There is one calculation engine behind this site, and the Ready Reckoner runs it twice: once with the published 2026/27 rules, once with the Budget's. Nothing is re-implemented for Budget Day, so the "before" column is the same figure the main calculator gives you.
That matters because the usual Budget-day failure is a second, hurried spreadsheet that disagrees with the main product. Running the same engine twice means the two can't drift apart.
A tax that stayed put is a real result, and most Budget coverage never tells you. The Chancellor announces a dozen measures; the useful question is which of them touch you — which is usually fewer than the headlines suggest.
There's an important distinction underneath that, and this tool keeps it visible: a £0 that was measured is not the same as a £0 that was never modelled. Anything the Budget announces that isn't a change to an existing rate or threshold — a genuinely new tax mechanism — is reported as not yet modelled, never quietly estimated at zero.
Until 28 October there is nothing to compare against, so this page shows your position under today's rules and the full list of taxes it will check. It does not show a prediction. Nobody knows what's in the Budget, and a calculator that guessed would be worth less than one that waits.