The money will be paid in a single lump sum but will not begin to arrive until the start of June at the earliest.
Self-employed people will be able to apply for a grant worth 80% of their average monthly profits over the last three years, up to £2,500 a month.
Mr Sunak said the grants would be available to people across the UK for at least three months, and longer if necessary.
In all, 95% of people who earn most of their income as self-employed would be covered, the chancellor added.
The Coronavirus Self-Employment Income Support scheme is targeted at up to 3.8 million of the 5 million people registered as self-employed, who earn under £50k. The money, backdated till March, will arrive directly into people's banks accounts from HMRC as a lump sum for all three months, but not until June. The grants will be taxable and will need to be declared on tax returns by January 2022.
At least half their income needs to have come from self-employment as registered on the 2019-20 tax return filed in January or averaged over the three previous years.
Company owners who pay themselves a dividend are not covered.