Building Society Mortgage Calculator: Monthly Repayments & Affordability
How much would a mortgage cost each month, and could you realistically afford it? Enter a house price, deposit, interest rate and mortgage term below to estimate the monthly repayment, total interest and loan-to-value (LTV).
You can also enter your income and household outgoings to make a basic budget check. The calculator is designed for UK homebuyers and remortgagers comparing building society mortgages; it does not represent an offer from any lender.
Editorial review: 14 October 2026. UK mortgage guidance and official calculator resources checked. This independent guide is not mortgage advice.
UK Mortgage Repayment Calculator
Start with your expected purchase price and cash deposit. For an existing mortgage, enter the property value and the equity amount that leaves the mortgage balance you wish to model. All amounts are in pounds sterling.
Calculate Your Monthly Mortgage Payment
Change the example figures to your own. No account, email address or personal identification is required.
10% deposit / 90% mortgage.
Assumes a constant 4.5% annual interest rate over the entire 30-year term, charged monthly, with 360 equal capital-and-interest payments. It excludes fees, insurance and rate changes during the term.
Mortgage Affordability Calculator: Income and Monthly Outgoings
Repayments tell you the cost of a particular loan. Affordability asks a different question: how much money will your household have left once the mortgage and ordinary living costs are paid?
Use your combined annual gross income for a rough borrowing multiple, then enter your actual monthly take-home pay and non-mortgage spending to check the household budget. They are separate checks and neither is an approval prediction.
Estimate Your Household Headroom
The budget calculation uses the loan, rate, term and repayment method selected above.
How This Mortgage Calculator Works
For a capital repayment mortgage, each monthly payment covers interest and repays some of the mortgage balance. At the start, a larger share of the payment normally goes towards interest. As the balance reduces, more of the monthly payment goes towards capital.
The Repayment Calculation
P is the loan principal; r is the monthly interest rate; n is the number of monthly payments. When the interest rate is zero, the calculator divides the loan by the number of monthly payments.
This standard amortisation formula assumes a constant rate, regular monthly payments and no additional charges. An actual lender may calculate interest daily, change your rate after an introductory period or alter payments after overpayments, so its payment schedule can differ.
Mortgage Repayment Examples: £150,000 to £350,000
The following examples use a 30-year capital repayment mortgage at an illustrative 4.5% annual rate. They show the effect of loan size without implying that this rate is currently available from any building society.
| Mortgage balance | Monthly payment | Total interest over 30 years |
|---|---|---|
| £150,000 | About £760 | About £124,000 |
| £200,000 | About £1,013 | About £165,000 |
| £250,000 | About £1,267 | About £206,000 |
| £300,000 | About £1,520 | About £247,000 |
| £350,000 | About £1,773 | About £288,000 |
How Deposit Size and Loan-to-Value Affect a Building Society Mortgage
Loan-to-value measures how much you borrow compared with the property value accepted by the lender. It is particularly important when comparing building society products because different LTV bands can carry different interest rates, fees or lending restrictions.
| Property price | Deposit | Mortgage | LTV |
|---|---|---|---|
| £300,000 | £15,000 (5%) | £285,000 | 95% |
| £300,000 | £30,000 (10%) | £270,000 | 90% |
| £300,000 | £45,000 (15%) | £255,000 | 85% |
| £300,000 | £75,000 (25%) | £225,000 | 75% |
| £300,000 | £120,000 (40%) | £180,000 | 60% |
Example: a buyer purchasing a £300,000 home with a £30,000 deposit needs a £270,000 mortgage. That represents 90% LTV. The same buyer contributing £45,000 instead would need £255,000 at 85% LTV.
Repayment vs Interest-Only: Why the Calculator Gives Different Answers
Capital Repayment Mortgage
You pay the lender interest plus a portion of the capital each month.
- The outstanding balance normally decreases with each payment.
- The loan is designed to be paid off by the end of the agreed term.
- The monthly payment is usually higher than on an otherwise equivalent interest-only mortgage.
- Term extensions can reduce monthly payments but increase overall interest.
Interest-Only Mortgage
You pay the mortgage interest but do not reduce the principal through those payments.
- Monthly interest payments are lower for an equivalent balance and rate.
- The full capital normally remains payable at the end of the term.
- A credible repayment strategy is essential.
- Residential interest-only mortgages can have stricter eligibility requirements.
How Building Societies Actually Assess Mortgage Affordability
A building society typically checks both the size of the proposed loan and whether repayments appear sustainable. The online calculator is only a starting point.
Can you borrow 4.5 times your salary?
Four-and-a-half times gross annual income is a common benchmark, not a right or guarantee. MoneyHelper describes it as a usual borrowing cap in general mortgage guidance, while individual lenders may apply different limits and exceptions.
| Annual gross household income | Illustrative mortgage at 4.5× |
|---|---|
| £30,000 | £135,000 |
| £40,000 | £180,000 |
| £50,000 | £225,000 |
| £60,000 | £270,000 |
| £80,000 | £360,000 |
| £100,000 | £450,000 |
Income-multiple examples are not lending limits or personalised borrowing recommendations. Approval depends on the lender’s complete assessment.
Costs the Monthly Mortgage Calculator Does Not Include
Mortgage affordability should leave room for the costs of buying, moving and maintaining the property. A low monthly payment alone does not tell you whether a deal is good value.
What Happens if Mortgage Interest Rates Rise?
Rate changes matter most when you are on a variable-rate mortgage or when a fixed-rate period ends. The calculator’s higher-rate test lets you model an increased annual rate on the current loan amount and remaining selected term.
It is a simple planning scenario rather than an official lender affordability stress test. It does not forecast future rates.
Why a longer mortgage term is not automatically better
Increasing a 25-year term to 35 years can lower your monthly repayment, but your mortgage can cost substantially more in interest overall. Extending a mortgage into retirement also needs a realistic plan for future income.
Official UK Mortgage Calculators and Verification Resources
Use these resources to check the estimate against an independent public calculator and then a building society’s own criteria. MoneyHelper is a free UK money guidance service backed by the Money and Pensions Service.
Independent UK guidanceMoneyHelper Mortgage Repayment Calculator
Useful for checking estimated monthly payments on repayment and interest-only mortgages.
After opening this link:- Start the mortgage repayment calculator.
- Enter the mortgage amount or the relevant property-price details.
- Choose repayment or interest-only and enter the mortgage term.
- Replace any default interest rate with the rate you want to test.
- Compare the resulting payment with the estimate on this page.
Independent affordabilityMoneyHelper Mortgage Affordability Calculator
Designed to explore borrowing, mortgage repayments and the money you may have left each month.
After opening this link:- Start the affordability calculator and enter household income.
- Add regular household spending and outstanding financial commitments.
- Use your payslips, bank statements and bills rather than guessing.
- Review the estimated borrowing and monthly budget.
- Change the mortgage term or interest rate to compare different scenarios.
Lender verificationFinancial Conduct Authority Register
Use the official register when checking the identity and regulatory permissions of a lender or mortgage intermediary.
After opening this link:- Search for the exact building society or mortgage intermediary.
- Review the legal entity, firm reference number and authorisation status.
- Check whether the firm has the appropriate permissions for the service being offered.
- Use verified firm contact information rather than contact details from an unsolicited message.
What to Do After Calculating Your Mortgage
Related Building Society Mortgage Guides
These published guides cover circumstances that a general repayment calculator cannot assess.
Building Society Mortgage Calculator FAQs
1. How accurate is this building society mortgage calculator?
It provides a mathematical estimate using your entered loan amount, annual rate, term and repayment method. It does not include changing rates, lender-specific interest calculations, fees or underwriting criteria, so your actual mortgage illustration may differ.
2. How much mortgage can I afford on a £40,000 salary?
At an illustrative 4.5 times annual income, £40,000 would suggest £180,000 of borrowing. That is not a guaranteed maximum or approval. Lenders also consider debt, spending, dependants, deposit, credit history, mortgage term and other circumstances.
3. How much deposit do I need for a building society mortgage?
Many standard UK mortgage options require a deposit of around 5% to 10% or more, although individual products can have different requirements. A larger deposit can improve your LTV and expand your available product choices. Check the specific lender’s current criteria.
4. What is a 90% LTV mortgage?
A 90% LTV mortgage means the borrowing represents 90% of the property value accepted by the lender. For a £300,000 home, that would be a £270,000 mortgage and a £30,000 deposit.
5. Is a 25-year or 35-year mortgage cheaper?
For the same loan and interest rate, a 35-year mortgage normally has a lower monthly capital repayment but a higher overall interest cost than a 25-year mortgage. Compare affordability now with total interest and the implications of borrowing later into life.
6. Does this mortgage calculator check my credit score?
No. This calculator does not check your credit file, verify your income, contact lenders or make a credit application. Its income-multiple and household-budget results are only illustrations.
7. Can a building society lend more than 4.5 times income?
Some lenders and products may allow higher income multiples for eligible applicants, subject to lending policy, affordability and regulatory constraints. A higher multiple is not automatically available to everyone.
8. Do mortgage repayments stay the same for the whole term?
Not necessarily. Fixed-rate payments generally remain stable during the agreed fixed period, but payments can change after that period ends. Variable and tracker mortgage payments can also change when the applicable rate changes.
9. Does this calculator include Stamp Duty and mortgage arrangement fees?
No. It models mortgage principal and interest only. Property taxes, arrangement fees, valuation costs, conveyancing, surveys, insurance and other buying costs should be calculated separately.
10. Is this calculator an official building society mortgage quote?
No. Building-society.org is an independent informational website and does not issue mortgage quotes, decisions or offers. Use a building society’s official calculator, affordability assessment and mortgage illustration to confirm product-specific details.
Sources, Accuracy and Financial Safety
This page uses standard mortgage amortisation arithmetic alongside UK educational guidance from MoneyHelper. It deliberately does not publish one “best” building society interest rate because product rates depend on the lender, loan-to-value, borrower and date of application.
Last editorial review: 14 October 2026. Calculations use an assumed constant nominal annual rate compounded through monthly payments. The borrowing-multiple comparison is illustrative and does not reproduce any particular lender’s affordability model.