Sixth Street Lending Partners is an externally managed, closed-end, private investment company focused on lending to upper middle-market companies.
The Company was formed on April 5, 2022. Since we began our investment activities in August 2022, through March 31, 2024, we have originated approximately $12.3 billion aggregate principal amount of investments and retained approximately $4.4 billion aggregate principal amount of these investments on our balance sheet prior to any subsequent exits and repayments. We seek to generate current income and long-term capital appreciation primarily by investing in U.S.-domiciled upper middle market companies through direct originations of senior secured loans and, to a lesser extent, originations of mezzanine and unsecured loans and investments in corporate bonds, equity securities, and other instruments.
By “upper middle-market companies,” we mean companies that have annual EBITDA (as defined below), which we believe is a useful proxy for cash flow, of greater than $75 million, although we may invest in smaller companies on occasion. “EBITDA” means a company’s earnings before interest, tax, depreciation and amortization. As of March 31, 2024, our core portfolio companies, which exclude certain investments that fall outside of our typical borrower profile and represent 98.2% of our total investments based on fair value, had weighted average annual revenue of $593.0 million and weighted average annual EBITDA of $201.0 million.
As of March 31, 2024, 99.7% of our debt investments based on fair value bore interest at floating rates, with 100.0% of these subject to interest rate floors, which we believe helps act as a portfolio-wide hedge against inflation. As of March 31, 2024 we had investments in 45 portfolio companies, with an average investment size of approximately $95.1 million based on fair value. As of March 31, 2024, the largest single investment based on fair value represented 7.7% of our total investment portfolio.
We have elected to be regulated as a business development company, or a BDC, under the Investment Company Act of 1940 and the rules and regulations promulgated thereunder. We are externally managed by Sixth Street Lending Partners Advisers, LLC an affiliate of Sixth Street and a Securities and Exchange Commission (“SEC”) registered investment adviser. We leverage the deep investment, sector, and operating resources of Sixth Street, a global investment firm with over $77 billion of assets under management.