You have found a promising buy-to-let, dropped the numbers into a free calculator, and it hands back a cash-on-cash figure of 11%. Tempting. The problem is that most of the calculators near the top of Google were built for a landlord in Ohio, and at least three of their assumptions do not apply to you in the UK.
Cash-on-cash return is one of the most useful numbers a landlord can track, because it answers a plain question: for every pound of my own money tied up in this deal, how many pence come back each year? Here is how to calculate it the UK way, and where the American versions quietly mislead.
What cash-on-cash return actually measures
Cash-on-cash return is your annual net cash flow before tax divided by the total cash you actually put into the deal. Nothing borrowed, nothing on paper.
Cash-on-cash return = (annual net cash flow before tax / total cash invested) x 100
Two things set it apart from yield. First, it is built on your real cash flow after the mortgage is paid, not the rent in isolation. Second, the denominator is the money that left your bank account, not the property price. A 6% gross yield can turn into a 2% or a 9% cash-on-cash return depending on how much you borrowed and at what rate.
What counts as "cash actually deployed"
This is the number people most often get wrong. Your cash in is every pound you spent to get the property tenanted, not the headline price:
- Deposit (the part you are not borrowing)
- Stamp Duty Land Tax, including the additional-property surcharge
- Legal and conveyancing fees
- Survey or valuation fees
- Mortgage arrangement and broker fees
- Initial refurbishment and any furnishings
The mortgage itself is not cash you deployed, so it never goes in the denominator. That is the whole point of the metric: it measures the return on your money, with the bank's money doing its own work.
A worked UK example
Take a £180,000 terraced house bought with a 25% deposit on an interest-only buy-to-let mortgage. Rates move often, so treat this as an illustration at the time of writing (mid-2026) and check live figures before you rely on them.
| Item | Amount |
|---|---|
| Purchase price | £180,000 |
| Deposit (25%) | £45,000 |
| Interest-only loan (75%) | £135,000 |
| Mortgage rate (illustrative) | 5.5% |
| Annual mortgage interest | £7,425 |
| Gross rent (£975/month) | £11,700 |
| Operating costs (management, insurance, repairs, voids) | £2,600 |
| Net operating income | £9,100 |
Cash flow before tax is £9,100 minus £7,425 of interest, which is £1,675 a year.
Now the cash you actually deployed:
| Cash deployed | Amount |
|---|---|
| Deposit | £45,000 |
| SDLT (incl. 5% surcharge) | £10,100 |
| Legal and survey fees | £2,000 |
| Broker fee and initial works | £2,900 |
| Total cash in | £60,000 |
Cash-on-cash return = £1,675 / £60,000 = 2.8%.
That is a realistic figure for a leveraged UK BTL while rates sit near 5 to 6%. It is not exciting, and that honesty is exactly why the metric earns its keep. A US calculator that shows 9% on the same house is almost certainly making assumptions that do not hold here.
Where US calculators get it wrong
1. Phantom property taxes
American calculators ask for annual "property tax", often defaulting to around 1 to 1.5% of value. On a £180,000 house that is roughly £2,000 a year of invented cost, because in England the equivalent, council tax, is paid by the tenant while the property is let. Leave that default in and your cash flow, and your cash-on-cash return, are understated. Your one-off tax is Stamp Duty, which belongs in the cash-in figure, not in annual costs.
2. Depreciation that does not exist here
US tax lets landlords depreciate a residential building over 27.5 years and deduct that against income. Some calculators fold this "depreciation shield" into an after-tax return, which flatters the number. UK residential property gets no such allowance. There is no depreciation deduction on the bricks, so any American after-tax cash-on-cash figure is simply not reachable here.
3. Full mortgage-interest relief
A US calculator assumes mortgage interest is fully deductible. For UK individuals it is not. Since April 2020, Section 24 has restricted relief on residential mortgage interest to a 20% basic-rate tax credit (gov.uk). A higher-rate taxpayer therefore keeps less of the rent than any US model predicts, which is why cash-on-cash before tax and after tax can look very different for the same person. Companies still deduct interest in full, which is one reason many landlords now hold through a limited company.
4. The wrong stamp duty
The additional-property surcharge rose to 5% on 31 October 2024 in England (gov.uk), on top of the standard SDLT bands. On the £180,000 example that is £10,100 of Stamp Duty, a large slice of the £60,000 cash in. US closing-cost assumptions of 2 to 3% badly understate this, and because SDLT sits in the denominator, understating it inflates the return.
Cash-on-cash is not the whole picture
Cash-on-cash deliberately ignores two things: capital growth and mortgage capital repaid. On an interest-only loan there is no capital being repaid, so your total return is cash-on-cash plus whatever the property appreciates. On a repayment mortgage, part of your monthly "cost" is actually building equity, so cash-on-cash understates your true return. Use it as your cash-flow reality check, and pair it with yield and total ROI for the fuller view.
If you want to pressure-test a deal quickly, the cash-on-cash return calculator uses the UK definition above, so you are not fighting phantom property taxes or depreciation.
A 30-second sense check
Before you trust any calculator, confirm that it:
- Excludes the borrowed money from cash in
- Includes the 5% SDLT surcharge in cash in
- Does not add annual US-style property tax
- Shows a before-tax number, then applies Section 24 separately
This is general information, not tax or financial advice. Figures for rates and stamp duty are current at the time of writing (mid-2026); always check the latest gov.uk guidance and speak to a qualified adviser before committing to a purchase.
