Offset Mortgage Calculator
An offset mortgage links your savings to your mortgage balance, so interest is only charged on the difference between the two — £200,000 owed against £30,000 in linked savings means interest is calculated on just £170,000. This calculator compares that arrangement directly against the alternative of keeping your savings separate and earning interest on them normally, so you can see which genuinely comes out ahead for your specific numbers.
Offset Mortgage vs Savings
Compare keeping savings in a savings account versus using them to offset your mortgage.
Why Offsetting Is Really a Tax-Free Return in Disguise
The real advantage of offsetting is not obvious at first glance: money sitting in a standard savings account earns interest that is potentially taxable, while money offset against a mortgage does not earn interest at all — it simply avoids mortgage interest instead, and interest you never earned in the first place cannot be taxed. This means offsetting effectively delivers a tax-free return equal to your mortgage rate, which can be worth considerably more than the after-tax return from an equivalent amount sitting in a standard taxable savings account, particularly once your Personal Savings Allowance for actual savings interest is exceeded.
This is a genuinely subtle point that is easy to miss when simply comparing headline interest rates side by side — the meaningful comparison is not the mortgage rate against the savings rate directly, but the mortgage rate against the after-tax savings rate, which can be a noticeably lower figure once tax is actually applied.
Seeing the Offset Advantage in Real Pounds
A £200,000 mortgage at 4.5% over 25 years, with £30,000 offset, cuts the monthly payment from roughly £1,112 to around £945 — a saving of about £167 a month, or roughly £20,025 in total interest across the full term. For a higher-rate taxpayer, keeping that same £30,000 in a standard taxable savings account at 4% instead would earn roughly £18,000 net of 40% tax over the same 25 years — meaning offsetting comes out around £2,025 ahead for this specific higher-rate taxpayer, purely from the tax-free nature of the arrangement.
The Higher Rate That Can Quietly Erase the Benefit
Offset mortgages typically carry a slightly higher interest rate than an equivalent standard mortgage, reflecting the additional flexibility and product complexity involved — this calculator flags this explicitly as a caveat, since the tax-free benefit of offsetting can be partially or entirely offset (in the other sense of the word) by simply starting from a higher baseline rate. It is worth comparing the actual specific offset product rate against a genuinely equivalent standard mortgage rate, not just assuming the offset concept alone guarantees a better outcome regardless of the rate attached to it.
This calculator uses the same rate for both scenarios to isolate the pure effect of offsetting itself, which is useful for understanding the underlying mechanic clearly — but a genuinely accurate real-world comparison should use the actual quoted rate for each specific product, since the rate gap between an offset and standard product varies by lender and can meaningfully change which option comes out ahead.
Why a Higher-Rate Taxpayer Benefits More Than a Basic-Rate One
Because offsetting’s core advantage is avoiding tax on savings interest, the size of that advantage scales directly with your marginal tax rate — a higher-rate taxpayer, who would otherwise lose 40% of savings interest above their Personal Savings Allowance to tax, benefits more from offsetting than a basic-rate taxpayer whose allowance likely covers most or all of their real-world savings interest already. This calculator adjusts its comparison based on your specified tax band precisely because the genuine appeal of offsetting is not identical for everyone — it depends meaningfully on how much of your alternative savings interest would actually have been taxed in the first place.
A basic-rate taxpayer with a relatively modest savings balance may find their real-world savings interest sits comfortably within their Personal Savings Allowance anyway, meaning little or no tax would actually have been paid on it regardless — in that specific situation, the tax-free advantage of offsetting matters considerably less, since there was little tax to avoid in the first place.
Checking an ISA Before Committing to an Offset Product
Before concluding that offsetting is the better tax-free option, it is worth checking whether a cash ISA offering a competitive rate is available, since ISA interest is also entirely tax-free, up to your annual £20,000 allowance, without needing an offset mortgage product at all. Our ISA Growth Calculator models that comparison directly. Our Savings Interest Calculator is useful too for the standard taxable savings account scenario this calculator compares offsetting against, if you want to explore that side of the comparison in more depth.
Frequently Asked Questions
Can I still access my savings if they are offset against my mortgage?
Yes, generally — offset savings typically remain accessible, similar to a standard savings account, though withdrawing reduces the offset benefit immediately, since a smaller linked balance means more of the mortgage is charged interest.
Does offsetting reduce my mortgage term or my monthly payment?
This depends on the specific product — some offset mortgages reduce the monthly payment while keeping the term the same, while others keep the payment the same and shorten the term instead, so it is worth checking which structure your specific product offers.
Is offsetting only useful for large savings balances?
It becomes more meaningful with a larger balance in absolute pound terms, but the underlying tax-free benefit applies proportionally at any balance size, so it is worth calculating your own specific numbers rather than assuming a minimum threshold is required for it to be worthwhile.
Do all lenders offer offset mortgages?
No — offset products are a more specialist offering compared with standard mortgages, available from a narrower range of lenders, so the choice of available rates and terms is typically more limited than the broader standard mortgage market.
Can multiple savings accounts be linked to one offset mortgage?
Many offset products allow several linked accounts, sometimes including accounts held by family members, which can be useful for combining savings across a household to maximise the offset benefit against a single mortgage.
Important Information
This calculator provides an estimate based on the figures and assumptions you enter and does not constitute financial advice. Actual offset mortgage rates and terms vary by lender and should be compared directly against your specific circumstances. For advice specific to your situation, consult a mortgage broker or FCA-regulated adviser. See our Disclaimer for further information.