What the calculator works out
Enter the price, refurb, mortgage and rent and the calculator returns gross and net yield, monthly cashflow, total cash in and cash-on-cash return, with stamp duty calculated automatically from the purchase price and the 5% additional-property surcharge.
The stress test and the post-tax view
Where it goes further than a spreadsheet is the stress test and the post-tax view: it shows what happens to your cashflow if interest rates jump, and it compares your post-tax return as a sole trader (under Section 24) versus in a Company, for both the 2026/27 and 2027/28 tax years.
That means you see the deal on your own figures, not the flattering gross number that ignores tax, voids and rate risk.