Work out your full return and download a summary mapped to the official HMRC box numbers, ready to file online at GOV.UK. Everything runs in your browser.
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A preparation aid, not tax advice, and not a filing service. 97% of people file Self Assessment online at GOV.UK. This tool prepares your figures and shows exactly which HMRC box each goes in, for you to file (online by 31 Jan, or paper SA100 by 31 Oct). Nothing is sent or stored.