https://www.irs.gov/pub/irs-pdf/p544.pdf

Exclusion of gain. You may be able to exclude from your gross income 50% of your gain from the sale or exchange of qualified small business stock you held more than 5 years. The exclusion can be up to 75% for stock acquired after February 17, 2009, and up to 100% for stock acquired after September 27, 2010. The exclusion can be up to 60% for certain empowerment zone business stock. 

Your gain from the stock of any one issuer that is eligible for the exclusion is limited to the greater of the following amounts. 

• Ten times your basis in all qualified stock of the issuer you sold or exchanged during the year. • $10 million ($5 million for married individuals filing separately) minus the gain from the stock of the same issuer you used to figure your exclusion in earlier years. 

More information. For more information on sales of small business stock, see chapter 4 of Pub. 550. See the Instructions for Schedule D (Form 1040) and the Instructions for Form 8949 for information on how to report the gain.