Mortgage porting calculator
When you port a mortgage and borrow more, you keep your existing rate on the amount carried over and take the extra as a top-up at a current-day rate. Your true cost is the blend of the two. Porting £180,000 at 2.0% and adding £40,000 at 4.5% over 22 years costs about £1,082 a month, a blended rate of roughly 2.45%. This free calculator shows the combined monthly cost and blended rate so you can compare porting against a full remortgage. No email, nothing stored.
Blend your ported deal with a top-up
Free. No email needed. Runs in your browser, stores nothing.
Total monthly
£1,082
Blended rate
2.45%
Total borrowing
£220,000
Worked example: £180,000 ported at 2.0% costs about £843 a month, and £40,000 of new borrowing at 4.5% adds about £239, a total of £1,082. The blended rate across the whole £220,000 is about 2.45%. Change the figures for your own move.
Simplified illustration for a repayment mortgage, with each rate held for the term shown. It ignores fees, any early repayment charge, and the different end dates the two parts may have. Not a quote and not advice. A regulated broker can confirm your real figures and compare porting with a full remortgage.
Why the blended rate is the number that matters
When you port and borrow more, you end up with two parts on one mortgage: the amount you carried over at your old rate, and the top-up at a current rate. Neither rate alone tells you what the mortgage really costs; the weighted average of the two does. If most of the debt is the low-rate ported part, the blend stays close to your old rate, and porting looks strong. If the top-up is large, the blend rises toward today\'s rates, and a full remortgage may compete. See porting a mortgage for how the process works.
Port, or remortgage the whole lot?
Porting protects a good existing rate and can dodge an early repayment charge, but the lender must accept the new property and reassess your affordability, and the extra borrowing is at today\'s rates. A full remortgage of the whole balance opens the wider market but loses your old rate. The blended rate here lets you line the two up honestly, and a broker can confirm which wins for your move.
Common questions
What does porting a mortgage mean?
Porting means taking your existing mortgage deal, at its current rate and terms, to a new property when you move, instead of paying it off and starting again. You keep the rate you have on the amount you are carrying over, which is valuable if that rate is lower than what is available today. The lender still has to approve the new property and reassess affordability.
How does borrowing more when porting work?
If the new home costs more, you usually borrow the extra as a separate top-up on a current-day rate, on top of your ported balance. That leaves you with two parts on one mortgage: the ported amount at your old rate, and the additional borrowing at a new rate. The blended cost sits between the two, which is what this calculator shows.
Why is the blended rate useful?
Because when you port and top up, your true cost is not your old rate and it is not the new rate; it is a weighted average of the two, based on how much sits on each. The blended rate tells you the real overall cost of the combined mortgage, so you can compare porting against simply remortgaging the whole amount to one new deal.
Is porting always the best option?
Not always. Porting protects a good existing rate and can avoid an early repayment charge, but the lender must accept the new property and your current affordability, and the top-up is at today's rates. Sometimes a fresh remortgage of the whole balance to a new lender works out better, or an early repayment charge changes the maths. A broker compares porting against the alternatives.
Founder, MortgageExplained, MortgageExplained
Adam spent nearly a decade as a mortgage adviser at Just Mortgages, with further experience in commercial finance. He is CeMAP and CF qualified. He built MortgageExplained to do one thing well: explain mortgages in plain English, then introduce you to a regulated broker when you are ready. Every page is written and reviewed by Adam.
Last reviewed: 21 July 2026