Jack Dorsey, the CEO of both Twitter and digital payments platform Square, said on Tuesday that he will donate $1 billion worth of equity in Square to his Start Small LLC to fund COVID-19 relief around the world. Dorsey made the announcement in a tweet, revealing that the sum equates to roughly 28 percent of his current net worth, or about $3.6 billion. The announcement marks the most significant philanthropic effort from the 43-year-old tech executive in his career.

Dorsey is far from as wealthy as tech moguls like Facebook CEO Mark Zuckerberg, Microsoft co-founder Bill Gates, and Amazon CEO Jeff Bezos. Yet, thanks to the performance of the shares he owns in both Square and Twitter, he is still a billionaire and until last year had not openly made plans to donate a majority of his wealth beyond the existence of the Start Small fund, which Forbes reports is a donor-advised fund that doesn’t have to disclose where its investments are directed.