Part-and-part mortgage calculator (part repayment, part interest-only)

Part-and-part mortgage calculator (part repayment, part interest-only)

A part-and-part mortgage (sometimes described as part repayment, part interest-only) lets you split your borrowing into two parts:

• Repayment portion: you pay monthly payments that reduce the balance over time.
• Interest-only portion: you pay the interest each month, while the capital for that portion is intended to be repaid at the end of the term.

Because the split affects how much of your loan is reducing each month, it also affects your monthly payment. This calculator helps you estimate what your payments could look like for different interest-only percentages.

Note: This is an educational calculator. The figures are estimates and do not reflect lender-specific underwriting, fees, or product rules.

How to use the calculator

Enter the details below to estimate your monthly payments for a part-and-part arrangement.

Inputs

• Loan amount: the total mortgage amount you’re considering.
• Interest-only amount: the part of the loan you want to keep as interest-only.
• Mortgage rate: the annual interest rate you want to model.
• Mortgage term: the length of the mortgage (in years).

Outputs

The calculator will show:

• Monthly interest payments (for the interest-only portion)
• Monthly repayment payments (for the repayment portion)
• Monthly total payment (repayment portion + interest-only portion)

What the results mean

A part-and-part mortgage can reduce monthly payments compared with a full repayment mortgage because the interest-only portion does not reduce the capital each month.

However, the trade-off is that you still need a credible plan for repaying the interest-only capital at the end of the term (or through an agreed alternative exit strategy).

When you adjust the interest-only amount, you’re effectively changing:

• how much of the loan is reducing each month
• how much is remaining for repayment later
• the balance between monthly affordability and end-of-term risk

Common scenarios to model

This calculator is useful for exploring different approaches, such as:

• A smaller interest-only slice to keep monthly payments closer to repayment levels.
• A larger interest-only slice to reduce monthly outgoings, while increasing the end-of-term repayment requirement.
• Different terms (e.g., 20 vs 30 years) to see how term length impacts monthly payments.

Important considerations for part-and-part mortgages

While this calculator estimates payments, lenders will typically focus on whether the overall structure is workable. In practice, that often includes:

• Repayment strategy for the interest-only portion: evidence that the capital can be repaid when due.
• Affordability for the full borrowing: lenders usually assess income and outgoings to ensure the mortgage is sustainable.
• Property and loan-to-value (LTV) factors: the acceptable split and maximum LTV can vary by lender and property type.
• Your circumstances and credit profile: underwriting requirements differ between lenders.

Part-and-part mortgages for buy-to-let (brief note)

Part-and-part structures can also be considered for buy-to-let in some cases. The modelling approach is similar, but the lender assessment is different and can depend on rental income, property type, and the lender’s buy-to-let criteria.

Next steps (content-only)

If you’re comparing options, it can help to model a range of interest-only percentages and terms, then consider how the end-of-term repayment plan would work in practice. This calculator is designed to support that comparison by giving you a clearer view of the monthly payment impact.