pre-estimate of the costs" of early repayment.

What the payment does not include

The mortgage is one bill among several. Budget separately for:

• Buildings insurance, usually a lender condition on a freehold house. For a flat, GOV.UK notes that "Your landlord will usually be responsible for insurance of the building (not the contents) - this will be part of your service charge."
• Service charge, and any reserve or sinking fund, on a leasehold flat.
• Ground rent. GOV.UK: "Ground rent is a payment to your landlord that might be included in your lease." Leases granted on or after 30 June 2022 usually cannot be charged more than a peppercorn.
• Council tax. GOV.UK: "You'll usually have to pay Council Tax if you're 18 or over."
• Maintenance and repairs — irregular, not optional. Landlords add letting costs, compliance and voids: see our tenancy deposits guide.

Fixed and variable, and the end of a fix

A fixed rate holds the interest rate, and so the payment, for a stated period. The fix is on the rate, not on the mortgage, which usually runs far longer. A variable rate can move, and the payment with it, up or down.

When a fix ends the loan does not. Unless something else is arranged it moves onto the lender's variable rate, and the payment is recalculated on the balance then outstanding over the term then remaining — higher or lower than before.

How lenders test affordability

Lenders must assess whether a borrower can pay, using evidenced income and expenditure including "basic essential expenditure and basic quality-of-living costs" of the household. They must also stress the loan against rate rises: the rules require a lender to consider "the likely future interest rates over a minimum period of five years" from the start of the contract, unless the rate is fixed for five years or more, and in any event "a mortgage lender must assume that interest rates will rise by a minimum of 1% over that period".

Put your own numbers in

Our calculator takes an amount, a rate and a term and returns the arithmetic above. Change one input at a time to see which your payment is most sensitive to.

Atlas Estates is an estate and letting agency in Manchester. We are not authorised by the Financial Conduct Authority and cannot give mortgage advice — for anything touching a product, rate, term or lender, speak to an FCA-authorised mortgage adviser. For a letting or sales view on an address, ring 0161 710 4510 or call in at 12–14 Midland Street, Manchester M12 6LB. Our fees are published, and we are a member of the Property Redress Scheme.

General information, current at October 2026. Not financial or mortgage advice. Atlas Estates is not authorised by the Financial Conduct Authority. The figures above are illustrative arithmetic, not quotations. Speak to an FCA-authorised mortgage adviser before acting.