Investor Warren Buffett has given away Berkshire Hathaway stock worth $13.5 million to five different charities since last summer.

The gifts of Class B Berkshire stock were disclosed in documents filed with the Securities and Exchange Commission on Tuesday. These relatively smaller gifts are part of Buffett's overall plan to give away his total fortune over time.

The five gifts disclosed this week were made between August and October. The gifts all went to unnamed charities.

Buffett plans to eventually give most of his Berkshire stock to five foundations with the biggest share going to the Bill & Melinda Gates Foundation.