Diagonalise sends your invoices, helps you get paid, and keeps your expenses and profit in one place, organised around the 6 April tax year, so your Self Assessment figures are ready long before 31 January.
Free to start, with 30 days of Professional included. No card needed.
Most bookkeeping tools think in calendar years. Diagonalise shows your profit for the UK tax year you're in, how many days are left, and when your next return is due.
Switch the dashboard, invoices, expenses and customers to any tax year in one click.
Each expense category maps to its HMRC box, so your totals line up with the self-employment pages.
Income counts on the day you were paid, not the day you invoiced.
Create an invoice, email it to your client, and Diagonalise watches the due date for you. Late invoices are flagged as overdue, and reminders go out on the schedule you choose.
Every invoice has its own secure link. Your client opens it to view the invoice, download the PDF and find your bank details, with no account or password to remember.
Log an expense in seconds, attach the receipt, and file it under a category you choose. See your spending as a list or on a calendar.
Turnover, allowable expenses and net profit, laid out the way HMRC asks for them. Use the simple three-line version, or the full SA103 breakdown.
Connect Diagonalise to Claude, ChatGPT or GitHub Copilot, and do your bookkeeping in plain English. You approve the connection, and it can only do what you allow.
Read only on Free. Read and write on Professional.
Invoice Hart & Co for 3 days of consulting at £450 a day, plus VAT.
Draft invoice INV-0045 is ready for Hart & Co: £1,350.00 plus £270.00 VAT, £1,620.00 in total, due in 30 days.
Want me to mark it as sent?
We're building a way to send your quarterly updates to HMRC from your Diagonalise account, using the same Self Assessment categories your expenses are already sorted into. It isn't available yet, and we'll only switch it on once HMRC has recognised Diagonalise. Until then, your figures are ready to copy into your return or pass to your accountant.
Contact details, payment terms and a full invoice history for every client.
Save the things you sell with their prices, then add them to invoices in a click.
Give them a free login to see your figures. Add a partner or assistant too, and choose exactly what each person can change.
Run separate businesses from one login and switch between them from the menu.
Names, addresses and bank details are encrypted before they're stored.
Check what you're owed or log an expense from wherever you are.
Sign up with your email. Your 30-day trial of Professional starts straight away.
Your details, bank information for invoices, and the clients you bill.
Email it from Diagonalise, then log expenses as they happen and watch the year add up.
Diagonalise is online invoicing and expense tracking for UK sole traders and freelancers. It keeps your invoices, expenses and profit organised by the UK tax year (6 April to 5 April) and gives you the figures for the self-employment pages of your Self Assessment return.
One-person businesses in the UK: sole traders, freelancers and contractors who send invoices, pay their own expenses and file a Self Assessment tax return. One-person limited companies can use it for invoicing and expenses too.
Yes, for invoicing, getting paid and tracking expenses. Add your company number and VAT number and they appear on your invoices. The tax reports are built for sole traders: they give Self Assessment (SA103) figures and an Income Tax estimate, not Corporation Tax figures, so a company's year-end accounts and tax return stay with your accountant.
Yes. Each expense category maps to its box on the SA103 self-employment pages, and the reports show turnover, allowable and disallowable expenses and profit on the cash basis, with an indicative Income Tax and Class 4 National Insurance estimate. Check your return before you file.
Yes. Diagonalise has an MCP server, so you can connect it to Claude, ChatGPT, GitHub Copilot and other AI apps, then create invoices, log expenses and check your figures by asking. You approve each connection. Read only on Free. Read and write on Professional.
Every invoice has its own secure link your client can open to view it, download the PDF and see your bank details for payment. You can email it with the invoice or copy it into a message.
There is a Free plan you can keep for as long as you like, with no card needed. Every new account also gets 30 days of Professional free; if you don't upgrade, you stay on Free and keep all your data.
Not yet. Quarterly updates for Making Tax Digital for Income Tax are under development, and we'll only switch them on once HMRC has recognised Diagonalise. Until then, your SA103 figures are ready to copy into your return or hand to your accountant.
Free to start, with 30 days of Professional included. No card needed.
Start free