An investor achieves a controlling interest in a business by owning any amount more than 50% of the shares in the entity. Doing so gives the investor significant influence over the actions of the company, including its strategic and operational decisions.
For example, 50% of all shares plus one share gives an investor a controlling interest. It is also possible to achieve a controlling interest when there is a separate class of voting stock, and the investor owns a majority of those shares. Yet another scenario is when ownership in a business is widely dispersed among many investors; in this case, an investor effectively has a controlling interest with fewer than 50% of the shares outstanding. For example, an active shareholder in a publicly held company could achieve significant influence over the firm with a stake of as little as 10% of the shares outstanding.