mortgage in principle calculator

What is a mortgage in principle?

A mortgage in principle (sometimes called an Agreement in Principle or Decision in Principle) is an estimate from a lender showing how much they may be willing to lend you. It is not a guaranteed mortgage offer, but it helps you set a realistic property budget before you start making offers.

This calculator gives you a practical starting point by combining two common checks: an income multiple method and an affordability method. The final estimate uses the lower of those two numbers, which is generally the safer approach.

How this calculator works

1) Income-based limit

Many lenders use a multiple of your gross annual income. A common range is around 4.0x to 4.5x income, with some cases going higher depending on profile and policy.

2) Affordability-based limit

The tool estimates your available monthly amount after typical commitments and then converts that amount into a loan size using your interest rate and mortgage term.

3) Conservative estimate

Your estimated borrowing amount is the lower of:

• Income multiple result
• Affordability result

That conservative rule reflects how lenders often decide the practical upper borrowing limit.

Inputs explained

• Annual income: Gross salary before tax. Add both applicants if buying jointly.
• Other income: Bonus, overtime, commission, rental income, or other provable income sources.
• Monthly debts: Loans, credit cards, car finance, student loans, maintenance payments.
• Monthly living costs: Essentials like utilities, travel, food, childcare, and subscriptions.
• Deposit: Cash deposit you can put down toward the property.
• Interest rate and term: Used to estimate monthly repayment capacity and loan size.
• Income multiple: Adjustable so you can model conservative vs. optimistic lender criteria.

Quick example

Suppose your household gross income is £70,000 and your lender multiple is 4.5x. That gives an income-based cap of about £315,000. If your monthly budget supports a smaller loan, say £285,000, then your working mortgage in principle estimate is £285,000. Add your deposit to estimate potential property value range.

Tips to improve your mortgage in principle result

• Pay down high-interest debt before applying.
• Avoid new credit applications in the months leading up to mortgage checks.
• Increase deposit size to reduce loan-to-value ratio (LTV).
• Check your credit report for errors and fix them early.
• Keep spending patterns stable so affordability looks consistent.
• If self-employed, keep clean and up-to-date accounts and tax filings.

What this calculator does not replace

A true lender assessment can include many additional checks, such as:

• Credit score and adverse credit history
• Employment type and probation period
• Dependants and household composition
• Property type and valuation
• Detailed underwriting and stress testing

So treat this as an educational estimate, not a formal lending commitment.

Next steps after getting your estimate

Speak to a broker or lender

They can match your profile to lenders that are more likely to approve your application.

Get documents ready

• Proof of income (payslips, P60, tax returns)
• Bank statements
• ID and proof of address
• Deposit evidence

Re-check affordability with real rates

Mortgage rates change. Before making an offer on a home, rerun your numbers using the latest available products and include full ownership costs.