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, since the investor can overturn or veto the decisions of other board members.
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.