D4 Free tools
Tools that do the sums.
Four free tools from William’s corner of the internet. They all run in your browser, so nothing you type is sent anywhere, and there is nothing to sign up for.
Take-home pay calculator
Enter a salary and see the Income Tax, National Insurance, pension, and student loan coming off it, and what is left for you, by year, month, and week.
Work out my payVAT & margin calculator
Add or remove VAT at any rate, work out profit margin and markup (they are not the same thing), and handle percentage changes without the head-scratching.
Do the mathsWillCel spreadsheet
A genuinely working mini spreadsheet: type numbers, write formulas like =SUM(A1:A5) or =IF(B2>0,"Profit","Loss"), and download the result as a CSV.
Open WillCelHow Excel-lent are you?
A quick-fire quiz on shortcuts, functions, and bookkeeping basics, with an explanation after every answer. Are you a #DIV/0! or a spreadsheet wizard?
Take the quizPrivate by design
Every calculation happens on your device. No accounts, no tracking, no data sent to a server.
Free, properly
No trials, no paywalls, no “premium tier”. Just tools that work.
Estimates, not advice
The calculators use published 2026/27 UK figures, but they are guides. Check GOV.UK for anything important.