Part and Part Mortgage Calculator — The Middle Ground, Modelled

A part and part mortgage — sometimes called a hybrid or part-repayment, part-interest-only mortgage — splits your loan into two separate portions under a single mortgage. One portion is on a capital repayment basis, paying down over the term as normal. The other portion is interest-only, meaning your payments cover just the interest, with the original capital still owed in full at the end.

You decide the split before applying — for a £250,000 mortgage, you might choose £200,000 as capital and interest and £50,000 as interest-only, or any other combination a lender will accept. This is exactly what a proper part and part mortgage calculator should let you explore before you ever fill in a lender's application form — testing different splits to see how each one changes your monthly outgoings and what's left owing at the end.

It helps to see a part and part mortgage calculator alongside the two options it sits between, rather than in isolation. A full repayment mortgage clears the entire loan by the end of the term, but at the highest monthly cost of the three structures. A full interest-only mortgage gives the lowest possible monthly payment, but leaves the entire original loan amount still owed at the end, with nothing paid down along the way. A part and part structure sits deliberately in between — a genuinely flexible middle ground, rather than a compromise that's worse than both alternatives.

What makes this genuinely useful in practice is that the split doesn't have to be fixed forever. Many borrowers start with a larger interest-only portion while income is tighter — early in a career, or while managing other costs — and shift the balance toward more repayment later, as circumstances improve, either at a remortgage point or through lender-permitted adjustments during the term.

The appeal is straightforward: a part and part structure gives you a lower monthly payment than a full repayment mortgage, while still clearing some of the capital each month — unlike a fully interest-only loan, where none of it reduces. The cost of that lower monthly payment is straightforward too: whatever sits on the interest-only side is still owed in full when the term ends.

This is worth being genuinely direct about. Lenders require a credible repayment strategy for the interest-only portion before approving a part and part mortgage — investments, savings, another asset, or a clear intention to sell the property or downsize. Many lenders expect the value of this repayment vehicle to be roughly matched to the amount owed, reviewed periodically over the life of the mortgage, not just assumed at the outset and forgotten about.

Many UK lenders cap the interest-only element specifically at somewhere between 50% and 75% loan-to-value, even if the overall mortgage LTV is higher. This means a meaningful deposit or existing equity is required against that portion of the loan, regardless of how the rest of the mortgage is structured — a detail worth confirming with a specific lender before assuming a particular split is achievable.

Here's how the numbers work through a real part and part mortgage calculator scenario. A £250,000 mortgage, 25-year term, 4.5% rate, split 50/50. The £125,000 interest-only portion costs roughly £469 a month, interest only. The £125,000 repayment portion costs roughly £694 a month. Combined: £1,163 a month — compared to roughly £1,389 a month on a full repayment mortgage for the same £250,000, a saving of around £226 a month.

At the end of the 25 years, the repayment portion is fully cleared — but the £125,000 interest-only portion is still owed in full, needing to be repaid from whatever vehicle was arranged at the outset.

Official sources & further reading: read general mortgage guidance at MoneyHelper, and verify any lender or broker on the FCA register. Compare against a full repayment mortgage with our residential mortgage calculator, or browse every tool on the mortgage calculators homepage.