Everything you need from invoice to tax return.

Here's what Diagonalise does, shown with the screens you'll actually use.

Invoices

Create an invoice in a couple of minutes, download it as a PDF, and let Diagonalise keep track of what's been paid.

  • Automatic invoice numbering
  • Pounds, dollars or euros
  • PDFs with PO numbers, payment details and attached documents
  • Draft, sent, part paid, paid and overdue status, with overdue flagged automatically
  • Deposits and part payments, with every payment and refund recorded
  • Mark a batch of invoices as paid in one go
  • Clone an invoice to bill the same work again
Invoice list filtered to the 2026/27 tax year, with three invoices selected and a Mark as paid button

Getting paid

Every invoice has its own secure link. Your client can view the invoice, download the PDF and find your bank details, without creating an account.

  • Send the link in the invoice email, or copy it into a message
  • Offer a deposit, or let clients pay in parts
  • Record each payment as it arrives and see what's still owed
  • Turn a link off or replace it whenever you like
Northwind Studio Invoice INV-0045
Amount due
£1,620.00
Due 7 November 2026
Pay by bank transfer
Sort code
04-00-04
Account number
12345678
Reference
INV-0045
Download PDF

Expenses

Log what you spend as you go, attach the receipt, and you'll never be hunting for it in January.

  • Your own categories, each mapped to an HMRC box
  • Receipts attached as photos or PDFs
  • Supplier tracking, search and filters
  • A calendar view of your spending
  • CSV export for your accountant
Expense list for the 2026/27 tax year showing £767.14 spent across 10 expenses, each with a category and supplier

Reports and Self Assessment

Every report follows the UK tax year, 6 April to 5 April. Pick a year and see the numbers you'll need for your return.

  • Turnover, allowable expenses and net profit on a cash basis
  • Three-line accounts, or the full SA103 breakdown
  • VAT registration threshold tracker
  • Tax and payment on account estimate
  • Monthly income and expense breakdown
UK Self Assessment summary: turnover £31,240.00, allowable expenses £6,892.40, net profit £24,347.60

Emails and reminders

Send invoices from Diagonalise and let it do the chasing.

  • Email an invoice with its PDF and online link
  • Sent from your business name, with replies coming to you
  • Automatic reminders before and after the due date, on your schedule
  • Pause reminders for any invoice or customer
  • Edit the wording of your invoice and reminder emails

Your brand

Make your invoices, emails and payment page look like they came from you.

  • Your logo and brand colours
  • Classic, Modern, Minimal or Bold invoice templates
  • Your own footer text on every invoice
  • Preview your payment page before clients see it

Customers and products

Keep your clients and what you charge them in one place.

  • Contact details, company and VAT numbers
  • Payment terms per customer
  • Invoice history and totals for each client, by tax year
  • A product and price list to add to invoices

Your team and your accountant

Bring in a partner, an assistant or your bookkeeper, and decide what each of them can do.

  • A free login for your accountant or bookkeeper to see your figures
  • Invite people with a personal sign-up link
  • Owner, admin, editor and viewer roles, or make your own
  • An activity log of who changed what
  • Run more than one business from a single login (Professional)

AI assistants

Connect Diagonalise to Claude, ChatGPT, GitHub Copilot or any app that supports the Model Context Protocol (MCP), and work with your books by asking.

  • Create and update invoices, customers, products and expenses
  • Record payments, share an invoice's payment link, and get the PDF
  • Profit, reports and Self Assessment figures for any tax year
  • Sign-in approval, with read-only or read-and-write access
  • Read only on Free. Read and write on Professional.
  • Limited to your role, and never settings, billing or HMRC
  • Personal access tokens for scripts and developer tools

See how it works →

Diagonalise connected

How much profit have I made this tax year?

Ran a financial summary in Diagonalise

So far in 2026/27 you've been paid £31,240.00 and spent £6,892.40, leaving a profit of £24,347.60.

September was your best month. Want the Self Assessment breakdown?

See it with your own numbers.

Free to start, with 30 days of Professional included. No card needed.

Start free